The Role of Micronutrients and Micronutrient Supplements in Vegetarian and Vegan Diets

Abstract

Vegetarian and vegan diets are becoming increasingly popular in Western countries. Numerous global nutrition bodies advocate that appropriately planned meat-free diets are suitable for all the life cycle stages. Nutritionally adequate vegetarian and vegan diets may provide substantial health benefits and reduction of disease states. However, many studies have identified that recommendations for certain micronutrients may be harder to achieve when following these diets. Micronutrient deficiencies can cause several serious health issues throughout life if not prevented and treated. The outcomes of micronutrient deficiencies are particularly severe in vulnerable individuals, including pregnant women and children. Given the large number of individuals now selecting to follow a vegetarian or vegan diet, it is important to address the challenge of achieving micronutrient requirements and to identify methods, such as supplementation, to improve micronutrient intakes in vegetarian and vegan groups.

1. Introduction

Global adoption of meat and animal product-free diets is increasing and, in 2021, around 14% of the UK population identified with these dietary preferences [1]. Vegan and vegetarian diets are characterized by varying levels of restriction of animal products and are recognized by different definitions (Table 1). Other subculture diets exist under the definitions of vegetarian and vegan categories. Subcultures are the groups with identities that differ from the dominant culture. In the context of vegan/vegetarian diets, subculture diets are described as those diets that share common dietary restrictions, for example, these do not consume meat but vary slightly from the primary definitions [2]. These include macrobiotic, raw, and fruitarian [2]. Variations in these classifications are significant when discussing the nutritional influence of vegetarian and vegan diets [2].

Type1	Definition
Flexitarian	Occasionally excludes meat and poultry but remains consumption of fish, eggs and dairy
Pescatarian	Excludes meat and poultry but remains consumption of fish, eggs and dairy
Lacto-ovo vegetarian	Excludes meat, poultry and fish but remains consumption of dairy and eggs
Lacto vegetarian	Excludes meat, poultry, fish and eggs but remains consumption of dairy
Ovo vegetarian	Excludes meat, poultry, fish and dairy but remains consumption of eggs
Vegan	Excludes all animal products (meat, poultry, fish, eggs and dairy)

Table 1.

Definition of common vegetarian and vegan diet types.

¹

Diet definitions from Phillips 2004 [2].

In 1847, the first vegetarian society was established in the UK [3, 4]. Later, in 1944, the term “vegan” was coined [4]. In the mid-twentieth century, definitions of vegetarian and vegan diets were homogeneous and consisted of those who did not consume animal products (vegans) and those who also may consume dairy and eggs (vegetarians) [2, 4]. Subtypes of the vegetarian diet were later classified based on the type of animal product chosen to consume (Table 1) [2]. At this time, individuals were typically consuming “traditional” vegetarian or vegan diets characterized by the intake of vegetables, fruits, beans, legumes, nuts, and seeds [5]. Descriptions of vegetarian and vegan diets further expanded in the 1960s and 1970s with influence from across cultures and religions [4]. In the 1970s and 1980s, micronutrients unfortified milk-alternative beverages and products mimicking meat became more readily available further diversifying vegetarian and vegan diets [4].

In many areas of the world, vegetarian and vegan diets are frequently adopted due to the low availability of animal foods and/or for monetary or religious reasons [6]. In western nations, individuals typically choose to follow vegetarian and vegan diets voluntarily or may adopt them due to food aversion, food allergies, or intolerances [7]. Adherence to vegetarian and vegan diets in Western countries often goes beyond food choice and is closely related to social identity [8]. The top motivations for following vegetarian and vegan diets are ethical and health reasons [9, 10, 11, 12]. Ethical motives include environmental and animal welfare concerns in relation with the production of animal-based foods [9, 10]. Health motives often involve the promise of improved health outcomes, weight loss, and reduction of chronic disease states [9, 10]. However, many studies have shown that the adoption of vegan and vegetarian diets may be due to a combination of different motives [9, 10, 11, 12, 13, 14]. The acceptance of dietary preferences among family and friends may also influence the uptake of vegetarian and vegan diets [5]. Population subgroups, such as young adults and women, are more likely to adhere to vegetarian and vegan diets [2].

A growing number of people in Western countries are reducing their meat consumption or pledging to follow short-term stints of meat-free diets for campaigns such as Veganuary or Meatless Monday [15, 16]. The rising interest in plant-based eating has amplified demand for commercially available alternative products and convenience items suitable for vegetarians and vegans [17]. Fast-food chains have designed products targeting vegetarian and vegan consumers [18]. Restaurants have expanded menu options to suit all dietary requirements, including an array of vegetarian and vegan products [18]. Additionally, dietary supplements have become more acceptable and available to consumers of vegetarian and vegan diets [19]. It is the position of the British Dietetic Association (BDA), and other societies worldwide, that appropriately planned vegetarian and vegan diets are healthful, nutritionally adequate, and suitable for all stages in the lifecycle, including pregnancy, lactation, infancy, childhood, adolescence, older adulthood, and athletes [20]. However, multiple studies have identified that the restrictive nature of vegetarian and vegan diets may act as a barrier to achieving adequate intake of certain micronutrients, including vitamin B12, vitamin D, iron, zinc, calcium, iodine, and selenium [21, 22, 23, 24, 25, 26, 27, 28].

Many plant foods are naturally lower in certain micronutrients in comparison with animal foods and plant-based alternative products are often not fortified with micronutrients to levels equivalent to those found in their animal counterparts [29, 30, 31, 32]. Some micronutrients, such as vitamin B12, are only provided by animal products, therefore, vegans and vegetarians with stricter restrictions must rely on fortified foods and/or supplements to achieve optimal B12 intake [33]. Calcium and other micronutrients can only be obtained in small quantities from plant foods, again resulting in reliance on other sources [34]. Furthermore, many micronutrients from plant sources (including iron and zinc) have lower bioavailability due in part to the presence of antinutrients, such as phytates and oxalates, found in whole grains, legumes, and spinach, which impair uptake [35, 36, 37]. All these factors may contribute to lowered micronutrient intake and status in individuals following vegetarian or vegan diets.

