Carbohydrates are one of the three macronutrients in our diet (fat and protein being the others). They exist in many forms and are mainly found in starchy foods such as bread, pasta, and rice, as well as in some beverages, e.g. fruit juices and sugar-sweetened drinks. Carbohydrates represent the most important source of energy for the body, and are vital for a varied and balanced diet.
Progress in scientific research has highlighted the diverse functions of carbohydrates in the body and their importance in the promotion of good health. The following review will expand on this research to give an insight into this macronutrient, without forgetting that a fair amount of our knowledge has been around for some time.
2. What are carbohydrates?
The building blocks of all carbohydrates are sugars and they can be classified according to how many sugar units are combined in one molecule. Glucose, fructose and galactose are prominent examples among the single unit sugars, also known as monosaccharides. Double units are called disaccharides, with sucrose (table sugar) and lactose (milk sugar) being the most widely known. The table below shows the major types of dietary carbohydrates.
CLASSIFICATION OF DIETARY CARBOHYDRATES and corresponding examples
Glucose, fructose, galactose
Sucrose, lactose, maltose
Isomalt, maltitol, sorbitol, xylitol, erythritol
Amylose, amylopectin, maltodextrins
Cellulose, pectins, hemicelluloses, gums, inulin
Glucose and fructose are monosaccharides and can be found in fruits, berries, vegetables, honey and glucose-fructose syrups. Table sugar or sucrose is a disaccharide of glucose and fructose and occurs naturally in sugar beet, sugar cane and fruits. Lactose, a disaccharide consisting of glucose and galactose, is the main sugar in milk and dairy products, and maltose, a glucose disaccharide occurs in malt and starch derived syrups. Both table sugar (sucrose) and glucose-fructose syrup contain glucose and fructose, either in free form (glucose-fructose syrup) or linked together (sucrose).
Polyols are sugar alcohols. They do occur naturally but most are made commercially by the transformation of sugars. Sorbitol is the most commonly used polyol; xylitol is frequently used in chewing gums and mints. Isomalt is another polyol used in confectionery and is produced from sucrose. Polyols are sweet and can be used in foods in a similar way to sugars although they can have a laxative effect when eaten in too large quantities.
The World Health Organization (WHO) defines oligosaccharides as carbohydrates formed of 3-9 sugar units (monosaccharides), although other definitions allow for slightly longer chain lengths. Fructo-oligosaccharides contain up to 9 fructose units and are produced commercially by the partial hydrolysis (or enzyme breakdown) of inulin. Raffinose and stachyose are found in small amounts in certain pulses, grains, vegetables, and honey.
Ten or more – and sometimes even up to several thousand – sugar units are needed to form polysaccharides. Starch is the main energy reserve in root vegetables and cereals. It comprises long chains of glucose and occurs as granules whose size and shape vary according to the plant in which they are contained. The corresponding equivalent in animals and humans is called glycogen (see section 3.1).
Non-starch polysaccharides are the main components of dietary fibre. They include; cellulose, hemicelluloses, inulin, pectins and gums. Cellulose is the major component of plant cell walls and consists of several thousand glucose units. The separate components of dietary fibre have different physical structures and properties. A hallmark feature of dietary fibre is that humans cannot digest it. Some types of fibre can, however, be metabolised by gut bacteria to give rise to compounds the human gut cells can utilise for energy production. Hence their lower average energy content compared to most other carbohydrates (see section 3.1)
3. Carbohydrates in the body
The main function of carbohydrates is to provide energy, but they also play an important role in the structure and function of cells, tissues and organs, as well in the formation of carbohydrate structures on the surface of cells. The different molecular classes are the proteoglycans, the glycoproteins and the glycolipids.
3.1. Energy source and storage
Starches and sugars are the main energy-providing carbohydrate sources and supply 4 kilocalories (17 kilojoules) per gram. Polyols provide 2.4 kilocalories (10 kilojoules) and dietary fibre 2 kilocalories (8 kilojoules) per gram, respectively. NB: The polyol erythritol is not metabolised at all and thus has 0 calories.
Monosaccharides are absorbed by the small intestine into the bloodstream, where they are then transported to their place of use. Disaccharides are broken down by digestive enzymes into monosaccharides. The body also needs the help of digestive enzymes to break down the long chains of starches into their constituent sugars which are then absorbed into the bloodstream.
