Open access peer-reviewed chapter - ONLINE FIRST

Sugar and Its Impact on Health

Written By

Roberto Ordoñez-Araque and Byron Revelo-Vizuete

Submitted: December 26th, 2021 Reviewed: March 10th, 2022 Published: April 20th, 2022

DOI: 10.5772/intechopen.104454

Malnutrition Edited by Farhan Saeed

From the Edited Volume

Malnutrition [Working Title]

Dr. Farhan Saeed, Dr. Aftab Ahmed and Mr. Muhammad Afzaal

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Consumption of foods containing free or added sugars continue to increase, causing the global prevalence of noncommunicable illnesses to rise year after year. The purpose of this chapter is to highlight the issues associated with excessive sugar consumption. The biochemical description of the major monosaccharides, disaccharides, and polysaccharides in the diet, as well as their metabolism and absorption in the organism, will be used to objectively understand how most of the carbohydrates we eat, regardless of their name, end up being used in the glycolysis pathway to produce energy. Excess sugar consumption will be converted to triglycerides and cholesterol in the body through de novo lipogenesis, increasing the prevalence of overweight and obesity, as well as other diseases. The necessity of eating fruits and vegetables with their matrix will also be emphasized, as these are linked to weight loss and obesity prevention. This does not include 100 percent natural juices, because when their matrix is broken, sugars are released and they act as sugary drinks, as well as food made with refined flours or white rice because the starch is quickly decomposed into glucose in our bodies because they are not accompanied by fiber.


  • carbohydrates
  • metabolism
  • absorption
  • non-communicable diseases
  • public health

1. Introduction

The prevalence of non-communicable diseases (NCDs) in the world is the highest it has ever been, both in developed and developing countries. The main risk factors for these diseases are poor diet and lack of physical exercise, with NCDs claiming the lives of 41 million people a year [1]. High consumption of free sugars is associated with obesity, overweight, and a high risk of non-communicable diseases [2]. For this reason, society must understand the risk of high sugar consumption, since people do not understand the metabolism of these biomolecules and how, regardless of the name given to the sugar, it will have the same metabolism in our body.

This is why it is important to understand the metabolic pathways that occur in our bodies when we eat carbohydrates. In general terms, glucose is used by our body to produce energy through the metabolic pathway of glycolysis. Fructose enters the body and is metabolized in muscle, adipose tissue, and liver to become part of glycolysis. Galactose is an epimer of glucose and through the galactose-glucose interconversion pathway enters glycolysis [3].

All sugars end up in the glycolysis cycle behaving like glucose; that is, all sugars end up metabolized in the same way. When sugar consumption is exceeded, it will be converted into pyruvate as the final product of the glycolysis pathway and from this, lactate or acetyl-CoA molecules can be formed. From the acetyl-CoA molecule, the organism can synthesize triglycerides and cholesterol through the biochemical route of de novo lipogenesis [4]. That is to say, all the free sugar or starch without fiber that enters our organism can end up being stored as fat in our adipose tissue or can be converted into low-density lipoproteins (LDL).

The consumption of sugar through the intake of vegetables is not associated with any pathology and can be eaten freely since it is consumed in its matrix and sugars are accompanied by all the fiber and other nutrients, this causes them to be assimilated very slowly, while the starch when it is not accompanied by the entire matrix of the food (fiber, vitamins, minerals, organic acids, etc..), such as refined flours, is quickly metabolized and absorbed by the body in the form of glucose, which is why people who eat foods made with refined flours (white bread, rice, pasta, etc.) are consuming sugar.

In the same way, the consumption of 100% natural juices should be avoided, since not being in its matrix at the time of processing, all the sugar is released and these juices become very similar to sugary drinks in terms of sugar content [5, 6].

This chapter aims to disclose the problems associated with excessive sugar consumption, to develop an understanding of how it behaves in our body and which sugar foods can be potentially unhealthy to consume.


2. Carbohydrates

Carbohydrates are molecules formed by several alcohol groups together with one more oxidized carbon (carbonyl group). The main functions of carbohydrates are the energy source for the cell, energy reserve in tissues (liver and muscle), a structural molecule in several tissues, and precursor for the formation of different biomolecules (anaplerotic pathways) [7]. The classification of carbohydrates is as follows.

