List of identified Nigerian indigenous plant foods (single foods) with anti-diabetic potentials/effects by food groups
Plants play important role in the cycle of nature. This is because life on earth basically depends on them. Plants provide man with all his needs as regards food, shelter, clothing, flavours and fragrance as well as medicine. They are naturally occurring substances that produce almost all the foods that animals as well as humans eat. They have unique potential to make their own food through photosynthesis. All foods that people eat naturally come directly or indirectly from plants. Food is fundamental to human survival. It is basic for averting hunger, satisfying one’s palate and maintaining health of every human being. It is used as a status symbol, making people happy, emotionally and socially content, and constitutes a form of cultural expression. It is also used in the performance of various rituals and rites as well as for therapeutic purposes. Culture has a lot of influence on the kind of foods people eat and how they eat them. Traditional foods, was adopted to describe all foods from a particular culture, available from local sources and culturally acceptable as appropriate and desirable foods . The direct and indirect food sources as regards nutrition are simply described as and categorized into two major sources - plant and animal foods, respectively.
Despite the unique role of plant foods as irreplaceable food sources for humans, not all plant foods are available or edible worldwide. Traditional plant foods are those plants grown for food in varying proportions within the farming system operating in any particular locality; or gathered in wild or semi-wild conditions; and are accepted by the community, through custom, habit and tradition as appropriate and desirable foods [1, 2]. They are categorized as those that are consumed as traditional dietary staples and those consumed as components of accompanying relishes and sauces. These traditional foods are often used in sciences as basis for ensuring and optimizing utilization of indigenous foods and their health benefits by individuals, households and communities. The use of plant foods for therapeutic purposes represents one of the biggest human uses of the natural flora of the world . This is because of their health-enhancing bioactive constituents . In the last four decades, there has been considerable interest in resurrecting health-promoting potentials of indigenous plant foods in developing countries and integrating their use into modern medical system. The reasons for this interest are varied and include, i) the negative impact of nutrition transition that increase epidemic of diet-related non-communicable chronic diseases (NCDs) in such regions; ii) high cost of medicare, which put modern health care out of reach of the poor; iii) low adherence to drug prescriptions; and iv) side effects of medicinal plant and herbal medicine extracts and/or preparations, because out of ignorance the consumers consume them as food supplements that are safe at any dose. This is not true for any of the high biologically active medicinal plants. Most of them cannot be consumed habitually as local foods without adverse effects. There was need to challenge reduced interest in and demand for drugs amidst poor health management in these poor regions. Pharmaceutical preparations from medicinal plants created a new expanding market as herbal components of health foods and preventive medicine under “nutraceuticals” or “nutriceuticals”. Culhane  defined them as products produced from food and sold in pills, powders and other medicinal forms not generally associated with food and are shown to have a physiological benefit or provide protection against chronic diseases. Chronic diseases are largely preventable diseases. Diet-related NCDs are diseases of long duration and generally slow in progression . They are relatively difficult to manage. The high cost and side effects of these supplements still limit their use and have made this approach an unsustainable alternative strategy for chronic disease management and prevention.
Carbohydrate food sources form the greatest percentage [50-60%) of the daily diet for different segments of the population . In Nigeria, starchy staples (cereals, roots/tubers) and legumes constitute the major part of the traditional diets, up to 70% and 25%, respectively [8, 9, 10]. Physiologically, chronic restriction of carbohydrate–rich sources, mainly supplied by plant foods, may pose a serious threat to Nigeria’s survival. Diabetics are particularly adversely affected due to misinformation and inability to seek dietetic intervention. Such dietary misinformation has led to starvation and the development of psychosocial problems in diabetics. This is because they feel denied and full of anxiety in anticipation of lifestyle changes .
Type 2 diabetes is more common in Nigeria. There was increase in the average prevalence rate from 2.7%  to 3.9%  and annual increase of 0.3%. Over the years, diet was implicated both as one of the aetiological factors to the development of diabetes  as well as a key component in diabetes management [7, 15]. Plant foods are the most important dietary sources to meet the nutritional needs of majority of the population in sub-Saharan Africa and Asian subcontinent . Dietary fibre is a component of carbohydrate in plant foods shown to modulate post-prandial blood glucose after discovery of the “dietary fibre hypothesis” by Cleave  and Burkitt et al. . Roles of phytochemicals and antioxidant constituents of plant foods were reported . On this basis, plant foods hold good promise for diabetes management. There are mounting scientific evidence to date on their various health-promoting properties.
The global concern for the diversification of the uses of plant foods to improve normal and therapeutic nutrition for diabetes control has shifted scientists’ interest to enhancing the potential sources of beneficial constituents in plant foods. Plant foods have generated increasing research interest because of their anti-diabetic potentials. There is need to integrate traditional plant foods within local research and innovative systems, in accordance with local needs, food habits and priorities. Currently, documentations on Nigerian indigenous plant foods with anti-diabetic properties are either scattered or lacking. Most reviews on anti-diabetic potentials of plants both locally [19, 20, 21] and internationally [22, 23, 24] were done on medicinal plants. Evidently, not all medicinal plants and herbs used locally are edible. Similarly, not all identified food crops have anti-diabetic potentials. Most of the plants assayed pharmacologically present some difficulties in their dietetic application in humans. They create a gap in optimizing their health-promoting potentials. This paper focuses on some Nigerian indigenous plant foods with anti-diabetic potentials, parts used, their individual effects as reported
2. Plant foods in human nutrition and health
Plant foods have remained the ultimate source of nutrients for larger population of the world. They are simply described as irreplaceable food resources for humans, which exclude animal sources. Besides, they are available, affordable and acceptable. They are used for socio-cultural, diabolic, nutritional and therapeutic purposes. These foods contain many chemical compounds needed for metabolic functions in varying proportions. Some of these chemical compounds are non-nutrients that are beneficial to man and others provoke some adverse reactions depending on the levels of intake, interrelationships of nutrients and food habits. Plant foods are classified as cereals, roots and tubers, legumes (pulses, oil seeds and nuts), vegetables and fruits . In order to accommodate these various constituents in foods provided by plants foods, three main categories were established according to the major nutrients they provide. They include the macronutrients (carbohydrate, protein, fat and water), micronutrients (minerals and vitamins) and non-nutrient components (dietary fibre, phytochemicals, anti-nutrients, food toxicants and additives). Metabolic functions of these nutrients and non-nutrients were reviewed by several authors. Bennett  observed that plant foods provide the bulk of daily calories for humans [85%) and about 65% of the protein, an assortment of minerals and vitamins, unsaturated fats and phytochemicals in a characteristic manner that reflects current dietary recommendations aimed at promoting health and reducing diet-related chronic diseases. Thomas  concluded that the contribution a food makes to nutrient intake of a person depends on the quantity consumed per 100g, the amount consumed per meal and the frequency of consumption.