Modern-day vegetarians and vegans are much more heterogeneous than in the past. Individuals now have a more diverse pool of foods to choose from that are suitable for their dietary preferences, permitting huge variations in food and micronutrient intake. Four distinct dietary patterns within the definition of a vegan diet in UK vegans have been identified [38]. These dietary patterns were termed “convenience” “health conscious,” “unhealthy,” and “traditional.” The least common diet pattern observed was the “traditional” vegan pattern comparable to the vegan diets followed in the 1980s [39]. Although there is yet to be an analysis of the patterns within the vegetarian diet, this suggests that innovation in the vegetarian and vegan food sector is beginning a shift from the healthful “traditional” diet. This change in eating could potentially compromise the micronutrient quality of vegetarian and vegan diets and risk micronutrient deficiencies if diets are not appropriately planned. For example, ultra-processed items, such as meat alternatives and convenience items, are often not fortified with micronutrients and are not nutritionally similar to the foods they are designed to mimic [29, 40, 41]. Alternative dairy products, such as beverages (soya, oat, rice, etc.) and yogurts, are now regularly fortified with vitamin B12, vitamin D, and calcium [30, 31, 32, 42]. However, concerns have been raised that these milk alternative beverages do not contain quantities of iodine comparable to dairy and fortification is not yet mandatory [29, 43]. Health-conscious vegetarians and vegans limiting their intake of convenience foods may miss out on the benefits of micronutrient fortification in these products [38]. Moreover, preoccupation with “healthy” eating and inflexibility in the diet has been shown to act as a barrier to micronutrient intake [38]. Supplement usage in vegetarian and vegan populations is variable but can be an efficient way of achieving micronutrient intake [44, 45, 46, 47].

Given that these diets are likely to only increase in popularity as we move to more sustainable plant-based diets globally [48, 49], it is paramount to understand the impact of following a vegetarian or vegan diet on micronutrient intake and status to prevent an increase in health problems associated with deficiency. In this chapter, we outline the current literature in this field and discuss the complex issue of obtaining adequate micronutrient intake for those adhering to vegetarian and vegan diets and how this has changed in recent times.

2. Micronutrients

Micronutrients are essential vitamins and minerals required to orchestrate a large proportion of biochemical functions within the body [50]. Most must be supplied in the diet and consequently specific dietary recommendations have been established for each micronutrient by country of origin, age, and sex [51]. In the UK, these recommendations are called Reference Nutrient Intakes (RNIs) [51]. The RNI value for a nutrient is the amount of that nutrient that is sufficient for 97.5% of the population of interest [51]. Lack of adequate provision of micronutrients from the diet can result in micronutrient deficiencies [50]. Although clinical deficiency states are relatively rare in Western countries, mild deficiencies can result in pathological and metabolic changes [50]. In the short-term, undesirable symptoms may occur, which may lead to more severe consequences long-term and potentially even death [52]. Single micronutrient deficiencies are easy to recognize and treat; however, subclinical deficiency states where multiple micronutrients are depleted are more serious and can result in severe complications [52].

3. Key micronutrients over the life cycle

Restrictive diets can pose a risk of micronutrient deficiency during all stages of life, however, the consequences of micronutrient deficiencies are often more serious during periods critical for growth and development (e.g., infancy and pregnancy) [6, 52, 53]. Here, we describe the key micronutrients over the human life cycle.

3.1 Infants and young children (0: 4 years)

There is debate as to whether vegetarian and vegan diets are suitable for infants and young children, for deficiencies of nutrients at this stage of life could have irreversible neurological and developmental consequences [54, 55, 56, 57, 58, 59, 60]. Micronutrients, such as iron, zinc, selenium, calcium, and vitamins A, D, and B12, may be problematic in these age groups [54, 55, 56, 57, 58, 59, 61]. Infants and children tend to consume diets chosen by their caregivers [2]; therefore, achieving micronutrient intake is subject to the nutritional knowledge of the food provider. Micronutrient consumption may therefore be impaired if strict ideologies and food choices are imposed [2].

In the UK, exclusive breastfeeding (feeding only breast milk) is promoted for the first 6 months of infancy [62]. The maternal diet influences the nutrient composition of breast milk, particularly B vitamins and vitamins A, C, and D [63]. Breast milk typically has low quantities of vitamin D, it is now recommended that all breastfed infants consume a vitamin D supplement of 8.5–10 mg day⁻¹ [64]. Generally, exclusively breastfed vegetarian children do not struggle to meet micronutrient requirements [6]; however, vegan and vegetarian mothers have been shown to produce milk with lower vitamin B12 concentrations [6], so B12 must be considered when vegetarian or vegan mothers select to exclusively breastfeed [6]. Studies of UK vegans have also found breast milk riboflavin content to be lower than omnivorous participants [65]. Riboflavin has a significant role as a co-enzyme in energy metabolism (flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD)) [66]. Riboflavin deficiency is not fatal but can cause clinical symptoms after several months of inadequate intake (RNI by age: 0–6 months; < 0.3 mg, 7–12 months; < 0.4 mg, 1–3 years; < 0.5 mg) [51]. Mild symptoms of riboflavin deficiency include sore throat and loss of hair [66]. Whereas anemia and impaired nerve functioning may present in more severe deficiency states [66]. Infants that are not breastfed before 1 year of age are recommended to consume commercially available infant formulas [67]. Formulas are usually made from modified cow’s milk [67] and are not appropriate for infants following a vegan diet. Infant formula made of methionine-fortified soya protein isolate is available for infants with cow’s milk allergy [67]. Infant formula milk is typically fortified with vitamins A, C, and D, but there are currently no vegan formulas available in the UK in part because vitamin D fortification is dependent on lanolin from sheep’s wool [64]. Soya formula has been extensively studied to ensure that infants grow and develop correctly. Studies have found no difference in infant development-fed soya formula compared to cow’s milk formula [68]. Cow’s milk and soya milk are not recommended prior to the first year of life as iron concentration and bioavailability are too low to meet requirements for growth [67]. Soya milk also has a low zinc concentration and bioavailability [69]. Nutritional concerns have been raised for vegetarian and vegan infants who have been given homemade plant milks (e.g., oat, soya, etc.), vegetable or fruit juices, or milks not recommended for individuals in the first year of life [70].