2.1 Monosaccharides

The simplest carbohydrates are called monosaccharides. A simple sugar or monosaccharide consists of a carbon chain, hydroxyl groups, and an aldehyde group (aldose) or a ketone group (ketose). They are classified according to the number of carbons into trioses and tetroses (metabolic intermediates), pentoses and hexoses (most important monosaccharides), heptoses (formed during photosynthesis), and octoses. Their chemical nature will allow having different types of monosaccharides, for example, there can be aldohexoses and ketohexoses. Their stereoisomerism is also important since they can present asymmetric carbons, and thanks to this a great number of monosaccharides can be formed, so we can name the most important monosaccharide: glucose, which can exist in the D and L form (configuration of the asymmetric carbon atom that is farther away from the aldehyde group) [8, 9].

With this same classification we can include D sugars that have a difference in their configuration only in one carbon atom, these are known as epimers, for example, D-glucose and D-galactose (epimers), they are only different in the configuration in carbon 4 [10].

The most important monosaccharides in the world of nutrition are hexoses. The main one is glucose (C6H12O6), A molecule that provides energy to the cells of all living beings, it is the main monomer of the disaccharides and polysaccharides, and is formed by plants during photosynthesis. One of the main characteristics of glucose is its rapid assimilation into the organism; it is absorbed by specific transporters and is also the substrate used by several microorganisms for fermentation. Fructose (C6H12O6), is found mainly in fruits (origin of its name), it does not need insulin for its metabolism. Galactose (C6H12O6), a monosaccharide that in the liver is converted into glucose to form part of the energy reserves, this is synthesized in the mammary glands of mammals; its contribution through the diet will be from the intake of milk [11, 12].

2.2 Disaccharides

Disaccharides are formed when two monosaccharides are connected by a covalent bond, this is known as a glycosidic bond, this bond can be of the α or β type depending on the configuration of the anomeric carbon atom of the bond. In general, this anomeric carbon atom is present in only one of the two monosaccharides that will form the final bond, for this reason, the final molecule still has a free aldehyde or ketone group and can behave as reducing sugar. The exception to this rule is sucrose since its two anomeric carbon atoms are bonded together [9, 13].

The most important disaccharides in nutrition are sucrose, which is a disaccharide formed by a bond between the anomeric carbon 1 of glucose and the anomeric carbon 2 of fructose (bond β (2 → 1)). It is known as table sugar and is mainly processed from sugar cane or beet. It is the most widely consumed sugar and is associated with processed and ultra-processed foods. Lactose is known as the milk sugar formed by the union of the anomeric carbon 1 of D-galactose with the anomeric carbon 4 of D-glucose, forming a β (1 → 4) bond (lactose can undergo mutarotation presenting two isomers: α and β). Maltose is a disaccharide resulting from the bonding of two glucose units at carbon 1 and 4, the anomeric carbon atom is in the α-form configuration, and thus forms an α (1 → 4) bond [13, 14].

2.3 Oligosaccharides and polysaccharides

Oligosaccharides are short chains of monosaccharides (up to 10 monomers) that are formed by glycosidic bonds, proteins (glycoproteins), or lipids (glycolipids) that can also be formed to these chains. One of the most famous oligosaccharides in nutrition is maltodextrin, obtained from the hydrolysis of starch and possessing between 5 and 10 glucose units, it is used in many processed foods and indifferent food mixtures, supplements, and medicines. It is rapidly metabolized in the body [15].

Polysaccharides are characterized by their large molecular size, insoluble in water, form colloidal solutions, and their bitter taste, unlike disaccharides and monosaccharides which have a sweet taste. Generally, their bonds are formed between the anomeric carbon and the hydroxyls found in carbons 4, 6, and 3. When there is an excess of glucose intake, animals store it in the form of glycogen (branched polysaccharide), when the organism requires energy, this reserve is released in the form of glucose to produce energy. In the food diet, mainly when we talk about malnutrition, the polysaccharide of greatest interest is starch. Starch is made up of two types of chains: amylose, the major component of starch, are linear chains formed by D-glucose that are formed by α bonds (1 → 4) and generally have a helical spatial structure. And of amylopectin: chains of branched order that are composed of α (1 → 4) bonds in their linear part, and α (1 → 6) bonds in the branches [9, 14].


3. Carbohydrate metabolism

3.1 Glucose metabolism

Sugar in the human body is metabolized from the metabolic pathway of glycolysis in the cytoplasm, this pathway aims to convert one molecule of glucose into two molecules of pyruvate, and this can be converted into lactate, ethanol, or acetyl-CoA (a molecule that can enter the citric acid cycle or substrate for the formation of fatty acids, ketone bodies, and cholesterol) [16].

