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August 2002
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Carbohydrates
This newsletters is about different types of carbohydrates
and their role in diet and health.
Carbohydrate, together with protein and fat, provides energy for the
content. Gram for gram, carbohydrates provide fewer calories in the diet
than fat.
Fat = 9 calories of energy per gram
Protein and carbohydrate each = 4 calories of energy per gram
Carbohydrate is formed from carbon and oxygen (from carbon dioxide) combining
with hydrogen (from water). This process is known as 'photosynthesis'
and is carried out by plants. Energy from the sun is stored in plant cells
in a form that all living things, i.e. plants and animals, can use to
fuel the processes of living. The green pigment chlorophyll traps energy
from sunlight and uses it to photosynthesise the carbohydrate in plants.
Consequently, foods high in carbohydrates are fruits, vegetables, grains,
seeds and pulses. In theory, these foods should make up the majority of
our diet. Animals (including man) consume carbohydrate-containing plants.
During digestion the energy is released from this food in a process known
as catabolism.
Non-plant carbohydrate in the diet comes from the lactose (milk sugar)
found in milk and dairy products and very small amounts from glycogen
found in meat-muscle (1).
Different Forms of Carbohydrates: (1)
Simple sugars - sugars are primary products in carbohydrate, they
are either mono-or disaccharides. They are soluble in water and easily
transported throughout tissue fluid in both plants and animals to be metabolised
in the cells. The most common simple sugars are glucose, fructose and
galactose. These are known as monosaccharides and joined together in two
unit combinations to make disaccharides. For example:
Sucrose = glucose + fructose
Lactose = glucose + galactose
Maltose = glucose + glucose
Simple sugars can be split into two groups:
'Intrinsic sugars' - are those sugars found naturally inside the
cellular structure of foods, for example in whole fruit and vegetables.
'Extrinsic sugars' - are sugars that are not incorporated inside
the cellular structure and are, for example, table sugar and fruit juice
(where the cell structure has been broken to allow release of the sugars);
and honey. Extrinsic sugars (except lactose) are a major cause of dental
caries and if consumed in sufficient quantity may contribute to High Cholesterol
levels and diabetes.
Complex carbohydrate - otherwise known as starches, complex carbohydrates
are polysaccharide structures and include fibre. Complex carbohydrates
provide more energy than people can derive from simple sugars. For example
wholegrains, beans, pulses and vegetables release their sugars gradually,
the starch having been stored by the plants as a source of energy. The
slow release of this energy in humans makes starches a better source for
stamina and continuous steady supply of energy. It is important not to
overcook starchy foods because they are digested more quickly, therefore
the starches are less likely to be slowly released. It may explain the
reason for serving pasta al dente; ensuring it is not overcooked when
served. There are two different types of starch molecule amylose and amylopectin.
These molecules are proportionally different depending on the plant from
which the starch is from, hence the difference when cooking wheat pasta
or corn, rice or buckwheat pasta (2).
Unrefined carbohydrate - natural, unprocessed form such as wholewheat
or wholegrains that contain nutrients within the cell structure.
Refined carbohydrate - processed in some way, for example wholewheat
grains that have been refined into fine white flour to make white bread,
scones, cakes and biscuits. Refined, processed carbohydrate foods contain
fewer nutrients than the unprocessed form (unless fortification has taken
place to replace certain nutrients that were removed during the refining
process). Pure refined sugar for example is just pure sucrose; it has
an energy value (4 calories per gram), but no vitamins or minerals, as
these are lost in the refining process.
Digestible carbohydrates - are sugars and starches.
Indigestible carbohydrate - are soluble and insoluble fibre -
indigestible polysaccharides in food, comprised mostly of cellulose, hemicellulose,
and pectin.
Soluble fibre - such as pectin, inulin and fructo-oligosaccharides
readily dissolve in water and form a gel-like substance. They help to
regulate the pH of the colon (3).
Insoluble fibre -The tough, fibrous outside of fruit, vegetables
and grains that does not dissolve in water, such as wheat bran.
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Carbohydrates and Blood
Sugar (4)
A relatively constant level of glucose in the bloodstream
is essential for optimum maintenance of all bodily functions. The main
hormones involved in glucose regulation are insulin and glucagon, which
complement each other as illustrated in the following diagram:
| HIGH CARBOHYDRATE INTAKE |
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HIGH BLOOD GLUCOSE LEVEL |
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Insulin released. |
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Glucose stored as glycogen in the liver and muscles. |
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OPTIMUM BLOOD GLUCOSE LEVEL
|
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|
|
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Glucagon released. |
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Glycogen converted back into glucose. |
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| LOW CARBOHYDRATE INTAKE |
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LOW BLOOD GLUCOSE LEVEL |
Starches produce a much more controlled and prolonged
rise in blood sugar levels. Simple sugars can produce sudden rises and
falls in blood sugar levels that may cause all sorts of symptoms of discomfort
ranging from Headaches and dizziness to mental confusion. This is because
the brain, which is dependent on glucose as a source of energy, is either
flooded or starved of fuel. It emphasises the importance of consuming
unrefined carbohydrates (as wholefoods) in the diet.