In the UK, infants can be introduced to solid foods (complementary feeding or weaning) at 6 months of age [71]. Advice for weaning is the same for vegetarian and vegan infants as for infants following an omnivorous diet [71]. Iron, calcium, zinc, iodine, and vitamin D and B12 must be carefully considered for weaning infants [57]. In infancy, vitamin D and calcium are essential for bone growth and bone mass attainment, whereas, iron, zinc, and iodine deficiency can have potential neurodevelopmental implications [70]. After 1 year, vegetarian and vegan infants can be weaned onto suitable diets containing cow’s milk or fortified (vitamin B12, D, and calcium) soy-based milk [57, 70, 71]. If fortified variants are not available, then micronutrient supplements (vitamins B12 and D) must be considered [72]. In 2021, there was a call for evidence in the UK to assess if dairy-free infant formulas should be mandatorily fortified with iodine (for infants 0–5 years) as these products contain lower quantities compared to cow’s milk-based products [73].

Young children, aged 1–4 years, experience large variations in growth rates, dietary patterns, and nutrient losses, making it difficult to assess micronutrient requirements and intake [58]. Studies have found that iron, calcium, and vitamin D intake tends to be lower in vegan children compared to vegetarian and omnivorous children [58]. Parents of vegan and vegetarian infants and young children may wish to seek support from a General Practitioner (GP) or Registered Dietician to ensure that micronutrient recommendations are achieved.

3.2 Childhood (4: 13 years)

Childhood is characterized by the rapid development of cognitive and physical function; therefore, sufficient micronutrient intake is critical [74]. Iron, zinc, vitamin D, iodine, calcium, and particularly vitamin B12, all have roles in growth and development at this stage [74]. Iron and zinc are particularly important in pubertal vegetarian and vegan girls, for menstruation losses to be replenished [75], while calcium and vitamin D are important for bone growth and metabolism [76]. Many cereals and alternative foods are now fortified with these key micronutrients and should be available to growing children by parents or caregivers [60, 77, 78]. At the time of this publication, there is limited research investigating vegan and vegetarian diets and micronutrient intake in childhood [78]. From the available research, childhood vegan diets have been reported to be lower in micronutrients required for optimal bone health cobalamin, calcium, and vitamin D. A study in 2021 found that vegan children aged 5–10 years tend to be shorter by 3 cm and have 4–6% lower bone mineral mass compared to omnivorous children [79]. Similarly, B12 intake was found to be lower in this particular study [79]. Adequate intake of micronutrients at this age is therefore essential to reduce the risk of fractures and potentially osteoporosis in later life [79].

3.3 Adolescents and young adults (14–25 years)

Adolescents and young adults (14–25 years) have more autonomy in their food choices, and this is typically the life stage where eating behaviors are adopted [80]. Adolescents may transition to a vegetarian or vegan diet as an expression of identity [8]. Adolescence is typically characterized by poor diet quality [81], especially for students and vegetarian or vegan individuals, increased consumption of unfortified and heavily processed vegan junk foods may limit the intake of iron, zinc, vitamin D, vitamin B12, iodine, and calcium [38]. Another concern is that adolescents may adopt vegetarian or vegan diets to mask eating disorders, further increasing the risk of inadequate micronutrient intake [82, 83]. This potentially poses a greater risk as young women approach childbearing years [28, 84, 85, 86]. For women of childbearing age, a well-planned diet prior to conception is essential for fertility and the maintenance of healthy pregnancy [87]. Women of childbearing age are consequently recommended to consume supplements during pre-conception and pregnancy to meet increased micronutrient requirements [87].

3.4 Pregnancy and lactation

Nutritional requirements increase to support maternal metabolism and tissue formation, along with fetal growth and development [87]. Supplementation of iron, zinc, folic acid, vitamin D, iodine, and B vitamins may be more necessary in individuals following vegetarian or vegan diets as inadequate micronutrient intake is more frequent [88]. In the UK, pregnant women are able to apply for free vitamin tablets as part of the NHS Healthy Start Scheme, which contain folic acid that lowers the chance of babies having spinal problems, vitamin C, and vitamin D [89]. The Healthy Start Scheme also provides cash cards for low-income households to buy certain types of milk, infant formula, fruit, and vegetables [89]. Adoption of a well-planned and micronutrient-rich diet is essential in pregnancy [6]. If achieved, pregnancy outcomes are no different from that reported in omnivorous individuals [6]. Individuals following vegetarian and vegan diets during pre-conception and pregnancy may benefit from direct support from health professionals as a preventative measure to ensure dietary recommendations are achieved [6]. Moreover, dietary behaviors adopted during pregnancy and post-pregnancy influence food choices and preferences of future offspring, which can have lifelong impacts on the child’s eating habits and micronutrient intake [6].

3.5 Adulthood (18: 60 years)

In adulthood and mid-life, micronutrients are required in adequate quantities to diminish the age-related cognitive and physical decline in later life [81]. Vegetarian and vegan adults may be at risk of deficiencies, including vitamin B12, vitamin D, iron, zinc, calcium, iodine, and selenium [81]. If individuals adopt vegetarian and vegan diets to lose weight, then inadequate food provision may further limit micronutrient intake if not balanced appropriately [24, 90, 91].