Obviously, plant foods have other values related to human nutrition and health apart from their nutritional importance. Plant foods are one of the elements in virtually all cultures, with symbolic and ritualistic values that link language and religion. In many cultures, especially the Asian countries, distinction between food and medicine as therapeutic agents is obscure as the traditional vegetable diets provide the people with disease fighting arsenals that serve both protective/preventive and curative functions. For this reason, foods are widely recognized as the primary health provider.
The bioactive compounds or secondary metabolites are the non-nutrient components in plant foods. They have some nutritional effects and health benefits. They are those substances contained in foods which supply no nutrients. They could contain some compounds that are beneficial to health or toxic to humans and/or act as antagonists to nutrients in foods . They include tannins and other phenolic compounds (phenols, flavonoids, isoflavonoids), saponins, glucosinolates, alkaloids [28, 29], phytate and dietary fibre . These chemical compounds are found in different classes and parts of plant foods in varying amounts. They are more concentrated in plant storage organs (leaves and seeds) than other parts of the plants . These constituents have their individual health-promoting qualities that compel people to combine the different food sources to achieve healthy eating and maintain good health. Several authors [27, 31-35] studied therapeutic potentials and metabolic effects of foods rich in dietary fibre and phytochemical constituents. These include lower risk of colon cancer, promotion of early satiety and normal laxation, moderation of post-prandial blood glucose responses and improved insulin sensitivity, reduction in total and low density lipoprotein (LDL)-cholesterol, regulation of appetite and enhancement of sodium and fluid balance. They are also used to treat constipation and prevent development of diverticulosis and diverticulitis. These are features of a dietary pattern to treat and prevent obesity and its co-morbidities (especially type 2 diabetes mellitus). These two conditions are closely linked because hyperglycaemia and hypercholesterolaemia are well-known cardiovascular risk factors in type 2 diabetes mellitus. Invariably, diets adequate in dietary fibre are usually rich in micronutrients and phytochemicals, and frequently less calorically dense and lower in fat and added sugars. However, environmental factors, cultural food habits and insufficient nutritional information about health benefits of traditional plant foods still pose a problem to healthy food choices. Drewnowski and Gomez-Carneros  reported that most of the bioactive compounds are bitter, acrid or astringent and aversive to the consumer and may be wholly incompatible with consumer acceptance. These caused increasing epidemic of diet-related diseases across the regions. They suggest the need to take sensory properties and food preferences into account when advocating for increased consumption and diversification of rich sources of these secondary metabolites in plant foods. The challenge of achieving adequate supply of energy and nutrient intake as well as the health-promoting compounds from plant-based foods/diets without compromising the health of an individual, forms the basis for current dietary recommendations aimed at promoting consumption of plant foods to reduce diet-related NCDs.
3. Plant foods utilization in Nigeria
Nigeria’s diverse ethnic groups (over 250] are accommodated under three main ethnic regions - Igbo (in the East), Yoruba (in the West) and Hausa-Fulani (in the North). The country has a beautiful climate, which endows it with very rich biodiversity. A variety of plant foods (maize, rice, sorghum, millet, yam, cassava, cocoayam and legumes) are mainly produced as subsistence food crops in Africa. They are more commonly and widely consumed in Nigeria and other developing countries than in the developed world. They are relatively available, affordable and acceptable. They contribute appreciably to the nutrient intake (energy, protein, fat, vitamins and minerals) of the less developed world. In Nigeria, starchy staples (cereals, roots/tubers) and legumes constitute the major part of the traditional diets (up to 70% and 25%, respectively) . However, cereals are the major staples in the north with much higher intake of animal protein sources (mutton, beef and milk), while starchy roots and tubers are the main staples in the south with relatively more consumption of legumes . These foods contribute appreciably to nutrient intake of southerners . The plant foods are cultivated, gathered wild or are semi-wild.
