Carbohydrates and Nutrition (English only)

There have been major advances in the understanding of how carbohydrates influence human nutrition and health in recent years. Progress in scientific research has highlighted the diverse functions of carbohydrates in the body and their importance in the promotion of good health. In fact, there is so much good news that it is time to take a closer look at carbohydrates.

The Importance Of Carbohydrates

Carbohydrates take the form of sugars, oligosaccharides, starches and fibres and are one of the three major macro-nutrients which supply the body with energy (fat and protein being the others). There is now good evidence that at least 55% of our daily calories should come from carbohydrates(1).

Whereas it is important to maintain an appropriate balance between calorie intake and expenditure, scientific studies suggest that:

A diet containing an optimum level of carbohydrates may help prevent body fat accumulation
Starch and sugars provide readily accessible fuel for physical performance
Dietary fibre, which is a carbohydrate, helps keep the bowel functioning correctly

Apart from the direct benefits of carbohydrates for the body, they are found in a wide range of foods which themselves bring a variety of other important nutrients to the diet. For this reason it is recommended that carbohydrates be supplied from diverse food sources to ensure that the overall diet contains adequate nutrients(1).

It is also important to remember that carbohydrates contribute to the taste, texture and appearance of foods and help to make the diet more varied and enjoyable.

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. Figure 1 shows the major types of dietary carbohydrates.

Sugars

Glucose and fructose are simple sugars or monosaccharides and can be found in fruits, berries, vegetables and honey. When two simple sugars combine, they form disaccharides. Table sugar or sucrose is a combination of glucose and fructose and occurs naturally both in sugar beet, sugar cane and fruits. Lactose is the main sugar in milk and dairy products and maltose is a disaccharide occurring in malt.

Polyols are so-called sugar alcohols. They do occur naturally but most are made commercially by the transformation of sugars. Isomalt is the most commonly used polyol and is derived 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 large quantities.

Oligosaccharides

When 3-9 sugar units combine they form oligosaccharides. Maltodextrins contain up to 9 glucose units and are produced commercially by the partial hydrolysis (or breakdown) of starch. They are less sweet than mono- or disaccharides. Raffinose, stachyose and fructo-oligosaccharides are found only in small amounts in certain pulses, grains and vegetables.

Polysaccharides

More than 10 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 it is contained.

Non-starch polysaccharides are the main components of dietary fibre. They include; cellulose, hemicelluloses, 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.

Carbohydrates In The Body

The main function of carbohydrates is to provide energy, but they also play an important role in:

The construction of the body organs and nerve cells
The definition of a person's biological identity such as their blood group

Energy source and storage

Starches and sugars are the main energy-providing carbo-hydrate sources and supply 4 kilocalories (17 kilojoules) per gram.

Simple sugars are absorbed directly 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 simple sugars. 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 also 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 needs to use glucose as an energy source, since it cannot use fat for this purpose. It is for this reason that the level of glucose in the blood must be constantly maintained above the minimum level. Glucose may come directly from dietary carbohydrates or from glycogen stores. Several hormones, including insulin, work rapidly to regulate the flow of glucose to and from the blood to keep it at a steady level.

The glycaemic index

When a carbohydrate food is eaten there is a corresponding rise and subsequent decrease in blood glucose level known as the glycaemic response. This response is relevant, for example, to appetite control, sports nutrition and those with diabetes. A number of factors influence the rate and duration of the glycaemic response. It depends on:

The specific food:

The type of the sugar that forms the carbohydrate
The nature and the form of the starch as some are more digestible than others
The cooking and processing methods used
Other nutrients in the food such as fat or protein

The individual person:

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 according to a standard food, such as white bread or glucose. This measurement is called the glycaemic index.

Table 1

The Glycaemic index (GI)
of common foods and sugars (2)

Food GI

Maltose 105

Glucose 100

Honey 73

White Bread 70

Mashed Potatoes 70

Cornflakes 84

Wholemeal Bread 69

Cakes 62

Sucrose 65

Rice (high amylose) 59

Banana 58

Muesli 56

Boiled Potato 56

Potato Crisps 54

Chocolate 49

Green Peas 48

Lactose 46

Orange 43

Pasta 41

Apple 36

Fullfat Milk 27

Fructose 23

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 and pectins and the oligosaccharides are fermented by the gut microflora. This also increases the overall mass in the bowel and has a beneficial effect on the make up of this microflora.