3.6 Elderly and old age (60+ years)

Elderly populations have lower requirements for most micronutrients as physical and metabolic demands are reduced [92]. However, they often exhibit deficiencies in folic acid, vitamin D, calcium, selenium, and vitamin B12 [92]. Older adults living in institutions, who are frail or overweight, consequently have a higher risk of micronutrient deficiency [92]. Additional barriers to achieving micronutrient intake in old age include limited ability to purchase food, poverty, loss of taste or smell, infections, and other diseases, such as dementia and drug interactions [93]. Micronutrient deficiencies may also be more common in elderly populations because of gastrointestinal complaints that reduce the absorption of fat-soluble vitamins [93]. Deficiencies may also occur due to a lack of diet diversity [92]. Frail elderly populations may be unable to cook for themselves, becoming reliant on ready meals and processed food items that are palatable and easy to prepare [92]. At present, the approximate number of elderly people following a vegan or vegetarian diet is unknown. In 2021, it was estimated that 1/5 of vegans in the UK are above the age of 40 years and are heading toward old age [94]. There is expected to be an increase in the number of elderly vegans and vegetarians in the coming years given the rise in popularity of these diets and the global shift to more sustainable diets. Whilst, there is little research covering micronutrients in elderly vegetarians and vegans, dependence on unfortified vegetarian and vegan convenience items, and disuse or avoidance of dietary supplements may further reduce micronutrient intake [43]. It is therefore important to promote the consumption of fortified foods and dietary supplements for the elderly who select to follow a vegetarian or vegan diet.

4. Prevalence of micronutrient deficiencies in adulthood

The study of micronutrients in vegetarian and vegan diets is a rapidly evolving field of nutrition, advanced in accordance with the global recognition of these diets. Much of the nutritional evidence focusing on these diets were collected in the 1980s [5]. In this section, we will address the prevalence of micronutrient deficiencies in the “second wave” vegetarian and vegan movement supported today (2010–2022).

4.1 Vitamin deficiencies

Adult (19–50 years) RNIs for key vitamins, their function, and symptoms of deficiency are shown in Table 2 [51]. Vitamins are classified into two groups according to their solubility [96]. Water-soluble vitamins (B vitamins and C) diffuse freely in plasma and cytoplasm, are not easily stored, and are excreted in urine [96]. Fat-soluble vitamins include vitamins A, D, E, and K which, once absorbed [97], are retained for longer than water-soluble vitamins.

Vitamin	RNI1	Functions2	Symptoms of deficiency2
A (Retinol, β-Carotene)	Male; 700.0 μg; Female; 600.0 μg	Regulator of gene expression and cell differentiation, creation of pigments in the retina for vision and antioxidant capacity	Night blindness (Xerophthalmia) and skin keratinization
D (Calciferol)	10.0 μg	Controls calcium balance, involved in bone mineralization and intestinal absorption of calcium (Ca^2+)	Rickets and Osteomalacia both related to bone mineralization.
E (Tocopherols, Tocotrienols)	Male; 4 mg; Female; 3 mg	Antioxidant capacity	(Rare) Neurological dysfunction
K (Phylloquinone, Menaquinones)	1 μg per kg body weight	Coenzyme required for formation of proteins used in blood clotting and bone formation	Disorders of blood clotting
B1 (Thiamin)	Male; 1.0 mg; Female; 0.8 mg	Coenzyme used in nerve transmission	Peripheral nerve damage (Beri beri) and central nervous system dysfunction (Wernicke-Korsakoff syndrome)
B2 (Riboflavin)	Male; 1.3 mg; Female; 1.1 mg	Coenzyme used in cellular growth and function by oxidation and reduction reactions	Injury to corner of mouth, lips and tongue (debhorroeic dermatitis)
Niacin (Nicotinic Acid, Nicotinamide)	Male; 16.4 mg; Female; 13.2 mg	Coenzyme in oxidation and reduction reactions that produce energy for cells	Pellagra
B6 (Pyridoxine, Pyridoxal, Pyridoxamine)	Male; 1.4 mg3; Female; 1.2 mg3	Coenzyme in amino acid and glycogen breakdown and has a role in steroid hormone action	Disorders of amino acid metabolism
B12 (Cobalamin)	1.5 μg	Coenzyme used to form red blood cells, DNA, brain and nerve cells. Also used in the metabolism of folate	Pernicious anemia (deficiency in production of red blood cells) and degradation of the spinal cord.
Folic Acid (Folate)	200 μg	Coenzyme used in cell development and metabolism such as the conversion of homocysteine to methionine and specific anticonvulsant drugs	Megaloblastic anemia (production of unusually large, immature red blood cells called megablasts)
C (Ascorbic Acid)	40 mg	Coenzyme with antioxidant functioning which enhances the absorption of iron	Scurvy

Table 2.

UK Adult (19–50 years) Reference Nutrient Intake (RNI) for key vitamins, functions, and symptoms of deficiency.

¹

Data provided by the British Nutrition Foundation [51].

²

Information from Introduction to Human Nutrition, 3rd Edition [95].

³

Based on protein providing 14.7% of Estimated Average Requirement (EAR) for energy.

4.1.1 Water-soluble vitamins

Water-soluble vitamins need a constant regular intake to prevent deficiency [96]. Deficiencies in water-soluble vitamins B2, Niacin, and B12 are common in vegetarian and vegan diets [98].

Riboflavin (vitamin B2) is naturally present in a variety of foods and commonly fortified in cereals, thus deficiency is relatively rare [66]. Milk and dairy products are important sources of vitamin B2 in the western diet, and these alongside other animal products contain B2 in greater quantities compared with plant foods [99]. Recent studies have identified absorption of vitamin B2 to be lower in vegans due to both inadequate intake and reduced bioavailability of B2 from non-animal sources [99]. The bioavailability of foods will be discussed in later sections. Further work has identified that 25% of vegans may be deficient in vitamin B2 compared to 14% of omnivores [99].