Roots and tubers are the thickened underground starch storage organs of some plants, propagated vegetatively from the underground stems and their stem cuttings. These edible roots and tubers belong to several families and are formed by both monocotyledons (yams and cocoyams) and dicotyledons (cassava and sweet potatoes) . Cereals are seeds of grass family (Gramineae). Cereals are important crops which serve as industrial raw materials and staple foods for the world over . Cereals are the most widely cultivated and consumed crops globally. World cultivated cereals include wheat, maize, rice, barley, oats, rye, sorghum, millet, wild rice and hungry rice (acha). In Nigeria, the starchy staples (maize, rice, sorghum, millet, yam, cassava, cocoayam and plantain) are utilized in many different forms for preparation of various dishes. Their nutritional importance is that they provide most of the energy, contain high carbohydrate, low protein and appreciable amount of minerals and vitamins, especially the B-complex vitamins. They form the base and usually constitute the major ingredients in the traditional dishes. Some dishes are light and serve as breakfast. Others are solid and frequently made palatable in combination with a variety of legumes and by the addition of palm oil, vegetables and fruits, a range of spices, various sauces and fish/meats. They serve as main meals and eaten later in the day. Processing and preparation methods of these starchy staples and their recipes vary with ethic groups and geographical locations. Each of these staples irrespective of the preparation methods produces a thick paste known as “foofoo” eaten with soups/sauces. These dishes are highly cherished and consumed daily in all parts of Nigeria . These starchy staples are also used to produce complementary foods and local snacks as roasted/fried/baked products or drinks. Some like yam, cocoyam, ‘okoho’ (
Legumes are flowering plants in the family Fabaceae (or Leguminosae) that have pods, which contain beans or peas. According to Okafor , a list of edible woody leguminous plants, consisting of 150 species covering 103 genera in 48 families was recorded in Nigeria. Legumes rank second to the grasses as source of fodder (for animal) and food for man. The plants are most commonly known as pulses, peas or beans . They contain more protein and less carbohydrate than cereals. There are two main types of legumes: those containing high protein [25-35%) and oil content [15-45%) and those containing moderate protein [18-24%) and less than 5% oil content. They are oil seeds and pulses, respectively. The most common forms in which legumes are used – they may be cooked alone as pottage, or eaten in combination with any starchy staple of choice. The more widely consumed legumes include cowpea, pigeon pea, soyabean, bambara-groundnut, african yam bean, groundbean and lima bean. It is due to their high protein content that legumes are widely used in combination with cereals for production of nutritious complementary foods for infants and young children both in Nigeria and other developing countries, in both industrial and household levels. This is because legumes are deficient in sulphur-containing amino acids (methionine and cystine) and contain high amount of lysine. Cereals are deficient in lysine and rich in the sulphur-containing amino acids. Legumes play very important culinary roles as soup thickeners. Many of the leguminous seeds are used for thickening soups, sauces and stews in Nigeria . These soup thickeners are condiments/ingredients used in preparation of local soups to alter their consistency and give them special flavour . Traditionally, roots/tubers (yam and cocoayam, ‘okoho’), vegetables (okro, ‘ogbamu’ leaves), legumes (grain legumes/oil seeds and nuts) and fungi are used for thickening soups. Soups made from them are basically prepared with meat and/or fish (when available), oil, vegetables, crayfish, pepper and other condiments and water to obtain watery or thick consistencies depending on their composition. These soups are traditionally consumed in combination with cereals and starchy roots/tubers, processed into a form of paste known as “foofoo”. Soups are very important accompaniment to main dishes in Nigeria. Ene-Obong et al.  recorded 110 soup recipes out of 322 recipes in all the 6 geo-political zones in Nigeria. The total number of recipes documented reflects the biodiversity of the Nigerian food system. The high viscous property possessed by these leguminous soup thickeners is associated with their dietary fibre content, implicated in attenuation of post-prandial blood glucose and control of lipid metabolism .
Vegetables are succulent herbaceous plants that are harvested and eaten whole or in part, raw or cooked as part of a main dish or salad . They can be classified into leaves (green, fluted pumpkin), stems/whole shoot (asparagus, elephant grass, ‘achara’), roots (carrot), flowers, immature fruit (fresh corn, okra), mature green fruits (peppers), ripe fruit (tomato), fresh pod (vegetable cowpea), bulbs (onions), tubers (irish potatoes, yams) and fungi (mushrooms, puffballs). There are over 500 known edible vegetables in Africa. Some of these are valued for their bulking effect/thickening power and others may be used as garnishes or spices. Vegetables are generally low in calories and protein. They are valued most for high vitamins and minerals contents. They are low in sodium and rich in potassium, which helps to lower blood pressure [44, 45]. They also contain dietary fibre and phytochemicals (pigments and other compounds that impart flavour to foods), which have heart-disease fighting properties. However, fruits and vegetables consumption is generally low in Nigeria despite their recognition as very important food items to reduce nutrient deficiencies from inadequate intake of minerals and vitamins .
Composition of vegetables depends on their species, part of plant and stage of maturity. Spices are vegetables used as ingredients, which contribute to the taste and flavour of foods. The flavour of vegetables is due to sugars, organic acids, minerals salts, volatile sulphur compounds and tannins. The strong flavour of some vegetables (onions and cabbage) is due to their sulphur-containing compounds. Non-volatile acids such as malic, citric, oxalic and succinic may also contribute to flavour in vegetables. The colour of vegetables depends on the pigments they contain. Spices are known to exert several physiological effects including antidiabetic influence . Anthocyanins impart the blue, purple and red colours to vegetables such as tomatoes, garden eggs and red peppers. Chlorophyll imparts green colour to green leafy vegetables and carotenoids are responsible for the yellow colour in carrots, sweet potatoes and maize. Green leafy vegetables also contain carotenoids but this is masked by the more intense green of chlorophyll. The indigestible dietary fibres – cellulose, hemicelluloses and lignin give the vegetables their characteristics structure. However, some vegetables also contain pectin as storage carbohydrate, which has been linked to reduction of post-prandial hyperglycaemia. Data collection and assessment of commonly consumed foods in all the 6 geo-political zones in Nigeria published by Ene-Obong et al.  showed that fruit consumption was conspicuously absent in all the areas. Fruits are the fleshy or dry ripened ovary of a plant, enclosing the seeds. They are commonly eaten raw and used for desserts. Green and yellow vegetables are rich sources of beta-carotene and essential minerals. Most fruits, particularly the citrus species are abundant in vitamin C. These vitamins play antioxidant roles in the body and needs to be supplied in the diet.
4. Diabetes and its public health importance
The term “Diabetes” has been defined by many authors from different perspectives. However, the similarity in these definitions is that diabetes is a chronic metabolic disorder, characterized by high blood glucose (hyperglycemia), associated with impaired carbohydrate, fat and protein metabolism, resulting from either insufficient or no release of insulin by pancreas in the body [48, 49]. Diabetes mellitus is a chronic life-long disease, which has been known to mankind for over 2000 years. It requires careful monitoring and control. Currently, diabetes ranks fourth worldwide among the NCDs (cardiovascular diseases, cancers, chronic respiratory diseases and diabetes) with prevalence rates of 30%, 13%, 7% and 2%, respectively WHO . It has been projected that by 2020 -2025, the number of people in the developing world with diabetes will increase by more than 2.5 fold; from 84 million in 1995 to 228 million in 2025  and that 70% of deaths due to type 2 diabetes will occur in developing countries. Diabetes mellitus is increasingly being recognized as a major public health problem in developing countries.
