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
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Carbohydrates take the form of sugars,
oligo-saccharides, starches and fibres and are one of the three major macronutrients 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.
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By GlobalDataWhat 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.
Figure 1 Classification of dietary
carbohydrates
| Monosaccharides: Glucose, fructose, galactose Disaccharides: Sucrose, lactose, maltose Polyols: Isomalt, Sorbitol, Maltitol, Oligosaccharides: Maltodextrins, Fructo-oligo-saccharides Polysaccharides: Starch, Amylose, Amylopectin Non-starch polysaccharides: Cellulose, pectins, hydrocolloids |
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 and 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 carbohydrate 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 (glucose) |
| 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 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
carbohydrates 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. - Fejershov O. Concepts of dental caries and
their consequences for understanding the disease. Community Dent. Oral Epidemiol. 1997;
25:5-12
Further reading:
- Dietary Starches and Sugars in Man: A
comparison (1989). Edited by J. Dobbing, ILSI Human Nutrition Review series. - ILSI Europe Concise Monographs Series:
Nutritional and health aspects of sugars: evaluation of new findings (1995) - ILSI Europe Concise Monographs Series:
Caries preventive strategies (1995)