Niacin is present in fish and meat, along with fortified foods such as cereals, legumes, and bread [95]. Small quantities can also be synthesized internally from the dietary amino acid tryptophan, but not at an amount to satisfy daily requirements. Several plant foods contain niacin in the form of niacin–glycosides, where niacin is bound to sugar molecules [100]. These are common in grains such as wheat and corn [100]. In this form, the bioavailability of niacin is diminished and may even be prevented altogether. Individuals reliant on plant foods therefore may find it more challenging to access niacin [100, 101, 102]. Moreover, individuals with low diet diversity and an over-reliance on wheat and corn may find it more challenging to meet requirements [100]. Studies have shown that vegans typically have the lowest intake of niacin, compared to other dietary groups, and may not be able to achieve recommendations [98].

Vitamin B12 (Cobalamin) is only provided in substantial quantities by the consumption of animal and dairy products, therefore adequate intake by individuals following vegan diets is dependent on the consumption of fortified foods, including cereals, soy or dairy alternatives (beverages, yogurts, etc.) along with supplements or food additives (e.g., nutritional yeast) [103]. Inadequate intake of vitamin B12 is common in vegan and vegetarian diets. Blood biomarkers, including serum cobalamin, can be used to assess vitamin B12 status [104]. A systematic review investigating blood serum concentration of Cobalamin (Cbl) in vegetarian adults and older adults found that deficiency was present in up to 86.5% of individuals [105, 106]. Further research identified longer-term vegans are more likely to experience vitamin B12 deficiency due to the depletion of bodily stores over time [105, 106]. Folic acid deficiency can occur consequent to vitamin B12 deficiency due to impaired methionine synthase action [107]. Methionine synthase is responsible for the restoration of methionine from homocysteine, inhibition of this enzyme traps folate as methyl tetrahydrofolate [107]. This phenomenon is called the “Folate trap” [107].

4.1.2 Fat-soluble vitamins

Vitamin D has been recorded to be significantly lower in vegetarian and vegan diets compared to omnivores [98]. Vitamin D can be endogenously generated via skin exposure to the sun [108, 109, 110]. Therefore, status does largely depend on non-food-related factors, including skin color, season, geographical location, duration of sun exposure, and quantity of sun exposure [108, 109, 110, 111]. Dietary sources of vitamin D are limited and are, for the most part, animal products [111]. Vitamin D increases the intestinal efficacy of calcium and phosphate absorption for proper formation of the bone mineral matrix [76, 108, 109]. Deficiency may therefore lead to reduced calcium absorption and heighten the risk of developing osteomalacia and other bone disorders [76, 108, 109]. Regardless of dietary preference, vitamin D deficiency is a UK-wide issue, recently around one in five adults have been identified as having levels of vitamin D in their blood (circulating 25(OH)D) corresponding to deficiency (< 25 nmol L⁻¹) [110].

Other vitamin deficiencies are uncommon in vegetarian and vegan diets due to greater consumption of plant foods compared to omnivores [99].

4.2 Mineral deficiencies

Adult (19–50 years) RNIs for each mineral, function, and symptoms of deficiency are shown in Table 3 [51]. Minerals can be grouped according to the quantity required in the body [95]. Calcium, phosphorus, and magnesium are vital in large amounts and are known as macrominerals [95]. Minerals needed in smaller amounts are called trace elements (iron, zinc, fluoride, selenium, copper, chromium, and iodine) [95]. Trace elements may be nutritionally essential or nonessential. All trace elements can be toxic if consumed in excess for long time periods [95].

Mineral	RNI1	Functions2	Symptoms of deficiency2
Calcium	700 mg	Required for growth and skeletal formation, muscle contraction, membrane transport and signal transduction	Hypocalcemia, Osteopenia and Osteoporosis
Phosphorus	550 mg3	Structural component of bones and teeth and DNA/RNA and contributes to bipolarity of lipid membranes and lipoproteins, with metabolic functions e.g. signal transduction and gene transcription.	Hypophosphatemia and non-nutritional cause of Rickets and Osteomalacia
Magnesium	Male; 300 mg; Female; 270 mg	Cofactor in energy metabolism, synthetizes of carbohydrates, lipids, nucleic acids, proteins and bone development	Reduced plasma and red blood cell magnesium, Hypocalcemia. Hypocalciuria and Hypokalemia
Sodium	1600 mg4	Regulation of osmotic and electrolyte balance involved in nerve conduction, cellular transport and formation of bone	Hyponatremia
Potassium	3500 mg5	Intra cellular fluid regulation, muscle contraction and blood pressure regulation	Hypokalemia
Chloride	2500 mg6	Intra cellular fluid regulation, muscle contraction, blood pressure regulation and balance of pH	Hypochloremia and metabolic alkalosis
Iron	Male; 8.7 mg; Female; 14.8 mg7	Blood cell and hormone production and cofactor for enzymes involved in oxidative phosphorylation	Iron deficiency anemia
Zinc	Male; 9.5 mg; Female; 7.0 mg	DNA and protien synthesis, cell growth, tissue and cell repair, and immune function	Bullous-pustular dermatitis, alopecia, diarrhea, mood disorders, weight loss, infections and male hypogonadism
Copper	1.2 mg	Growth, cardiovascular and lung integrity, neovascularization, neuroendocrine function, and iron metabolism.	Hypocupremia, anemia (microcytic, normocytic, or macrocytic), neutropenia, thrombocytopenia and myelopathy and peripheral neuropathy
Selenium	Male; 75 μg; Female; 60 μg	Component of selenoproteins performing metabolic functions e.g. thyroid hormone activation and immune system functioning	Cardiovascular disease, infertility, myodegenerative diseases, and cognitive decline
Iodine	140 μg	Component of the thyroid hormones regulating growth and metabolism	Hypothyroidism, goiter cretinism (infants only), and impaired cognitive development (infants only)

Table 3.

UK Adult (19–50 years) Reference Nutrient Intake (RNI) for key minerals, functions, and symptoms of deficiency.

¹

Data provided by the British Nutrition Foundation [51].

²

Information from Introduction to Human Nutrition, 3rd Edition [95].