In glycolysis glucose by the enzyme, hexokinase is phosphorylated by ATP to glucose-6-phosphate with a molecule of ATP, this is transformed into fructose-6-phosphate by the action of phosphoglucose isomerase (here an aldose is converted into ketose). ATP together with the enzyme phosphofructokinase phosphorylates fructose 6-phosphate to fructose 1,6-bisphosphate and ADP. The enzyme aldolase catalyzes the cleavage of fructose 1,6-bisphosphate which has six carbons into two molecules with three carbons respectively: glyceraldehyde 3-phosphate (only this molecule will follow in glycolysis) and dihydroxyacetone phosphate (this molecule can be converted to aldehyde 3-phosphate by the action of triosephosphate isomerase). The enzyme glyceraldehyde 3-phosphate dehydrogenase catalyzes the reaction of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate using inorganic phosphate and NAD+. The phosphoryl group of 1,3-bisphosphoglycerate is transferred to ADP to generate ATP along with the 3-phosphoglycerate molecule via the enzyme phosphoglycerate kinase, this molecule is converted to 2-phosphoglycerate by the action of phosphoglycerate mutase. The 2-phosphoglycerate is dehydrated to form phosphoenolpyruvate by the action of the enolase enzyme converting the low-energy phosphate ester bond into a high-energy bond. Finally, the enzyme pyruvate kinase causes the irreversible transfer of the phosphoryl group from phosphoenolpyruvate to ADP forming ATP and pyruvate (see Figure 1) [16, 17, 18].

Figure 1.

The pathway of glycolysis. Modified from: Hames and Hooper [19].

3.2 Fatty acid metabolism from glucose: de novo lipogenesis

One of the major problems of eating foods with excess free sugars is that glucose metabolism will trigger the conversion of this excess into fatty acids.

Lipogenesis is the synthesis of fats (triglycerides) in the liver and adipose tissue. De novo lipogenesis is a biochemical pathway capable of converting carbohydrates into triglycerides when glycogen stores are full. Fatty acids can be synthesized through an extramitochondrial system where complete synthesis of palmitate from acetyl-CoA occurs in the cytosol (aqueous substance surrounding the nucleus and organelles of the cell), in most mammal’s glucose will be the main substrate for de novo lipogenesis to occur. In general, when there is a high availability of ATP and acetyl-CoA (accompanied by a low rate of the tricarboxylic acid cycle, better known as the Krebs cycle - citric acid cycle), the body can synthesize fatty acids, using acetyl-CoA as the carbon source that comes mainly from carbohydrates and even from ketogenic amino acids [20, 21].

As mentioned above, pyruvate is obtained from glucose in the mitochondrion and can be converted to acetyl-CoA, which must be transferred to the cytosol. This occurs due to its condensation with oxaloacetate to form citrate. The citrate in the cytosol is regenerated back to acetyl-CoA and oxaloacetate by the enzyme ATP-citrate lyase (oxaloacetate returns to the mitochondrial matrix as it is converted to malate and pyruvate), the energy generated is used in fatty acid synthesis.

Acetyl-CoA undergoes carboxylation to form malonyl-CoA (reaction catalyzed by the enzyme acetyl-CoA carboxylase using biotin as a prosthetic group). Both acetyl-CoA and malonyl-CoA are converted to their ACP (acyl carrier protein) derivatives. These compounds will have an elongation pathway: 1) Condensation of acetyl-CoA and malonyl-CoA into acetoacetyl-CoA (together with the release of free CoA and CO2) by the enzyme acyl-malonyl-CoA. 2) Reduction by NADPH forming D-3-hydroxybutyrate-ACP (reaction catalyzed by β-ketoacyl-ACP reductase enzyme). 3) Dehydration to crotonyl-ACP (the enzyme acting is 3-hydroxy acyl-ACP). 4) Reduction by NADPH which forms butyryl-ACP (thanks to the action of enoyl-acyl carrier protein reductase). As this happens there are several successive rounds of elongation that add more carbon atoms to the hydrocarbon chain that is growing from the malonyl-ACP, this happens until palmitate (16-carbon fatty acid - C16,0) is formed [20, 22].

