|
t_Articles
Carbohydrates and Nutrition
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 to 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 structure and function of the body organs and
nerve cells
Energy source and storage
Starches and sugars are the main energy-providing carbohydrate sources
and supply 4 kilocalories (17 kilojoules) per gram. To utilise this
energy all carbohydrates need to be broken down to glucose after which
they are 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.
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 (GI)
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 nature and the form of the carbohydrate
as
some are more digestible than others
- The cooking and processing methods used which can change the
digestibility.
- Other nutrients in the food such as fat or protein that can slow
the digestion rate.
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 standardised
amount of glucose. This measurement is called the glycaemic index, (GI).
|
Table 1
|
The Glycaemic index (GI)
of common foods and sugars (2) |
|
Food |
GI |
|
Maltose |
105 |
|
Glucose |
100 |
|
Rice (low amylose) eg
white rice |
88 |
|
Cornflakes |
84 |
|
Honey |
73 |
|
White Bread |
70 |
|
Mashed Potatoes |
70 |
|
Wholemeal bread |
69 |
|
Soft drink |
68 |
|
Ice cream |
61 |
|
Sucrose |
65 |
|
Rice (high amylose) eg
Basmati |
59 |
|
Papaya |
58 |
|
Rice vermicelli |
58 |
|
Boiled potato |
56 |
|
Sweet corn |
55 |
|
Mango |
55 |
|
Potato crisps |
54 |
|
Banana |
53 |
|
Chocolate |
49 |
|
Green Peas |
48 |
|
Instant noodles |
47 |
|
Lactose |
46 |
|
Orange |
43 |
|
Pasta |
41 |
|
Apple juice |
41 |
|
Apple |
36 |
|
Kidney beans |
27 |
|
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 (broken down)
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
fats. Fibre-rich foods also tend to be bulky and physically filling
therefore less calories may be consumed.
- 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 used an energy by the body.
It is increasingly 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, in several studies, 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 (Type II) 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 of any fermentable carbohydrate
- 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 other
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 Organization and the Food and
Agriculture Organization 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. 7. 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 Preventative
Strategies (1995).
|