³

Phosphorous RNI is equal to calcium in molar terms.

⁴

1 mmol sodium = 23 mg.

⁵

1 mmol potassium = 39 mg.

⁶

Corresponds to sodium 1 mmol = 35.5.

⁷

Insufficent recommendation for individuals with high menstrual losses, iron supplements may be required.

4.2.1 Macrominerals

Calcium has been identified as a problem micronutrient in vegetarian and vegan diets as it is less bioavailable from plant foods than from animal sources [95]. Absorption is limited in plant foods by inhibiting substances oxalate, phytate, and dietary fiber [95]. Water is, however, a valuable source of dietary calcium and can be consumed by all diet types, and many food products targeted at vegetarians and vegans, including alternative milk beverages, yogurts, and spreads, are now fortified with calcium [31, 32, 42, 112]. Despite this, calcium intake tends to be lowest in vegans compared with non-vegans, and a recent systematic review showed that 76% of vegan participants had intakes that were less than the RNI for calcium [98]. Additional studies investigating blood plasma calcium levels show that vegans have significantly lower plasma calcium concentrations compared to non-vegans and a growing number of papers indicate that vegans and vegetarians are at increased risk of bone fractures and bone disorders in later life [34, 113, 114].

4.2.2 Trace elements

Dietary iron comprises two forms, haem and non-haem ferric (Fe³⁺) iron [115, 116]. Only haem iron and reduced ferrous (Fe²⁺) iron are absorbed [115, 116]; haem iron via the heam transporter and Fe²⁺ via the divalent metal transporter 1 (DMT1) present on the apical membrane of the small intestinal epithelial cell (enterocyte) [115, 116]. Plant foods only contain non-haem iron which must be converted to Fe2⁺ and consequently, is less effectively absorbed compared with haem iron. Fe³⁺ is reduced to Fe²⁺ via the ferric reductase enzyme duodenal cytochrome B (Dcytb) which oxidizes vitamin C in the process [115]. Non-haem is rendered less bioavailable because its absorption i additionally inhibited by naturally occurring molecules such as phytate, oxalate, and polyphenols [95]. The Institute of Medicine has stated that iron requirements for vegetarians should be 1.8x greater than omnivores for this reason [117]. Recent studies have suggested that vegans may have greater iron intakes than other dietary groups, but the elevated level may still not be sufficient to overcome the bioavailability issues [118, 119]. In support of this, circulating ferritin, a biomarker of iron availability, is low in vegans [118]. Some studies have failed to show that vegetarians and vegans are any more likely to develop iron deficiency anemia than omnivores, however, young women adhering to vegetarian and vegan diets tend to have blood hemoglobin (Hb) concentrations that are indicative of anemia [118].

Many recent studies have identified that intakes of iodine within vegan and vegetarian populations are suggestive of mild to severe deficiency [25, 27, 28]. Individuals following omnivorous diets tend to have significantly greater iodine intake [26], as iodine is found in fish, seafood, eggs, and dairy products [120]. Plant foods have a low iodine concentration, and their iodine content is dependent on the content of the soil in which it has been grown [121]. However, even in high iodine areas, plants contain very low levels as it is not actively absorbed [122]. Across much of the globe, salt is mandatorily fortified with iodine (iodized salt), but this is not so in the UK, hence the capacity for individuals living in the UK to achieve dietary adequacy is more limited without the consumption of animal products and is reliant upon supplements or fortified foods which are limited in number [123, 124].

A small number of studies have found selenium intake to be significantly lower in vegan diets compared to vegetarians [28, 125]. Vegetarians can consume selenium from dairy products, such as eggs, yogurt, cheese, and milk [95], whilst vegans may be more vulnerable to selenium deficiency because of excluding them from the diet [28]. A recent exploratory study in the UK found that selenium intake was below the lower RNI of 50% [28]. Both selenium and iodine are found in similar foods and are linked to thyroid hormone production and thyroid hormone action [126]. These deficiencies are often coupled and can exert similar deficiency states [126].

5. Characteristics potentiating micronutrient deficiencies

As with all diets, there are certain factors that may increase the risk of micronutrient deficiencies.

5.1 Diet

Diet is the main component that needs to be considered when evaluating the adequacy of vegetarian/vegan diets. Individuals following diets with less extensive avoidance of animal products or those that eat larger quantities of fish, seafood, dairy products, and/or eggs are likely to have improved micronutrient intake [98]. Individuals with greater restrictions must consume other micronutrient-rich foods, fortified foods, or dietary supplements [2]. A select proportion of individuals may choose to further avoid fortified foods, dietary supplements, and processed alternatives due to personal, ideological, or religious reasons [2]. Additional avoidance of specific food items, for example, only eating raw or organic foods or experiencing food neophobia may further risk micronutrient deficiency [2].

Food consumption may also be limited for involuntary reasons. Individuals living in urban areas may have better availability and access to specialist vegetarian and vegan foods and food outlets compared to those living in smaller rural areas [127]. Similarly, there may be a lack of suitable supplements and fortified foods to meet requirements for certain micronutrients [29]. The price of vegetarian and vegan foods may further act as a barrier to achieving adequate micronutrient intake [127]. Socio-economic status may therefore further influence food intake and availability [127].

5.2 Bioavailability and absorption of micronutrients

The European Food Information Council (EUFIC) describes bioavailability as “the proportion of a nutrient that is absorbed from the diet and used for normal body functions” [128]. Consumed food is digested and absorbed in the intestine. The bioavailability of micronutrients in different food stuffs is affected by various factors (Table 4) [35]. Plant foods tend to have significantly lower bioavailability of micronutrients compared to animal products.

Gender Age and life stage (e.g. pregnancy/children) Body composition Health status Nutrient status Structure of foods Cooking and processing Chemical form of micronutrients Interactions between micronutrients Non micronutrient inhibitors and enhancers (factors that decrease/increase absorption)

Table 4.