Similarly, animals can synthesize cholesterol from acetyl-CoA from a series of reactions that will form the 27 carbon atoms of cholesterol (acetate units are converted to five-carbon isoprene units, which condense to form the linear precursor of cyclic cholesterol). Although the body needs cholesterol for its daily functions, an excess of cholesterol-containing low-density lipoproteins can lead to atherosclerosis (hardening of the arteries due to the presence of cholesterol, which causes arterial thickening) [23].

People, in general, do not realize that excessive sugar consumption can increase triglyceride, cholesterol, and LDL levels, which can lead to several pathologies starting with overweight and obesity. Many doctors, upon seeing laboratory tests with high levels of these markers, prohibit the intake of fat in the diet, when the first thing that should be prohibited is the intake of foods with free sugars in their composition (see Figure 2).

Figure 2.

De novo lipogenesis. Modified from: Ameer et al. [20].

3.3 Fructose metabolism

As already mentioned, fructose is one of the most consumed sugars in the human diet, since it is present in countless fruits and is the molecule that, together with glucose, forms part of sucrose. Fructose can be metabolized in the muscle, adipose tissue, and liver. When metabolism occurs in muscle and adipose tissue, it is phosphorylated by the enzyme hexokinase to form fructose 6-phosphate, which then becomes part of glycolysis [3].

While when it is metabolized in the liver, the enzyme glucokinase interacts instead of hexokinase, with the disadvantage that the enzyme is only capable of phosphorylating glucose. For this reason, fructose enters the fructose-1-phosphate pathway. The pathway starts with the enzyme fructokinase which converts fructose to fructose-1-phosphate, this is then cleaved from fructose-1-phosphate into glyceraldehyde and dihydroxyacetone phosphate (a molecule that enters glycolysis in the triose phosphate isomerase step). Finally, glyceraldehyde is phosphorylated by the action of triose kinase to glyceraldehyde 3-phosphate and thus can also enter glycolysis (see Figure 3) [3, 24].

Figure 3.

The pathway of fructose. Modified from: Hames and Hooper [19].

3.4 Galactose metabolism

Galactose is one of the monomers from which lactose is formed, and is an epimer of glucose (they differ in the carbon 4 configuration). Galactose will enter glycolysis, but it must first undergo an epimerization reaction via the galactose-glucose interconversion pathway.

This pathway begins with the enzyme galactosidase which phosphorylates galactose to produce galactose 1-phosphate, this molecule is catalyzed by the action of the enzyme galactose-1-phosphate uridylyltransferase, transferring a uridyl group from uridine diphosphate glucose (UDP-glucose) to galactose 1-phosphate, and thus forming uridine diphosphate galactose (UDP-galactose) together with glucose 1-phosphate. UDP-galactose is converted to UDP-glucose by the action of the enzyme UDP-galactose 4-epimerase. Finally, glucose 1-phosphate is converted to glucose 6-phosphate by catalysis of the enzyme phosphoglucomutase to enter glycolysis [25, 26].

After understanding how dietary monosaccharides at the end of the metabolic pathways behave in the same way, as they access glycolysis to generate energy and their excess in unhealthy molecules in our organism, we must understand how they enter our body, and how by consuming some starch-rich foods we end up consuming free glucose indirectly (see Figure 4).

Figure 4.

The pathway of galactose. Modified from: Hames and Hooper [19].


4. Digestion of carbohydrates

Carbohydrate digestion is a complex system that starts from the mouth. In this case when ingested through the diet: starch, glycogen, or glucose polymers, these will begin to break down by the action of the enzyme α(1 → 4) glycosidase (salivary amylase) that is present in the saliva, this enzyme partially degrades the linear chains of amylose and those present in amylopectin, thus beginning to break down the starch. This action is not very extensive since it will be prolonged during the time that the food is in the mouth, the enzyme accompanies the food and is denatured when it enters in contact with the acid pH of the stomach [27, 28].

Digestion continues in the duodenum (first portion of the small intestine), here the enzyme α-pancreatic amylase is found, which is synthesized by the pancreas and has the same action as salivary amylase. At this point the starch molecules and other polymers formed, will be metabolized to oligosaccharides, mainly to: maltose, maltotriose, and in limit dextrins (oligosaccharides of approximately 8 glucose units branched from starch amylopectin and glycogen containing α(1–6) branch points that cannot be cleaved by α-amylase enzymes).

Finally, hydrolysis to monosaccharides of all oligosaccharide and disaccharide molecules that were formed by α-amylases and of disaccharides that are naturally ingested in the diet occurs through the action of several oligosaccharides and disaccharidase enzymes found in the enterocyte apical membrane [27, 29]. Table 1 shows the main enzymes that break glycosidic bonds in the body.