Aetiological classification of diabetes mellitus includes type I diabetes (immune-mediated and idiopathic), type 2 diabetes, gestational diabetes and other specific types [48, 49]. However, types 1 and 2 diabetes mellitus appeared to have gained much more popularity among researchers and have generally been considered as the two major categories. In Africa, type 2 diabetes is the predominant form of diabetes in sub-Saharan Africa, accounting for over 90% of cases. The IDF Atlas  estimated that 10.8 million people have type 2 diabetes in sub-Saharan Africa in 2006 and this would rise to 18.7 million by 2025, an increase of 80%, as such exceeding the predicted worldwide increase of 55% . In the past, the estimates on mortality of diabetes as the leading cause of death for sub-Saharan Africa were amongst the lowest for all regions globally . This was attributed to Africa’s rich biodiversity and high consumption of natural foods, which are rich in complex carbohydrates, proteins and dietary fibre.
There is enormous and escalating economic and social cost of treating type 2 diabetes. Sifelani  observed that the number of people seeking medical assistance for diabetes is rising in Africa at a time when health experts reported the continent’s overburdened health systems are ill equipped to diagnose the disease. Majority of the poor cannot afford the cost of treatment. Cost of treating diabetes accounts for about 10% of the national income of most countries in Sub-Saahara Africa. National surveys in most parts of Africa indicate that diabetes cases are on the rise due to rapid urbanization as well as fast changing diets which are displacing the traditional ones in favour of the western diets . This makes a compelling case for attempts to reduce the risk of developing diabetes.
Nigeria is among the top 5 countries that have the highest number of people affected by type 2-diabetes in sub-Saharan Africa. Nigeria has about 1.2 million people; South Africa, 841,000; the Democratic Republic of Congo, 552,000; Ethiopia, 550,000 and Tanzania, 380,000 living with diabetes. A national survey had an average prevalence rate of 2.7% with similar pattern in both sexes. There are slightly varying prevalence rates in different geographical locations . Some sporadic figures on prevalence rates of diabetes in Nigeria were published. Cooper
5. An overview of pathogenesis of type 2 diabetes mellitus
Several pathogenic processes are involved in the development of diabetes. Type 2 diabetes is characterized by excessive hepatic glucose production, reduced insulin action and insulin resistance. These alter the utilization of endogenously produced insulin at the target cells. These precipitate hyperglycaemia . The pathogenesis of type 2 diabetes rests on the relationship between carbohydrate metabolism and insulin action. Consumed complex carbohydrates are metabolized to their monosaccharide constituents (glucose, galactose and fructose) in the gut. These monosaccharides have roles in nutrition. Glucose metabolism and absorption receive more attention in relation to diabetes. Glucose, absorbed into the blood stream, elevates blood glucose level. This rise in blood glucose stimulates the secretion of insulin from the beta-cells of the pancreas to regulate blood sugar levels by increasing active transport of glucose into fat and muscle cells. Post-prandially, blood glucose is absorbed and transported via the portal vein to the liver. The liver maintains blood glucose levels by converting glucose into glucose-6-phosphate and glycogen (glucogenesis). The increased insulin secretion from the pancreas and subsequent cellular utilization of glucose lowers blood glucose levels. Lower blood glucose level decreases insulin secretion. In diseased condition, insulin production is decreased to inhibit glucose uptake into the cells, which precipitates hyperglycaemia. The insulin secreted by the pancreas at this time is not used by the target cells. Hyperglycaemia is a common effect of uncontrolled diabetes and overtime, it causes serious damage to many body systems, particularly, the blood vessels and nerves. Common symptoms include glucosuria, frequent urination (polyuria), excessive thirst (polydypsia), excessive hunger (polyphagia), sudden weight loss, extreme tiredness and blurred vision.
Decreased amount of insulin in circulation decreases lipogenesis and increases lipolysis. Increased lipolysis releases fatty acids from adipose tissues. Fatty acids are also absorbed from the intestinal tract. The rapid release of fatty acids in the blood leads to hyperlipidaemia. The blood level of cholesterol increases, causing the development of atherosclerosis to occur at an earlier age than in non-diabetics and is more pronounced . Elevated circulating levels of free fatty acids derived from adipocytes are the most likely link between insulin resistance and type 2 diabetes . They contribute to insulin resistance by inhibiting glucose uptake, glycogen synthesis and glycolysis, and increasing hepatic glucose production. This leads to a decrease in intracellular concentration of glucose by a reduction in glucose-6-phosphate levels, implicating the glucose transport system as the rate-controlling step for free fatty acid-induced insulin resistance .
Free radicals and enhanced oxidative stress with reduced blood levels of anti-oxidants were implicated in the pathogenesis of diabetes and more importantly, in the development of diabetic complications [60, 63]. Free radicals are highly reactive molecules (charged superoxide, hydroxyl radical and nitric oxide). The uncharged hydrogen peroxide species is capable of damaging cellular molecules, DNA, proteins and lipids. These produce altered cellular functions. Oxidative stress is a serious imbalance between the production of free radicals and antioxidant defences. This causes potential tissue damage . Antioxidants play protective role against the effects of hyperglycaemia and free fatty acids
6. Dietary approaches to the management of type 2 diabetes mellitus
A diet is a combination of foods individuals or community habitually eat to exist or live, or use for therapeutic purposes. Diet is known for many years to play a key role as a risk factor for chronic diseases . Conventionally, type 2 diabetes is controlled with diet alone or diet and hypoglycaemic drugs (in combination with insulin in few cases where pancreas produces no insulin because of age). Objectives of dietary management of diabetes are to: achieve optimal blood glucose and blood lipid concentrations; provide appropriate energy for reasonable weight, prevent, delay and treat diabetes-related complications and improve health through optimal nutrition . The general practice is to counsel people with diabetes on moderation of their food intake through proper food selection with incorporation of low GI foods that contain slowly digestible carbohydrates and increased consumption of dietary fibre-rich foods in combination with increased physical activity. Most often, diabetics are misinformed and advised to consume only those foods with low GI values. The restriction has two adverse effects: either the diabetics become too rigid with their food selection which might cause starvation and frequent hypoglycaemic attacks  or consume more carbohydrate per meal from the low GI diets. These precipitate poor diabetic control (hyperglycaemia). This has resulted in reduced intake or total avoidance of starchy staples and overall poor diabetes management in Nigeria with its attendant high cost of medical treatment. The types of foods/diets eaten in Nigeria vary tremendously and depend on several factors. Climate, environment, cost and availability of foods, and religious beliefs are among the factors. There are also significant differences in eating habits between the rural and urban populations. The latter increasingly adapt to western food habits . The promotion of indigenous mixed diets still remains a powerful weapon in the fight against diabetes.