The human body uses carbohydrates in the form of glucose. Glucose can be converted to glycogen, a polysaccharide similar to starch, which is stored in the liver and the muscles and is a readily available source of energy for the body. The brain and the red blood cells need glucose as an energy source, since they cannot use fat, protein, or other forms of energy for this purpose. It is for this reason that glucose in the blood must be constantly maintained at an optimum level. Approximately 130g of glucose are needed per day to cover the energy needs of the brain. Glucose may come directly from dietary carbohydrates, from glycogen stores, or from the conversion of certain amino acids resulting from protein breakdown. Several hormones, including insulin, work rapidly to regulate the flow of glucose to and from the blood to keep it at a steady level.
3.2. The glycaemic response and glycaemic index
When a carbohydrate-containing food is eaten there is a corresponding rise and subsequent decrease in blood glucose level known as the glycaemic response. This reflects the rate of digestion and absorption of glucose as well as the effects of the insulin action to normalise the blood glucose level. A number of factors influence the rate and duration of the glycaemic response:
The specific food:
- The type of the sugar that forms the carbohydrate, e.g. fructose, sucrose and polyols have a reduced glycaemic response compared to glucose and maltose
- The form of the starch as some are more readily digestible than others
- The cooking and processing methods used
- Other nutrients in the food (or meal) such as fat (most important), protein, and fibre
The individual person:
- The extent of chewing (mechanical breakdown)
- The rate of gastric emptying and small bowel transit time (partly influenced by the food)
- Their metabolism
- The time of day the carbohydrate is ingested
The impact of different carbohydrate-containing foods on the glycaemic response of the body is classified relative to a standard food, usually white bread or glucose, within two hours after consumption. This measurement is called the glycaemic index (GI). A GI of 70 means that the carbohydrate-containing food or drink causes 70% of the blood glucose response observed with the same amount of carbohydrate from pure glucose or white bread.
High GI foods cause a greater blood glucose response than low GI foods. Foods with a low GI are digested and absorbed more slowly than foods which have a high GI. Evidence suggests that a diet based on low GI foods is associated with a reduced risk of developing metabolic diseases such as obesity and type 2 diabetes mellitus.
THE GLYCAEMIC INDEX OF SOME COMMON FOODS
(using glucose as standard)
Foods with a very low GI (≤ 40)
Foods with a low GI (41 – 55)
Noodles and pasta
Raw oranges/orange juice
Specialty grain bread
Foods with an intermediate GI (56 – 70)
Sucrose (table sugar)
Foods with a high GI (> 70)
Bread (white or wholemeal)
White rice (low amylose or "sticky rice")
3.3. Gut function and dietary fibre
The body is unable to digest dietary fibre and some oligosaccharides in the small intestine. Fibre helps to ensure good gut function by increasing the physical bulk in the bowel and stimulating the intestinal transit.
Once the indigestible carbohydrate passes into the large intestine, some types of fibre such as gums, pectins and oligosaccharides are fermented by the gut microflora. This increases the overall mass in the bowel and has a beneficial effect on the make-up of this microflora.
4. Body weight regulation
People who eat a diet high in carbohydrates are less likely to accumulate body fat compared with those who follow a low-carbohydrate/high-fat diet. The reasons for this observation are threefold:
- Carbohydrates have less calories weight for weight than fat (and alcohol), and thus, high-carbohydrate diets are comparatively lower in energy density. Fibre-rich foods also tend to be bulky and physically filling.
- The inclusion of plenty of carbohydrate-rich foods appears to help regulate appetite. Studies have found that carbohydrates, both in the form of starch and sugars, work quickly to aid satiety. As a result, individuals consuming high carbohydrate diets may be less likely to overeat. In addition, many foods with a lower GI may be particularly satisfying as they are slowly digested.
- Dietary carbohydrate is preferentially burned for fuel, or stored as glycogen for future use. Very little dietary carbohydrate is converted to body fat due to the inefficiency of this process in the body.
Evidence now indicates that, in comparison to high-fat diets, diets high in carbohydrates reduce the likelihood of developing obesity.
Diabetes mellitus is a metabolic disorder whereby the body cannot regulate blood glucose levels properly. Based on the reasons why this control fails, two types of diabetes are distinguished. In type 1 diabetes, the body becomes unable to produce the hormone insulin, which is the major regulator of blood glucose levels. Between 5-15% of all diabetics are affected, commonly from under the age of 40. In type 2 diabetes, the body either does not produce enough insulin or the target cells become unresponsive to the hormone (a phenomenon called insulin resistance). This form affects the vast majority of diabetics, 85-95%. While it tends to occur after the age of 40, an increasing number of adolescents and younger children are being diagnosed with type 2 diabetes. Treatment for both diabetes types involves a healthy, balanced diet and physical activity. Type 1 diabetics also require daily insulin injections.