Secretion and originEnzymesSubstrateActionProducts
Saliva from the salivary glands of the mouthα-amylaseStarch / α-linked polysaccharidesHydrolysisDextrins - maltose
Exocrine secretions of pancreatic acinar cells (site of action: duodenum)α-amylaseStarch and dextrinsHydrolysisDextrins - maltose
Small intestine (brush border membrane)SaccharaseSucroseHydrolysisGlucose - fructose
α-dextinase (siomaltase)Dextrin - isomaltoseHydrolysisGlucose
LactaseLactoseHydrolysisGlucose - galactose

Table 1.

Enzymes that allow starch and glucose polymers to be rapidly broken down into oligosaccharides, disaccharides, and monosaccharides during digestion.

Adapted from the book: Krause. Diet Therapy [27].


5. Free and intrinsic sugars

Everyone needs to understand the difference between free sugars and intrinsic sugars. According to the World Health Organization in its Guideline: Sugars intake for adults and children, free sugars are all monosaccharides and disaccharides that the food industry intentionally adds to their products, and sugars that are found naturally in different foods such as honey, 100% fruit juices, syrups, etc.

While intrinsic sugars are those found in whole vegetables, i.e. unprocessed fruits and vegetables, these types of sugars are not related to any adverse health effects, while free sugars are associated with several pathologies as we will describe later in this chapter [2].

The industry tries to create new products and often masks free sugars with different names unfamiliar to people (including honey, which has about 80% free sugars in its composition) [30]), Table 2 shows some of the names under which sugar is labeled in some food products.

Other names for sugar that you may see on food labels
Agave nectarEvaporated cane juiceMaltodextrin
Brown sugarFructoseMalt syrup
Cane crystalsFruit juice concentratesMaple syrup
Cane sugarGlucoseMolasses
Corn sweetenerHigh-fructose corn syrupRaw sugar
Corn syrupHoneySucrose
Crystalline fructoseInvert sugarSyrup

Table 2.

Compounds are formed by different monosaccharides that provide a sweet taste and their final metabolic effect in the organism is the same as glucose.

The food industry uses other names instead of sugar to deceive the consumer. Source: Harvard T.H. Chan School of Public Health [31].


6. Food matrix (starch can also be sugar)

The matrix of food is the global structure that a food has, it is the support and the joint union of all the nutrients of which it is constituted, this union allows us to identify each food with a certain thickness, texture, density, hardness, color, porosity, crystallinity, etc. Each food in nature has its matrix that provides certain characteristics when consumed, such as the bioavailability of nutrients or the sensation of satiety (solid foods rich in fiber will provide more satiety than liquid or semi-solid foods [32].

During the processing of food (industrial or at home), there will be a release of nutrients on a large scale since its matrix (its global structure) is being broken, this will allow the slow metabolism and absorption that the molecules had before to become much faster. In case of sugars, they will go from being intrinsic to free and will be absorbed immediately, since the organism will not have to use biochemical mechanisms to break the matrix, which in many cases can pass through the stomach and intestine intact since the structure can be accompanied by fiber (polysaccharide not digestible by the human body) [33].

All sugars or starch in the organism will be metabolized as monosaccharides (glucose, fructose, and galactose). It will depend on the dietary intake for this to be fast or slow when starch is consumed that is not accompanied by its fiber matrix, the absorption will be fast, this group includes refined flours, with which countless products can be made such as bread, white rice and pasta [34, 35]. Consuming this type of food is the equivalent of eating monosaccharides (sugar). Precisely many of these products may be accompanied by more components that are not recommended for a healthy daily diet, for example, bread made with refined flours has as ingredients: sugar, salt, and generally trans fats. People should be aware that just because a food does not have a sweet taste, such as white rice, does not mean that it does not have glucose in its composition [36].


7. Health problems due to consumption of free sugars

Current evidence indicates that the consumption of free sugar through food is associated with several diseases, observational studies have shed several lights on the problems associated with sugar consumption, so we can find relationships between the intake of sugar-sweetened beverages with the association of adverse effects on markers of cardiovascular risk, especially in the increased risk of stroke [37], or we can cite the analysis conducted in 75 countries where the relationship between the consumption of sugar-sweetened soft drinks and its positive association with the prevalence of overweight, obesity, and diabetes was found, these data do not vary regardless of the income of the household [38].