7. Dietary recommendations for type 2 diabetes mellitus
Nutrients are needed in certain amounts for good health. The type of food/diet people eat, in all their cultural variety, is a key universal factor that defines and affects people’s health, growth and development. However, due to some environmental, genetic, physiological and metabolic factors, humans become susceptible to conditions that influence the body’s homeostasis, nutrient intake and metabolism. To meet the challenges of optimizing nutrient utilization for specific conditions, dietary recommendations are made for various diet-related diseases. These are based on the causal factors and aim of dietary intervention in a particular disease condition. These involve consideration for the key nutrients in metabolic functions, their daily required amounts with reference to age and sex, their food sources and individual quantities of selected foods. These foods must offer desirable short or long-term health benefits without adverse effects. Dietary recommendations for prevention of diet-related chronic diseases and for people with diabetes were published by WHO  and American Diabetes Association , respectively. One major emphasis in these recommendations is that these diets should be close to normal (family) diet. In diabetes mellitus, the three energy-giving nutrients are involved. Based on this, adequate supply and intake of carbohydrate, protein and fat as well as micronutrients (vitamins and minerals) becomes an important factor to consider in diet and glycaemic control. The diet should contain 50-60% carbohydrate, 30-35% and 10-15% protein relative to the individual’s total daily caloric allowance. Fruits, vegetables and spices are rich in bioactive constituents [68, 69]. Bioactive constituents (dietary fibre, phytochemicals and anti-oxidants) of plant foods are receiving attention with regard to their roles in diabetes management. Most studies used different fibre sources (legumes, cereals and leaves) at different levels of incorporation [5-15%) into single meals of subjects [70, 71, 72, 73]. These studies tend to support the American Diabetes Association (ADA) position  that the public should consume adequate amounts of dietary fibre from a variety of plant foods. Unfortunately, recommended dietary fibre intakes 20-35g per day for healthy adults and the aged , and 5g per day for children , respectively, are not being met. Recent studies have shown that high carbohydrate, high fibre diets have low glycaemic index and reduce blood cholesterol and triglyceride levels. This more flexible glycaemic index concept replaces the therapeutic distinction between simple versus complex carbohydrates. Quantification of bioactive compounds
One limitation of dietary recommendations is the problem of translating them into practical terms. This appears to be a major gap in knowledge, which should be explored to translate the accumulated scientific evidence of plant foods benefits into practical terms. FAO/WHO/UNU  recommended the food-based dietary guideline (FBDG) to address problem of dietary inadequacies and poor nutrient intake. This ensures supply of nutrients per 1000kcal (nutrient density) of the traditional diet as consumed relative to standard values. The concept is used to express required, desirable nutrient intake and population goals relative to energy intake. It strongly emphasizes that food-based approaches are the only sustainable strategy to improve the nutritional status of all. Its major advantages are - it is easy to adopt and implement; emphasizes changes that promote adequate vitamins and minerals, and reduce risks of food-borne illness, obesity, diabetes, cancers and CVD. It also portrays long term effects of diets as consumed . For instance, a total of 15-20g dietary fibre/1000kcal and sodium 1000mg/1000kcal are recommended. McCarty  proposed a dietary ‘phytochemical index’ as a means of improving phytochemical nutrition of clients/patients. This index is the percentage of dietary calories derived from foods rich in phytochemicals. Calories derived from fruits, vegetables, legumes, whole grains, nuts, seeds, fruit/vegetable juices, soy products, wine, beer and cider- and foods compounded therefrom, would be counted in this index.
Harden et al.  advocated direct evaluation of GI of many meal combinations based on indigenous, more widely and frequently consumed foods. Two principal approaches were used to study the effects of increasing carbohydrate and fibre in diabetes management:
supplementation of the low fibre foods with fibre-rich sources (for instance, guar gum, flours from tallow tree, counter wood tree seeds and locust bean); and
selective use of dietary fibre–rich foods.
Selective and holistic use of dietary fibre content of a mixed meal than the soluble non-starch polysaccharides (NSP) supplements has currently become of research interest. It is because the supplementation approach compromises sensory properties of indigenous food products with anti-diabetic potentials at clinically effective and acceptable levels . Translation of the nutrient recommendations for specific conditions into foods for easy culinary application and appropriate nutrient intake is an integral part towards achieving the goals of dietary recommendations.