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Digestion of Carbohydrates
(1)
Once carbohydrate enters the mouth, it is chewed. The action of chewing
combines the food with saliva and increases the surface area of the food
for the saliva to mix with. Saliva contains the enzyme amylase (ptyalin).
This starts to digest, or break down, amylose in the carbohydrate. In
the stomach, gastric acids inhibit the carbohydrate digesting process.
Nothing further happens until the amylase from the pancreas breaks down
(hydrolyses) starch into disaccharides (smaller units of sugars). Thorough
digestion of the carbohydrate takes place in the small intestine where
it is further broken down into monosaccharides (smallest units of sugars)
in the brush border of the digestive tract.
Converting Carbohydrate (as glucose) to Fat (5)
The area of knowledge surrounding 'what makes us fat'
is very imprecise. Apart from genetics, medication, ill-health and gender,
the main relationship between food and excess content fat is simply that
calorific output must be equal to or more than calorific input. In this
respect complex carbohydrates are invaluable because they are lower in
calories than fats. However, by consuming too many carbohydrate-rich foods
it is possible to increase the content fat levels. The liver converts extra
energy compounds into fat, which is a more permanent and unlimited energy-storage
substance than glycogen.
Carbohydrate and Real Life
Imagine you have been out for a long walk and you
now fancy some chocolate. Just one or maybe two pieces of course, lovely,
now sit down and enjoy. Half the bar of chocolate later, you begin to
feel full (and guilty?). Your digestive tract, in the meantime, has delivered
glucose molecules to your liver and other content cells for instant use to
replace the energy expended during your activity. More glucose keeps coming
(the other half of the bar looks very attractive, doesn't it?) but the
muscles and liver are now full to capacity with glycogen (glucose linked
with glucose). Somehow, the liver has to cope with the excess glucose
- and it does - by breaking it down and reforming the fragments (carbon,
hydrogen, oxygen and perhaps a bit of Phosphorus) as the longer term energy
storage compound - FAT. Worse than this, is when the carbohydrate food
already has fat in it or on it, for example - crisps or chocolate.
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Carbohydrate for Health
We know from the news there are more clinically obese
people in the world, mostly in the affluent West, than there has ever
been before. The question is, why?
- Lack of exercise: there are more machines to carry out tasks that
were once done by people, in the form of electric lawn mowers,
washing machines, cars etc. Not that anyone would want to bring back
the scythe, washboard or horse and cart.
- Abundance of food choices in 'easy to get to' supermarkets, ready-meals,
instant TV dinners, snack foods etc.
- Convenience food contains more salt, fat, sugars, and is generally
more highly processed than homemade food made from simple ingredients.
- More food is consumed, especially in the 'affluent areas' of the world
for fun, taste or fashion than because a person genuinely feels hungry.
- Consumption of highly processed, refined foods that may also be high
in processed carbohydrates and hydrogenated fats that can be harmful
to health in the long term.
- People mistake the feeling of being thirsty for hunger. If we drank
more water, we would feel better and not need to 'comfort' eat.
- The role of home entertainment and advent of the 'couch potato'.
As far back as 1988 the advice in America given by the Surgeon General
was that people should reduce their intake of fat and protein and increase
their intake of unrefined, complex carbohydrates and fibre. Foods that
are good sources of unrefined, complex carbohydrates are: wholegrains,
dried beans, peas and pulses, root vegetables, nuts and seeds.
Closer to home, the 1991 COMA report, "Dietary Reference Values for Food,
Energy and Nutrients in the UK" recommended that anyone obtaining less
than around 37% of their energy from carbohydrate is relying too heavily
on fat and protein as an energy source. The report also recommended that
no more than 10% of the energy content of the diet should be derived from
extrinsic sugars, for example: sweets, confectionery, juices and sugar
(4).
We have the knowledge. Do we have the will power?
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References
- Human Nutrition and Dietetics", James, Ralph & Garrow et al. Churchill
Livingstone 2000.
- "On Food and Cooking" Harold McGee. Harper Collins, 1996.
- Adv Exp Med Biol, 1997, 427:211-9.
- Quest Vitamins Professional Product Manual, 1998.
- "Nutrition Concepts & Controversies", Sizer & Witney, West, 1994.
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