It is also interesting to cite the study in which 26,190 people without pathologies (diabetes and cardiovascular diseases) were followed for 17 years, in this case, it was associated that people who consumed more than 15% of their energy intake from sucrose intake in their meals and drinks could be more likely to have coronary events [39]. Although observational studies often do not provide the real causality of the results, there are intervention studies that present the same conclusions on sugar consumption, for example, a meta-analysis of randomized controlled trials and cohort studies showed that the consumption of free sugars in food and sugar-sweetened beverages is the most important and determining factor in weight gain, while people who reduce sugar consumption show a decrease in body weight [40].

Another meta-analysis and cohort study conducted in children and adults indicated that consumption of sugar-sweetened beverages is associated with increased body mass index, whereas reduction of these beverages showed a reduction in weight gain especially in children (an interesting conclusion was that reduction of sugar-sweetened beverages in children causes more effect on weight loss than school-based good nutrition programs) [41].

The relationship of dietary sugar intake on blood pressure and serum lipids has also been analyzed in a meta-analysis of randomized controlled trials. The conclusions found a positive association, including this association regardless of the bodyweight of the person, which indicates that it is not always necessary to be overweight or obese and to be prone to suffer from diseases associated with sugar consumption [42]. Finally, we can refer to a study carried out after six years of follow-up where the quantity and quality of abdominal adipose tissue were analyzed using a consumption frequency questionnaire, data were obtained on the consumption of sugar-sweetened beverages and their association with the change in the volume of visceral adipose tissue, while consumers of diet drinks were also analyzed, which were not associated with changes in abdominal adipose tissue [43].

It should also be mentioned that several studies have determined that the consumption of foods with sugars in their composition is highly related to the increased risk of dental caries since different bacteria found in the mouth are capable of transforming monosaccharides into acids that will subsequently affect dental enamel [44].

To complement all this information we can refer to the most current report of the European Food Safety Authority (EFSA) [45], which was developed following the request to establish a maximum tolerable sugar intake level by 5 countries. Although EFSA does not make guidelines or recommendations that influence public health, it does give nutritional conclusions based on scientific evidence, for this reason, the scientific experts analyzed 120 scientific studies (the scientific publications that were analyzed met different inclusion criteria, these were selected from more than 25,000 studies in 2018 and 7500 in 2020) that linked the intake of sugars in the diet with chronic metabolic diseases, and pathologies that were related to pregnancy and tooth decay. This meta-analysis concluded that it is not possible to establish a maximum tolerable intake level, nor a safe level of dietary sugar intake. This is because after the analysis of all the research and with the available scientific evidence, it can be determined that the intake of free and added sugars should be as low as possible. After all, it is associated with chronic metabolic diseases and tooth decay, the consumption of sugar is not essential because what the body needs can be consumed in fruits if they are accompanied by their matrix, for this reason, there should not be a recommendation.

The World Health Organization (WHO) already pronounced this issue in 2015, likewise, after the analysis of randomized controlled meta-analyses in adults and children, concluded that the consumption of free sugars should be reduced throughout the life cycle. This consumption should not be more than 10% of total caloric intake and it is recommended to reduce this consumption to 5% [2]. Table 3 presents the metabolic diseases related to the consumption of different types of sugar and the pathologies related to obesity.

Metabolic diseasesSugar typeObesity-related pathologies
ObesityFree and added sugars, sweetened beverages, and 100% fruit juicesAll causes of death (mortality), hypertension, dyslipidemia, type 2 diabetes, coronary heart disease, stroke gallbladder disease, osteoarthritis, sleep apnea and breathing problems, low quality of life, mental illness such as clinical depression, anxiety, and other mental disorders, body pain and difficulty with physical functioning and many types of cancer: Meningioma (cancer in the tissue covering the brain and spinal cord), adenocarcinoma of the esophagus, multiple myeloma (cancer of blood cells), kidneys uterus, ovaries, thyroid, breast (postmenopausal women), liver, gallbladder, upper stomach pancreas, colon and rectum
Liver diseasesFree and added sugars, sweetened beverages, and sugar-sweetened beverages
Diabetes type 2Free and added sugars, sugar-sweetened beverages, and 100% fruit juices
LDL cholesterolFree and added sugars and sugar-sweetened beverages
HypertensionFree and added sugars and sugar-sweetened beverages
Cardiovascular diseasesFructose and sugar-sweetened beverages
GoutFructose, sweetened beverages, and 100% natural juice
Diabetes during pregnancySweetened beverages

Table 3.