8. Problems associated with use of medicinal plants/traditional medicine and diabetes control
WHO reported that in several countries and areas, about 80% of the third world countries and 40-60% of total world population use traditional medicine instead of orthodox medicine for health-care. The process of extraction and purification of plant extracts affect the unit cost . These expensive products are unaffordable for the low income countries. The low demand for drugs led to the use of medicinal plants as herbal components of health foods and for preventive medicine. This new expanding market gave rise to nutraceuticals (nutriceuticals). Culhane  reported nutraceuticals as “foods derived from a naturally occurring substance (in pills or powder or other medicinal forms), which can and should be consumed as part of the daily diet, and serves to regulate or otherwise affect a particular body process when ingested”. These foods are known as medicinal foods, phytofoods, phytonutrients and functional foods. All of these terms meant substances that may or may not be considered as foods or parts of food but provide health benefits when eaten. However, more people prefer traditional medicine. This is because it provides holistic treatment (treatment that caters for the spirit, body and soul). The need for traditional medicine arose from the indigenous knowledge. This showed that aetiology of diseases does not only come from physiological imbalance and psychological causes as in orthodox medicine, but can come from spiritual causes, astral influence, esoteric causes and imbalance in cosmic natural forces. This could be associated with the belief by some diabetic patients that their condition is diabolical (personal communication). Every region has had, at one time or the other in its history, a form of traditional medicine. However, distinction exists between the use of traditional medicine and traditional plant foods as regards therapeutic agents used in diabetes management and health promotion. The misconception in the use of these indigenous medicinal plants precipitated self-medication when such plants are used. This poses serious problem in diabetes control in Nigeria. Consumers take medicinal plants, herbal products and nutraceuticals as food and think they are safe at any dose. Concerns raised about the interaction between herbal preparation and drugs are that herbal remedies might – i) increase the effect of an antidiabetic drug and plunge blood sugar to dangerously low level; ii) decrease the effect of blood pressure medication and cause high blood pressure and stroke; iii) decrease the effect of an anti-infection agent, letting the infection get out of control; iv) increase or decrease the effect of a blood thinner such as warfarin and cause either bleeding episode or formation of a dangerous clot. The combination of orthodox and traditional medicines has negative impact on people with diabetes in Nigeria. This may explain high incidence of cases of uncontrolled diabetes and associated complications in patients admitted in hospitals in Nigeria. The major link between the use of traditional medicine and traditional plant foods is that their uses are community-based and indigenous. Each community has its own peculiar approach to health and diseases even at the level of ethno-pathological perception of diseases and therapeutic behaviours. This gives indigenous knowledge in agriculture, medicine and health its diverse and pluralistic nature. The implication is that success for use of any indigenous plant foods that possess antidiabetic potential when incorporated into the family diets, could offer sustainable health benefits to the community/consumers of such plant foods.
9. Bioactive constituents and their effects on diabetes
Several terms have been used to describe the therapeutic potentials of plants. Health-promoting activities, bioactive constituents, medicinal properties (often used in orthodox and traditional medicine), natural plant products and biological response modifiers are examples of such terms. All plants, especially the wild ones, are potential sources of biological active molecules. These compounds protect plants against predators and other damage but are not directly essential for growth. They are known as secondary chemical compounds or metabolites. The plant constituents useful extractable substances in their storage organs (leaves and seeds/roots) in quantities sufficient to be economically useful as raw materials for various scientific technological and commercial applications .
Some plant constituents appear to be disease specific. The plants for consideration when a hypoglycaemic property of a plant is elucidated were reported. Ivorra et al.  studied different compounds isolated from plants with attributable hypoglycaemic activity. They gave a broad classification as follows: polysaccharides and proteins; steroids and terpenoids; alkaloids; flavonoids and related compounds. Drewnowski and Gomez-Carneros  and Noor et al  reported phenols and polyphenols, flavonoids, isoflavones, terpenes and glucosinolates in vegetables and fruits; Thompson et al.  included phytate in bean flour, which reduced GI. Iwu et al.  found alkaloids in yam. Several studies have published similar effects with dietary fibre (non-starch polysaccharides, NSPs) [31, 73]. A new classification of dietary fibre (water-soluble and insoluble dietary fibre) was based on their solubility characteristics . The soluble dietary fibre is highly viscous and has added viscosity as functional property in the evaluation food/diets. These NSPs lower blood glucose level by impeding glucose absorption from the gastrointestinal tract and reduce post-prandial hyperglycaemia. The water-insoluble NSP are mainly obtained from structural carbohydrates (cellulose and lignin of cell walls) of starchy roots/tubers and cereals. The water-soluble NSP are obtained from storage carbohydrates (gum and hemicellulose) of legumes and as pectin from fruits and vegetables. The phytochemicals are basically sourced from fruits and vegetables.
Widely used parameters for clinical studies in diabetes are post-prandial blood glucose, glycaemic index (GI) and glycaemic load (GL). Jenkins et al.  introduced the concept of GI to classify both single foods and mixed meals on the basis of their potential for increasing the blood glucose concentration. The starchy foods have high GI (>70%) compared to the low values (<50%) for vegetables, fruits, legumes. The GL is defined as the product of the amount of available carbohydrate in a certain amount of food and its GI divided by 100 . Low GI-based diets tend to maintain glucose and insulin at a moderate level avoiding the hyperglycaemic state . This is based on advantages of the GI concept: i) GI and GL are determined both on single and mixed meals; ii) they permit proper selection and combination of low and high GI foods to achieve good glycaemic control and prevent hyperglycaemia; iii) they prevent fear of excessive intake of low or high GI foods and resultant hypoglycaemia/hyperglycaemia; and iv) hyperinsulinaemia is abated when both GI and GL are considered in dietary counselling. The link between GI and GL simply means that although both GI and GL elicit information on carbohydrate content of foods in relation to their blood glucose response. GI is associated with the quality of carbohydrate in a food/meal while GL relates to the overall quantity of carbohydrate in the same food/meal . However, when these low GI (low GL) foods are habitually consumed in large amounts, high glycaemic effect may manifest over time. Based on this, attention is drawn to researchers on the limitation of the use of glycaemic index values alone in diabetes management. The practical application of the Gl, used in combination with other information on food composition, is promising in guiding good food choices, especially for people with diabetes. Englyst et al.  proposed the inclusion of rapidly digested starch (RDS) and slowly digested starch (SDS) to account for limitations of GI. More recently, resistant starch (RS) has received attention as one of the components of carbohydrate foods that are closely linked to glycaemic control . Salijata
Evans et al.  has reported that pathogenesis of diabetes is better appreciated and should be discussed in line with hyperglycaemia and elevated free fatty acid in the blood in relation to oxidative stress and production of free radicals. Some plants with hypolipidaemic activities were included by several authors as those that may assist in diabetes control. This is because diabetes and cardiovascular disease are closely related. The active ingredients identified
10. Documentation of anti-diabetic potentials of selected Nigerian indigenous plant foods
The 1978 Alma Ata declared that traditional medicine is the surest means to achieve total health care coverage of the world’s population. This has encouraged the use of medicinal plants. Currently, the active ingredients in most plants with anti-diabetic potentials are elucidated pharmacologically. Consequently, it was a bit difficult to identify edible Nigerian indigenous plants from huge database on medicinal plants. Information on their constituents of nutritional importance was lacking. Invariably, not all traditional medicinal plants qualify as plant foods. Bosch et al.  as cited in Ogbu et al.  reported that the total number of species of edible plants in primary use in Africa is 1004. However, a little above 50% of the number of medicinal plants was reported as 1975. In Nigeria, Federal Environmental Protection Agency (FEPA) shows that 20 species of the plants were in extinction since 1950. Another 431 species are endangered, 45 species are classified as rare, 20 species are vulnerable and 305 species are endemic . Not all plants with antidiabetic potentials published online may be available or edible in Nigeria. The local herbalists keep secret the information on health benefits of indigenous crops to enhance their patronage. Preliminary identification of the anti-diabetic potentials of these plant foods becomes necessary to improve their food diversification and consumption.
