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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 four 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.
Read and learn more
Sources:
1. WHO/FAO (1998) Carbohydrates in human nutrition. FAO food and nutrition
paper no. 66. FAO, Rome.
2. Foster-Powell, K., Brand Miller, J. (1995), International tables
of glycaemic index. American Journal of Clinical Nutrition. 62: 871S-93S.
3. 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.
4. World Health Organisation (1998) Obesity - preventing and managing
the global epidemic. Report of the WHO consultation on obesity. Geneva,
June 97.
5. Bolton-Smith C & Woodward M (1994). Dietary composition and fat to
sugar ratios in relation to obesity. Int J Obesity 18;820-828. 6. 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.
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