In previous decades, starchy carbohydrates such as bread, pasta,
potatoes and rice were considered to be fattening, stodgy and generally
unimportant foods. The modern view is, however, that they are an
essential part of a balanced diet and we should be eating more of them.
We should be aiming to increase our carbohydrate intake to about 50 per
cent of our total calories consumed, whilst at the same time,
decreasing the amount of fat that we eat, in order to help reduce the
risk of coronary heart disease.
Carbohydrates are converted by the body into glucose and glycogen (the
animal equivalent of starch in plants). During exercise, our muscles
are fueled by glucose in the blood and by glycogen, stored in the liver
and in the muscles themselves. Glucose and glycogen are inter
convertible - if the body has enough glucose, carbohydrates will be
converted into glycogen, and if there is a shortage, glycogen will be
turned into glucose. The digestion of carbohydrates helps to maintain
the balance between the level of glucose in the blood and stores of
glycogen.
There are three main forms of carbohydrates: sugars, starch and fiber.
Both starch and fiber are complex carbohydrates. Table sugar and sugars
that are added to food and drinks are simple carbohydrates. They are
digested and absorbed rapidly - although only glucose is readily
available for use by the body. Other simple sugars, such as fructose
(from sucrose and fruit) and galactose (from milk sugar - lactose),
cannot be used quite as fast since they must first be converted to
glucose.
Complex carbohydrates, such as the starch found in bread and potatoes,
are broken down more slowly than simple sugars. By the time they have
been digested, the body's need for glucose has often been satisfied by
simple sugars provided by other foods, and so they tend to be converted
into glycogen - ready for future energy needs.
A measure of how quickly the energy from carbohydrates is made
available for use by the body is the glycaemic index. Generally, foods
with a high glycaemic index are quickly broken down into glucose and
provide a fast energy fix, while those with a lower index take longer
to break down and tend to boost long-term energy stores rather than
meet immediate energy needs.
Muscles normally contain enough glycogen to fuel about 90-120 minutes
of intense physical activity. Glycogen stores can be boosted in
preparation for prolonged periods of physical exercise, such as a
climbing weekend, or endurance sports such as long-distance running, by
eating a carbohydrate-rich diet - containing about 600g of
carbohydrates a day, or around 70 per cent of the daily calorie intake
- for about three days beforehand.
After taking a lot of exercise, when glycogen levels are reduced,
uptake of glucose by the muscles can be increased by a factor of three
or four. A high-carbohydrate diet composed of sugars and starches
replenishes and increases the body's reserves of glycogen, which
enhances the capacity for endurance exercise. This is best achieved by
eating more fruit and vegetables and more complex carbohydrates, which
are found in foods such as potatoes, yams, rice, pasta, bread, pulses
(such as peas and beans), breakfast cereals, tortillas, chapattis and
starchy root vegetables.
Eating more dietary fiber, found in fresh fruit and vegetables -
particularly those of the cabbage family - is now thought to help
protect the body against various cancers, including cancer of the
colon. Some types of carbohydrates, such as resistant starch, are not
digestible by the small intestine and pass into the large intestine, or
colon. There, along with other forms of dietary fiber, they increase
stool weight and speed the passage of food residues through the
digestive tract. This is thought to contribute to warding off cancer of
the colon.
Although the useful role of most carbohydrates is now widely
recognized, sugar continues to come under fire - because it has few
nutritional benefits and can contribute to tooth decay if it is eaten
too frequently. Many experts believe it is best to spread carbohydrate
consumption throughout the day as evenly as possible, so that there are
not vast swings in blood sugar levels. This is particularly important
for people with diabetes.
Common sugars and some of their sources are:
Glucose, found in honey, fruit, vegetables and some soft
drinks.
Fructose, which is found in fruit and honey.
Lactose, in milk and dairy products.
Maltose, which is found in sprouting grains, malted wheat
and barley, and malt extract.
Sucrose, from table sugar, as well as fruit, vegetables and
many foods and drinks which contain added sugar.