11. Sequence for literature review
Surveys on traditional food systems are commonly used to document information on indigenous foods with health-benefits . Google search was used to review literature on published works on ethnobotanical surveys in northern and southern regions of Nigeria. These were used to identify commonly consumed indigenous plants. Some related unpublished works were also used to supplement published data. Information obtained include botanical names of the plant foods where available, habitat (cultivated (C), wild (W) or semi-wild (S)), part used, culinary uses, anti-diabetic potentials/effects of single foods and diets and corresponding references. From the profile of edible indigenous plant foods, those that have antdiabetic potentials were selected based on their bioactive constituents, proven to be active ingredients in attenuating post-prandial blood glucose. Additional information on anti-diabetic effects of some of the plant foods/diets evidenced by
The concept of food groups of plant origin was used to present the results on documentation of Nigerian indigenous plant foods and those with anti-diabetic potentials. This was to encourage increased diversification and culinary uses. These food groups include fruits, vegetables, spices and condiments, grain legumes, nuts and oil seeds as well as starchy roots/tubers/fruits and cereals. Forty-eight families of plant foods were identified. Out of 241 plant species (spp) identified, 80 fruits, 77 vegetables, 68 spices and condiments, 22 grain legumes/nuts/oil seeds and 19 starchy roots/tubers/fruits and cereals were documented. Twelve out of 22 grain legumes/nuts/oil seeds and 11 out of 19 roots/tubers/cereals were soup thickeners. Only 3 spp of fruits, 16 spp of vegetables, 27 spp of spices/condiments/grain legumes and 10 spp of starchy staples have anti-diabetic potentials
|Oleaceae||W||Fruit||Dietary fibre||Not determined|||
|Myrtaceae||C||Fruit and fruit juice||[96, 97]|
|Chrysobalanaceae||W||Fresh fruit, paste||High viscosity||Not determined|||
|Cucurbitaceae||C||Seeds flour||Dietary fibre||Not determined|||
|Cucurbitaceae||C||Ground leaves pulp||High phytochemical constituents (alkaloids, tannins, flavonoids, steroids, anthacyanin, carotenoides)||Antioxidant; anti-hyperglycaemic||[101, 102]|
|Cucurbitaceae||S||Fruit extract||Phytochemicals||Anti-oxidant, anti-hyperglycaemic||[45, 24]|
|C||Ground leaf pulp||In vitro||High phytochemical constituents (alkaloids, tannins, flavonoids, steroids, anthacyanin, carotenoides)||Hypoglycaemic||[102, 103]|
|Labiatae||S||Ground leaf pulp||The same constituents||Not determined|||
|S||Ground leaf pulp||The same constituents||Not determined|||
|Tiliaceae||W||Ground leaf pulp||The same constituents||Not determined|||
|Fabaceae||W||Ground leaf pulp||The same constituents||Not determined|||
|S||Ground leaf pulp||The same constituents||Not determined|||
|W||Pod||Mucilage, dietary fibre||Antioxidant anti-hyperglycaemic||[99, 104, 105, 106]|
|S||Leaves and seeds extracts||Phytochemicals||Anti-hyperglycaemia; anti-lipidaemia|||
|SPICES AND CONDIMENTS/ GRAIN LEGUMES, NUTS, OIL SEEDS|
|Alliaceae||W||Dried bulb powder||Anthocyanins||Anti-hyperglycaemia||[24, 47]|
|Alliaceae||C||Bulb extract||Anthocyanins (allicin)||Anti-hyperglycaemia|||
|Euphorbiaceae||C||Seeds extract||Not determined||Anti-hyperglycaemia|||
|C||Ground leaves pulp||High phytochemical constituents (alkaloids, tannins, flavonoids, steroids, anthacyanin, carotenoides)||Not determined|||
|W||Leaves extract||Not determined||Hypoglycaemic||[24, 47]|
|W||Fresh or dried rhizome powder||Not determined||Antihyperglycaemia||[24, 47]|
|Vitaceae||W||Stem flour||High viscosity, dietary fibre||Hypoglycaemic|||
|Basidiomycetes||C||Sclerotium flour||Dietary fibre||Not determined|||
|Caesalpiniaceae||W||Seeds flour||Dietary fibre||Hypoglycaemic||[42, 73]|
|Caesalpiniaceae||W||Seeds flour||Dietary fibre||Not determined|||
|Caesalpiniaceae||W||Seeds flour||Dietary fibre||Hypoglycaemic||[42, 73]|
|Fabaceae||C||Seeds flour||Dietary fibre||Not determined|||
|Fabaceae||C||Seeds flour||Dietary fibre||Not determined|||
|Fabaceae||C||Seeds flour||Dietary fibre||Antihyperglycaemic, anti-hyperlipidaemic||[106, 109]|
|Fabaceae||C||Seeds flour||Not determined||Antihyperglycaemia, anti-hyperlipidaemia|||
|Fabaceae||C||Seeds flour||Mucilage||Not determined|||
|Fabaceae||C||Seeds flour||Dietary fibre||Not determined|||
|Irvingiaceae||C||Seeds flour||Soluble dietary fibre||Anti hypoglycaemic||[106, 111]|
|Irvingiaceae||S||Seeds flour||Soluble dietary fibre||Anti-hypoglycaemic||[106, 111]|
|Lauraceae||W||Seeds flour||High viscosity, dietary fibre, low GI||Not determined|||
|Papilionaceae||C||Seeds flour||Dietary fibre||Antihyperglycaemia, anti-hyperlipidaemic|||
|Papilionaceae||C||Seeds flour||Dietary fibre||Antihyperglycaemia, anti-hyperlipidaemic||[106, 109]|