Relationship between sugar consumption and health problems.

LDL: Low-density lipoprotein. Information was obtained after the evaluation of 120 meta-analyses selected from more than 32,000 studies from 2018 and 2020 after meeting inclusion criteria. Excessive sugar consumption is related to overweight and obesity, in turn, this disease is associated with other pathologies.

Adapted from: EFSA explains draft scientific opinion on a tolerable upper intake level for dietary sugars [45] and Centers for Disease Control and Prevention: Cancer and obesity [46].


8. White rice and refined flours

Throughout the chapter, the health problems that the consumption of free sugars can cause have been described, but it must be taken into account that many times people assume that the consumption of rice and flour does not mean eating sugar. As already mentioned, rice and flours are formed by starch, starch is formed by amylose and amylopectin which are linear and branched chains of glucose, if foods that are obtained from flour or rice, are not accompanied by their matrix, the degradation and absorption of glucose in the body will be rapid, and therefore the intake of this type of food will be very similar to the consumption of products with free sugars. Whole rice is composed of an outer layer which is responsible for wrapping the grain, the bran (made up of pericarp, this or integument, and aleurone layers), the germ, and the endosperm (here the reserve nutrients are stored: starch) [47].

In general, white rice is not important from the point of view of nutrition, since the germ and the bran (which contains mainly fiber in its composition) are removed in the process of production. For this reason, white rice has been the subject of several investigations and is related to the increased risk of type 2 diabetes (during consumption a large amount of glucose is obtained in a very short time after the metabolism of starch, with this there is a significant increase in blood glucose, causing the pancreas to secrete a high amount of insulin. When this process is repeated throughout life, insulin resistance can develop) [48], it is also important to note that replacing white rice with brown rice in the diet is associated with a decreased risk of type 2 diabetes [49].

If we talk about flour, we can use wheat as an example (in flour we can include all cereals, and in general there can be whole wheat flour and refined flour), which is composed of bran (pericarp, testa, and aleurone), germ and endosperm (main component composed of starch as in all cereals) [50]. Like rice, refined flours are composed mostly of starch and are not accompanied by fiber, which will cause the same outcome of glucose absorption after consumption, for this reason, the consumption of refined cereals is associated with a higher incidence of metabolic syndrome (group of risk factors for heart disease, diabetes, and other pathologies) [51], and among the most important problems of high consumption of refined grains in the long term is their association with the risk of coronary heart disease [52].


9. Natural fruit juices (naturally sweetened)

As already mentioned in Section 6 of this chapter, it is important to consume food in its matrix, when we break this matrix, we release nutrients that will be much faster to absorb in our organism. Fruit juices 100% natural are no exception, if we break the matrix of fruit, we release all the sugars it may have (fructose, glucose, sucrose, etc.,) and these will go from intrinsic to behave as free sugars, behaving like any sugary drink [53]. In the United States, fruit juices have been ranked 5th of the 6 beverages recommended for consumption according to their health risks and benefits, with water ranked first and sugar-sweetened beverages sixth [54].

There is ample evidence on 100% natural fruit juices, their health implications can be seen in Table 3, these associations to different pathologies have been collected for years, so there are meta-analyses that indicate that the intake of sugary drinks and fruit juices have a high association with the possible development of type 2 diabetes, and conclude that the consumption of fruit juices is not a healthy alternative to sugar-sweetened beverages [55].

Type 2 diabetes is the pathology that is most related to excessive consumption of fruit juices, has a high association when analyzed in a large number of people, so it is emphasized that nutritional recommendations should be followed to reduce the consumption of natural juices in the diet [56]. Sugar is one of the main causes for overweight and obesity, that is why the consumption of fruit juice is closely related to these pathologies since in its composition it has as many free sugars as a sugary drink, the metabolism of fruit is not the same as that of juice, there is no feeling of satiety since it is not chewed, in the juice there are several servings of fruit that are consumed immediately, raising blood glucose levels just as a drink with glucose, fructose or sucrose does, it is also important to mention that in fruit juices an interesting amount of fiber is lost during processing [57]. We can also mention three prospective cohort studies where the relationship of changing the consumption of sugary drinks and fruit juices with water in the long term was examined, finding a positive association, that is, if in the diet the consumption of sugary drinks (including juices) is changed by water, a marked decrease in weight is observed, it is important to emphasize that water does not have compounds that intervene in the metabolism of a person to lose weight, but the simple consumption of water prevents people from consuming unhealthy beverages [58]. Finally, it should be mentioned that the consumption of natural juices is associated with dental caries in adults, which has been corroborated in several studies [59, 60].