There is no evidence that sugar consumption is linked to the development of any type of diabetes in healthy individuals. However there is now good evidence that obesity and physical inactivity increase the likelihood of developing type 2 diabetes mellitus.
Weight reduction is usually necessary and is the primary dietary aim for people with type 2 diabetes mellitus, and for those at high risk of developing this type of diabetes. Consuming a wide range of carbohydrate foods is an acceptable part of the diet for diabetes management, and the inclusion of low GI foods is beneficial as they help regulate blood glucose levels. Recommendations for the dietary management of diabetes emphasise increasing dietary fibre intake and allow the inclusion of modest amounts of table sugar in the diet. The inclusion of a small amount of sugar with a meal has little impact on either blood glucose or insulin concentrations in people with diabetes.
Foods containing fermentable carbohydrates (e.g. sugars or starch) can be broken down by the enzymes and bacteria in the mouth to produce acid which attacks the enamel of the teeth. After an acid challenge, saliva provides a natural repair process which dilutes and neutralises the acid and rebuilds the enamel. When fermentable carbohydrate-containing foods are consumed too frequently, or nibbled over time, this natural repair process is overwhelmed and the risk of tooth decay is increased.
Recent research indicates that a more rational approach to the role of sugar and other carbohydrates in dental caries should be applied. It is now recommended that programmes to prevent dental caries focus on fluoridation, adequate oral hygiene, frequency of eating and drinking, and a varied diet, as opposed to sugar intake alone.
The availability of fluoride and the widespread use of good oral hygiene practices have resulted in a low rate of tooth decay in today's children and adolescents. This improvement has occurred independent of any change in the intake of sugar or fermentable carbohydrates. Keeping plaque bacteria at bay and strengthening the teeth with fluoride reduces the risk of decay.
There is now substantial evidence that carbohydrates can improve the performance of athletes. During high-intensity exercise, carbohydrates are the main fuel for the muscles. By consuming high levels of carbohydrate before, during and after exercise, glycogen stores are maintained. Glycogen is a reservoir of glucose that can help an athlete perform for longer and help their bodies sustain the effort.
The vital role of physical activity in maintaining health and fitness in the general population is now recognised. For those who want to keep fit and active, a well-balanced high-carbohydrate diet is recommended.
8. Approved Health Claims
The following are examples of health claims related to carbohydrates that have undergone an assessment of their scientific validity by the European Food Safety Authority (EFSA) and are approved for use in the European Union (EU):
- Barley grain fibre contributes to an increase in faecal bulk.
- Beta-glucans contribute to the maintenance of normal blood cholesterol levels.
- Consumption of pectins with a meal contributes to the reduction of the blood glucose rise after that meal.
- Lactulose contributes to an acceleration of intestinal transit.
- Chewing gum sweetened with 100% xylitol has been shown to reduce dental plaque. High content/level of dental plaque is a risk factor in the development of caries in children.
9. Carbohydrate recommendations
Carbohydrates are a vital component of a healthy and balanced diet. They can help to control body weight, especially when combined with exercise, are vital for proper gut function and are an important fuel for the brain and active muscles. Neither starch nor sugar has been found to have any special role in the development of serious diseases such as type 2 diabetes, and the role of sugar in the development of tooth decay is less important in today's fluoride and oral hygiene aware populations. The WHO/FAO report on carbohydrates in human nutrition and the scientific opinion on dietary reference values for carbohydrates and dietary fibre from EFSA hold key information for health professionals and research scientists.
The most important messages for the public are:
- The many health benefits of dietary carbohydrates should be recognised and promoted. Carbohydrates provide more than energy alone.
- An optimum diet contains 45 to 60% of energy from carbohydrates per day for all those over two years of age.
- A wide range of carbohydrate-containing foods should be consumed so that the diet is sufficient in essential nutrients and dietary fibre.
- Adults should aim to consume 25 g dietary fibre per day. A fibre intake of 2 grams per megajoule of dietary intake (1 megajoule equals 239 kilocalories) is considered adequate for children from the age of one year.
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