Body Weight Regulation

People eating 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:

It could be due to the lower energy density of high carbohydrate diets, as carbohydrates have less calories weight for weight than fat. Fibre-rich foods also tend to be bulky and physically filling.
Studies have found that carbohydrates both in the form of starch and sugars work quickly to aid satiety and that those consuming high carbohydrate diets are therefore less likely to overeat. The inclusion of plenty of carbohydrate rich foods appears to help regulate the appetite. Many foods with a lower glycaemic index may be particularly satisfying as they are slowly digested(1).
It has also been confirmed(3) that very little dietary carbohydrate is converted to body fat mainly because it is a very inefficient process for the body. Instead carbohydrate tends to be preferentially burnt for fuel.

It is now more and more evident(1) (4) that diets high in carbohydrate, as compared with those high in fat, reduce the likelihood of developing obesity. Starch and sugars have not been found to have different effects on weight control(1). In fact high sugar consumers have been found to be slimmer than low sugar consumers(5).

Diabetes

Diabetes is a metabolic disorder whereby the body cannot regulate blood glucose levels properly. There is no evidence that sugar consumption is linked to the development of any type of diabetes(1). However there is now good evidence that obesity and physical inactivity increase the likelihood of developing non-insulin dependent diabetes, which usually occurs in middle age(1) (6).

Weight reduction is usually necessary and is the primary dietary aim for people with non-insulin dependent diabetes. Consuming a wide range of carbohydrate foods is an acceptable part of the diet of all diabetics, and the inclusion of low glycaemic index foods is beneficial as they help regulate blood glucose control. Most recommendations for the dietary management of diabetes allow a modest amount of ordinary sugar as the inclusion of sugar with a meal has little impact on either blood glucose or insulin concentrations in people with diabetes(1).

Dental Health

The incidence of tooth decay is influenced by a number of factors(7). These include:

degree of oral hygiene and plaque removal carried out,
availability of fluoride,
type of food eaten,
frequency of consumption,
genetic factors

Foods containing 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. However it is not the amount of sugar or carbohydrate that is important but how often they are consumed. After an acid challenge, saliva provides a natural repair process which rebuilds the enamel. When 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.

However in recent years the availability of fluoride and the widespread use of good oral hygiene practices have been widely heralded as responsible for the low rate of tooth decay in today's children and adolescents. This improvement has happened independent of any change in sugar or fermentable carbohydrate intake(1). Keeping plaque bacteria at bay and strengthening the teeth with fluoride reduces the risk of decay.

The research now available in the 1990's allows a more rational approach to the role of sugar and other carbohydrates in dental caries. It is now recommended that programmes to prevent dental caries focus on fluoridation, adequate oral hygiene and a varied diet, and not on sugar intake alone(1).

Getting Active

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 training or an event, glycogen stores are kept well stocked. These stocks help the athlete to perform for longer and help their bodies sustain the effort(1).

The vital role of physical activity in maintaining health and fitness in the general population is now recognised(6). There is no doubt that many people would benefit from increasing their activity level as it helps in the regulation of body weight. It also reduces the risk of developing diseases such as heart disease and diabetes(6). For those who want to keep fit and active, a well-balanced high-carbohydrate diet is recommended.

Carbohydrate Recommendation

Carbohydrates in all shapes and forms are good for your health. 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 have been found to have any special role in the development of serious diseases such as 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 recent report from the World Health Organisation and the Food and Agriculture Organisation of the United Nations on Carbohydrates in Human Nutrition(1) makes many recommendations for health professionals and research scientists, but the most important messages for the public are:

That the many health benefits of dietary carbo-hydrates should be recognised and promoted. Carbohydrates provide more than energy alone.
An optimum diet contains at least 55% of energy from carbohydrates 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.

References

WHO/FAO (1998) Carbohydrates in human nutrition. FAO food and nutrition paper no. 66. FAO, Rome.
Foster-Powell, K., Brand Miller, J. (1995), International tables of glycaemic index. American Journal of Clinical Nutrition. 62: 871S-93S.
Hellerstein, M.K., Christiansen, M., Kaempfer, S. et al (1991). Measurement of de novo hepatic lipogenesis in humans using stable isotopes. J. Clin. Invest. 87: 1841-1852.
World Health Organisation (1998) Obesity - preventing and managing the global epidemic. Report of the WHO consultation on obesity. Geneva, June 97.
Bolton-Smith C & Woodward M (1994). Dietary composition and fat to sugar ratios in relation to obesity. Int J Obesity 18;820-828.
US Department of Health and Human Services (1996). Physical activity and health: a report of the Surgeon General, Atlanta, Georgia, USA. 7. Fejershov O. Concepts of dental caries and their consequences for understanding the disease. Community Dent. Oral Epidemiol. 1997; 25: 5-12.