|Papilionaceae||C||Seeds flour||Dietary fibre||Anti-hyperglycaemic|||
|Sterculiaceae||W||Seeds flour||High viscosity, dietary fibre, low GI||Not determined|||
|Sterculiaceae||C||Nuts||Not determined||Anti-oxidant, hypoglycaemic||[113, 114, 24]|
|STARCHY/ROOTS/TUBERS/FINGERS & CEREALS|
|Dioscoreaceae||C||Tuber flour||Dietary fibre||Not determined|||
|Seeds flour||Dietary fibre||Anti-hyperglycaemia||[116, 117, 118]|
|Poaceae||C||Seeds flour||Dietary fibre, polyphenols and phytic acid||Anti-hyperglycaemic|||
|Poaceae||C||Seeds, ||Dietary fibre, low GI||Not determined|||
|Poaceae||C||Seeds, ||Dietary fibre, low GI||Not determined|||
|Euphorbiaceae||C||Roots, ||Dietary fibre, low GI||Not determined|||
|C||Leaf and stem extracts; stem||Flavones ; dietary fibre||Anti-oxidant, hypoglycaemia;||[24, 121]|
|Musaceae||C||Unripe fruit flour||Saponins, flavonoids, tannins & alkaloids||Anti-oxidant, hypoglycaemia|||
|Seeds flour||Not determined||Hypoglycaemic||[24, 114]|
|Traditional soup prepared with||Dietary fibre||Hypoglycaemic|||
|Moi-moi (||Dietary fibre||Anti-Hyperglycaemic|||
|Gari with ||Gari||Not determined||Anti-Hyperglycaemic|||
|Pounded yam with ||Not determined||Anti-Hyperglycaemic|||
|Cocoayam ||Not determined||Anti-Hyperglycaemic|||
|Plantain porridge with beans||Not determined||Anti-Hyperglycaemic|||
|Yam flour with ||Cooked, thick paste made from ||Low GI||Hypoglycaemic|||
|Fermented cassava flour with ||Cooked, thick paste made from ||Low GI||Hypoglycaemic|||
|Maize flour with ||Cooked, thick paste made from ||Low GI||Hypoglycaemic|||
|Fermented cassava flour with “ogbono soup||Cooked, thick paste made from ||High viscosity; reduced digestibility||Hypoglycaemic|||
|Plantain flour with ‘ogbono’soup||Cooked, thick paste made from ||High viscosity; reduced digestibility||Hypoglycaemic|||
|Fermented cassava flour with “egusi soup’||Cooked, thick paste made from ||High viscosity; reduced digestibility||Hypoglycaemic|||
|Plantain flour with ‘egusi soup’||Cooked, thick paste made from ||High viscosity; reduced digestibility||Hypoglycaemic|||
|Fermented cassava flour with pepper soup||Cooked, thick paste made from ||High viscosity; reduced digestibility||Hypoglycaemic|||
|Plantain flour with pepper soup||Cooked, thick paste made from ||High viscosity; reduced digestibility||Hypoglycaemic|||
|Boiled yam with fish stew||Boiled yam (||Dietary fibre||Anti-hyperglycaemic; anti-hyperlipidaemic|||
|Cassava flour with vegetable soup||Cooked, thick paste made from ||Dietary fibre||Anti-hyperglycaemic; anti-hyperlipidaemic|||
|Yam flour with vegetables stew||Cooked, thick paste made from ||Dietary fibre||Anti-hyperglycaemic; anti-hyperlipidaemic|||
|Boiled beans and boiled plantain with beef/fish stew||Boiled cowpea (||Dietary fibre||Anti-hyperglycaemic; anti-hyperlipidaemic|||
|Fermented cassava/||Dietary fibre, protein||Anti-hyperglycaemia|||
|Fermented maize/||Dietary fibre, protein||Anti-hyperglycaemia|||
|Dietary fibre, protein||Anti-hyperglycaemia|||
13. Conclusions and recommendations
Diet therapy, undisputedly, has long been recognized as key tool in the clinical management of diabetes. The challenge centres on how to translate the existing scientific evidence into practical terms for individual and household application in the management of diabetes mellitus. Relatively, a few clinical studies were published in Nigeria. Widely used parameters for diabetes-related clinical studies in Nigeria are post-prandial blood glucose response, anti-oxidant activity, lipid profiles and GI of diets. Most of the studies conducted on single foods were on dietary fibre. Much more indepth research is needed on phytochemical constituents of the rich national flora as regards phytochemical index and diabetes control. This is because health benefits of these indigenous plants are of public health importance. Based on recent documentation, Nigeria is rich in varieties of plant foods. Only a few varieties or cultivars were captured relative to diverse nature of Nigeria. It has generated a database for future investigations. Relevant research outputs on the plant foods would encourage diversification and increase consumption to maintain dietary adequacy and optimize their health benefits/ therapeutic potentials.
Much of the
The goal for optimizing the use of indigenous plants with anti-diabetic potentials to control and prevention type 2-diabetes rests on conducting controlled clinical trials using food-based approach as the last step to establish their efficacy in humans. Evidently, these diets adequate in macronutrients, micronutrients and phytochemicals can retard and prevent diabetic complications. Optimum utilization of these indigenous anti-diabetic plant foods in Nigeria and their diversification would enhance management of diabetes and improve health of people in the communities.