For this reason, one of the nutritional guidelines in the world is the reduction of this type of beverage, but the lack of knowledge of people about sugar leads them to think that 100% natural juices are a good nutritional alternative, this can be corroborated in a study conducted in California with data from 2003 to 2009 in children, where it was identified that during those years the intake of sugary drinks decreased drastically but the consumption of 100% natural juices increased in the same way [61].


10. Fruits: The only sugar we need

The human body needs glucose to properly perform its daily functions, this sugar is found in fruits and vegetables intrinsically accompanied by its matrix. For this reason, we can eliminate the consumption of free sugar (remember that one of the WHO recommendations is not to exceed 5%, while the EFSA does not suggest any limit since its consumption is associated with several pathologies). The WHO is one of the most important recommendations it has given in the area of nutrition is that a healthy diet should include the consumption of 5 servings (at least 400 grams) of fruits and vegetables per day (this recommendation does not include starchy tubers such as potatoes, sweet potatoes, cassava, etc.), its recommendations are based on scientific evidence and emphasize that eating 5 servings of fruits and vegetables per day can reduce the risk of the onset of noncommunicable diseases [62].

It should also be clarified that the intake of fruit juices does not replace fruit consumption under any circumstances, even though the fruit juice industry tells us otherwise. It is interesting to mention that the consumption of vegetables (mainly fruits) not only does not cause overweight and obesity but also is associated with the prevention of these pathologies [63]. It has been found that the consumption of fruits and vegetables has a positive relationship with the improvement of anthropometric parameters and the risk of increased body adiposity, for this reason, nutritional and governmental agencies should seek all the necessary mechanisms for people to increase the consumption of fruits and vegetables [64].

For this reason, nutrition is increasingly seeking to find the best way to convey healthy guidelines to the population and the nutritional pyramid that has been used for many years is increasingly in disuse because its interpretation can cause several nutritional errors, for this reason, the new trend proposed by one of the best schools of nutrition in the world is the healthy eating plate (the Harvard plate) where the interpretation is clear and simple, no superfluous foods without nutritional importance (sugary drinks, alcoholic beverages, desserts, sausages, trans fats, etc.) and it is established that the main pillar of the nutritional pyramid is the healthy eating plate,) and it is established that the fundamental pillar of nutrition is fruits and vegetables [65].

11. Conclusions

It is important for the whole society to understand in a general way the metabolism of sugar in the human body, this will allow people to make better decisions when choosing their food since they will understand what type of nutrients are found in the products and if they are recommended or not.

Glucose is the main monomer of carbohydrates, the body uses it to generate energy, its excess is stored as glycogen in the liver. Fructose and galactose (important monosaccharides in the diet) are metabolized in the body, generating compounds to enter the glycolysis pathway. Once the glycogen reserves are completed, pyruvate (molecule resulting from glycolysis) can be transformed into the acetyl-CoA molecule, and this in turn is used by the organism to generate fatty acids and cholesterol. In other words, the excessive consumption of carbohydrates can be transformed into adipose tissue in the body.

Sugar should always be consumed in the food matrix (intrinsic sugar), i.e., accompanied by all the fiber and other nutrients. When food is processed, sugar is released and behaves as free sugar, i.e. like any sugary drink or any processed or ultra-processed food with sugar or refined flours in its composition.

Excessive sugar consumption is associated with several non-communicable diseases, starting with overweight and obesity. A strong association of pathologies has been found after the consumption of free or added sugar in the diet, among the most important ones: Liver diseases, type 2 diabetes, cholesterol (LDL), cardiovascular diseases, hypertension, gout, and gestational diabetes.


The authors are very grateful to Universidad de las Américas (UDLA) for all the support to carry out this chapter. We are also very grateful to: Julio Bazulto, Aitor Sánchez, José Miguel Mulet and Juan Revenga, their books and publications have helped us to understand the world of nutrition in a great way.

Conflict of interest

The authors declare no conflict of interest.


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Written By

Roberto Ordoñez-Araque and Byron Revelo-Vizuete

Submitted: December 26th, 2021 Reviewed: March 10th, 2022 Published: April 20th, 2022