Factors influencing the bioavailability of micronutrients.

5.3 Health status

Micronutrient status is linked to health status and age. Life stages that require rapid growth (fetus, infant, children, and those going through puberty) as well as pregnant and lactating women who need to support rapid growth, need greater micronutrient intake [52]. Individuals who are gaining weight and are experiencing rapid anabolism (e.g., bodybuilders and regular gym goers) or those with high energy requirements for exercise have greater requirements [52]. Diseases such as anorexia and chronic alcohol misuse, digestive diseases, and intolerances or allergies reduce food intake and consequently micronutrient intake along with any condition that causes unintentional weight loss of more than 5% of weight per month [52]. Individuals with increased requirements for metabolism caused by surgery, infection, or trauma will demand a greater supply of water-soluble vitamins (vitamins B and C) and minerals [52]. Loss of body fluids leads to a reduction in micronutrient stores [52]. This may be from excessive diuresis (either from disease or consumption of diuretic drugs), hemorrhage (menstruation or trauma), or diarrhea [52].

5.4 Personal characteristics and beliefs

Deeply ingrained philosophical, ethical, or religious beliefs may dictate food choices and may not be planned according to nutritional recommendations provided by expert bodies [2]. Vegetarianism and veganism are strongly linked with a number of religions, including Hindus, Buddhists, Rastafarians, Seventh-Day Adventists, and Jains [5]. Seventh-Day Adventists following a lacto-ovo vegetarian diet are among those who make a conscious effort to follow the guidance of nutritional professionals and are often used in epidemiological studies of the vegetarian diet [5]. However, some regimes may demonize certain food groups and eating practices or reject nutritional recommendations [2]. Conformity and group pressure may reinforce negative beliefs around food choice [2, 7]. Nutritional knowledge and education are linked to adequate micronutrient status [84, 129, 130, 131]. Individuals with good knowledge of how to correctly balance and plan a vegetarian or vegan diet are likely to have more adequate micronutrient intake [25]. Improving knowledge of how to achieve micronutrient recommendations would therefore be beneficial to those choosing to follow vegetarian and vegan diets [25, 28].

6. Achieving micronutrient recommendations for individuals

A well-planned vegetarian or vegan diet can provide an adequate intake of all micronutrients [20]. The UK government advises that a vegetarian diet should be planned according to the Eatwell Guide (Figure 1) [132]. The Vegan Society has created a vegan-specific version of the Eatwell Guide to support individuals who choose a diet that excludes animal products (Figure 2) [133]. The Eatwell Guide and Vegan Eatwell Guide are divided into sections and are comprised of example foods and portion sizes required for a healthy diet. There is no standard universal healthy diet for those following vegetarian and vegan diets, nevertheless, the Eatwell Guide and Vegan Eatwell Guide direct individuals to consume dietary patterns that include higher consumption of fruit and vegetables, wholegrains, pulses, nuts and seeds, and a lower intake of fatty/processed foods, refined grains, sugar-sweetened foods and beverages, salt, and saturated fat [133].

Figure 1.

The Eatwell Guide UK 2016 [132].

Figure 2.

The Vegan Eatwell Guide adapted from the Eatwell Guide by The Vegan Society 2020 [133].

The Eatwell Guide does not give specific guidance on how to achieve RNI of micronutrients within a vegetarian or vegan diet. Conversely, the guide does include notes to lead users to select micronutrient-fortified alternative products, sprinkles, and supplements. However, as discussed, without substantial dietary planning certain micronutrients may be missed from vegetarian and vegan diets.

6.1 Consumption of fortified foods

Food fortification describes the process of adding nutrients to foods either to return nutrients that have been lost through processing or to improve the nutrient content of foods [134]. Fortified foods are useful to ensure adequate micronutrient intake when certain food groups are excluded from the diet [134]. In the 1940s, the UK introduced mandatory fortification of white flour with vitamin B2 and other B-complex vitamins to improve its nutrient profile following processing [135]. In 2022, there was a call for evidence launched by the UK Government to identify methods for improving the vitamin D status of the population [136]. Fortification of staple food products, such as cereals and flour, has been identified as a technique to improve vitamin D intake for at-risk population groups, such as vegetarians and vegans [110, 136].

Some vegetarian and vegan-appropriate foods are already fortified with key micronutrients including vitamin D, calcium, and B-complex vitamins, for example, yeast extract, nutritional yeast, grains, cereals, alternative dairy products (e.g., dairy-free cheeses, fat spreads, yogurts, etc.) and alternative milk beverages [133]. Nutritional yeast is a good source of micronutrients providing 6 mg per portion (1 tbsp) of zinc [137]. One serving of fortified alternative milk beverage (200 ml) may provide an average of 1.5 μg of vitamin D, 0.2 mg of B2, 0.76 μg of B12, and 240 mg of calcium (30% of nutrient reference values in the UK) [137]. However, alternative milk beverages fortified with tricalcium phosphate as a method of calcium fortification may not be a like-for-like comparison to cow’s milk [137]. Additionally, iodine-fortified milk alternative beverages are also a good source of iodine, with one portion serving at 45 μg per 200 ml. Currently, most alternative milk beverages on the UK market are not iodine fortified [29]. The British Dietetic Association (BDA) is currently petitioning to change the governmental policy around alternative milks to ensure that all are required to be fortified with iodine to be sold in the UK to help improve iodine intake in populations that do not consume cow’s milk [138]. Therefore, it is important to check nutrition labels carefully during purchasing of products to ensure the selection of products adequately fortified with micronutrients.

6.1.1 Iodized salt

In many countries worldwide, iodine is added to table salt in the form of potassium iodate or iodide [123, 124]. Universal Salt Iodization (USI) is the most cost-effective and simple route to improving population iodine intake [123, 124]. In the UK, no USI is present and iodized salt is not used in commercial food processing [123]. Iodized salt brands are available to purchase in a very limited number of UK supermarkets [123]. A 1.5 g portion provides approximately 20% of the RNI for iodine [123]. Iodized salt intake must agree to UK recommendations whereby adults should eat no more than 6 g of salt a day [123].

6.1.2 Supplementation

The Food Standards Agency UK defines a supplement as “any food the purpose of which is to supplement the normal diet and which is a concentrated source of a vitamin or mineral or other substance with a nutritional or physiological effect, alone or in combination and is sold in dose form” [139]. Micronutrient supplementation is usually the provision of single or multiple micronutrients in consumable form (capsules, tablets, drops, etc.) with the aim of correcting or maintaining adequate micronutrient intake for optimal human health.

Micronutrient supplements are easily accessible, and being sold in a range of shops including supermarkets, pharmacies, and health shops [140]. Popular micronutrient supplements include vitamins D, C, and B12, along with minerals iron and calcium [140]. Most individuals can achieve adequate micronutrient intake by improving their diet quality, however, the BDA suggests that people following a vegan diet may benefit from taking micronutrient supplements [140].

Information on supplement consumption and its impact on vegans and vegetarians is relatively sparse. A recent systematic review found that only 39% (55/141) of studies assessed the contribution of supplements to dietary micronutrient intake [36]. The authors found that, for the most part, micronutrient intake in vegans did not differ considerably between those whose intake relied exclusively on foods and those who additionally consumed supplements [36]. Vitamin B12 and D were the exceptions to this, whereby intake and status tended to be lower in non-supplementing vegans and vegetarians. Most studies show that individuals following a vegan diet consume supplements more frequently than omnivores [36], particularly those providing B12 [25, 113]. Greater B12 supplementation in vegan populations is likely due to the general awareness that people following a vegan diet have regarding the higher risk of vitamin B12 deficiency [33, 141]. However, it is apparent from other work that awareness of potential deficiencies for other micronutrients such as iodine, is less common [25, 142].

There is still little robust evidence on the supplement intake of vegans and vegetarians in the modern day. This is likely to be due to methodological issues associated with accurate reporting of micronutrient supplements in dietary surveys. Patterns of supplement consumption can vary substantially over a period of time and varies considerably between individuals. The dose and type of supplement consumed differ widely and various other parameters, including life-stage, also need to be considered before introducing supplements to the diet [143]. There is a need for more comprehensive studies of micronutrient supplement use in vegan and vegetarian diets.

In the UK, the population is recommended to consume a daily 10 μg vitamin D supplement between October and March when sun exposure is lower regardless of dietary practice [136, 144]. Studies have identified that the rate of vegans supplementing with vitamin D is approximately 50% in some countries and is an effective way to improve vitamin D status all year round [99]. Appropriate supplementation, along with a balanced and varied diet, is required for vegans and vegetarians [99, 145]. B12 supplements are recommended for vegans and vegetarians who consume small quantities of animal products as adequate B12 is not provided by plant foods. The Vegan Society UK recommends individuals take one 10 μg B12 supplement daily or take a weekly 2000 μg B12 supplement. Daily iodine supplements (150 μg) are available and are useful for individuals not consuming iodized salt or other sources of iodine in the diet (e.g., fortified foods) [133]. Kelp or other seaweed supplements are not recommended to improve iodine intake as these products have variable levels of iodine and can lead to iodine excess [146]. Individuals following a vegan or vegetarian diet must seek nutritional guidance from a health professional before considering dietary supplements.

6.2 Altering cooking, food serving, and storing practices

The cooking method used, and the way food is served and stored can influence micronutrient content [35, 115]. Using iron as an example, cooking in iron-based pans can significantly improve iron intake by transferring small quantities of iron into compound foods like sauces, soups, and stews [35, 115]. Serving iron-rich foods with foods rich in vitamin C (e.g., oranges and other fruits) can improve absorption of non-haem iron sources at each meal by facilitating conversion to Fe²⁺ [35, 115, 119]. There are various compounds that further reduce the absorption of iron, including tannins (in tea), polyphenols (in coffee), phytates (in cereals and grains), and oxalates (in spinach) [35]. These substances do not need to be avoided, however, reducing the consumption of tea and coffee during meals may help to improve iron absorption. Phytates also prevent the absorption of zinc and other divalent ions [35]. Soaking foods containing phytates can reduce the abundance of phytate and diminish their effects [35]. Other serving practices to improve micronutrient intake include spreading the intake of vitamin B12 fortified foods throughout the day to improve absorption and shaking bottles of fortified alternative milk beverages helps to mix micronutrients such as calcium that may have sunk to the bottom. A study in the US found that unshaken alternative milks had 30% less calcium than described on the label [137]. Storage can also affect the micronutrient quantity of foods with factors such as temperature and exposure to UV light being important [35, 37]. Vitamin B2 is UV sensitive and exposure to foods to sunlight can result in a significant decrease in its concentration [35].

7. Conclusions

In conclusion, vegans and vegetarians are at risk of micronutrient deficiencies. Different life stages may struggle to achieve recommendations for different micronutrients. The risk of deficiency may be more prevalent in vegan and vegetarian individuals at life stages with greater micronutrient demands. Further research into the influence of vegan and vegetarian children (0–4 years) on micronutrient nutrition is required. There are many additional factors that may affect micronutrient intake, absorption, and status at an individual level. However, all individuals following vegan and vegetarian diets, irrespective of the level of restriction chosen, should be able to achieve the RNI for all micronutrients with appropriate dietary planning, knowledge of the limitations for attaining micronutrients, and implementation of suitable precautions to improve intake, including supplementation, fortified food consumption, and implementation of other techniques to improve micronutrient bioavailability.

Acknowledgments

The Division of Food, Nutrition, and Dietetics at The University of Nottingham for backing and the BBSRC for funding. This book chapter was supported by a BBSRC doctoral training program studentship, grant number BB/M008770/1. The Vegan Society UK for the king permission to use the Vegan Eatwell Guide in this publication.

Conflict of interest

The authors declare no conflict of interest.