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Many people do not really understand which foods are classified as carbohydrates and which are not. Because the Carbohydrate Issue is always under the spotlight, it may be useful to revisit carbohydrates.
Carbohydrate chemistry
a) Monosaccharides
Without getting too technical, one can say that all carbohydrates are made up of carbon, hydrogen and oxygen atoms, which are combined by green plants to produce starches and sugars.
The most basic carbohydrates, the so-called "Monosaccharides" namely glucose, fructose and galactose (free sugars), contain only one chain of these atoms and are not often found in large quantities in foods. Free glucose occurs in small amounts in fruits and vegetables, especially grapes and onions, while fructose is the primary monosaccharide in fruits, honey and commercially manufactured high-fructose corn syrup. Both glucose and fructose have a sweet taste. (Mahan & Escott-Stump, 2000).
It is important to keep in mind that all carbohydrates are broken down to glucose and fructose during digestion and transport into the bloodstream and to our body cells to serve as a primary source of energy for physical activity and to keep our basic body processes going. The human brain is particularly sensitive to a lack of glucose and can sustain permanent damage if deprived of this energy source for too long.
b) Disaccharides
In nature, monosaccharides are usually linked together by plants to form more complex carbohydrates. If two monosaccharides are combined, they form so-called "Disaccharides", for example sucrose which contains the monosaccharides glucose and fructose (Mahan & Escott-Stump, 2000). During digestion sucrose is broken down to glucose and fructose and absorbed into the bloodstream and transported to our body cells.
Common disaccharides are sucrose (table sugar), lactose (milk sugar), and maltose (a combination of glucose and galactose found in sprouted or malted grains). Up to this point all the carbohydrates we have discussed are forms of sugar.
c) Polysaccharides
If more than two monosaccharides are combined then we speak of polysaccharides. The number of monosaccharides that are linked up in a polysaccharide can either produce so-called "short-chain carbohydrates" like oligosaccharides and inulin which are fermented in the large bowel and promote the growth of "good microorganisms". Nowadays many food products such as breakfast cereals contain inulin and other oligosaccharides, collectively know as prebiotics, to keep the good microorganisms or probiotics in our digestive tract healthy.
When a great many monosaccharides are strung together then complex molecules are formed which are called starches. Starches can in turn be divided into Rapidly Digestible Starch (RDS) (maltodextrins, white and brown bread, potatoes, etc) and Slowly Digestible Starch (SDS) (brown rice, high-fibre foods, sweet potatoes, etc) (Garrow et al, 2000).
All starches are broken down during digestion into glucose which is absorbed into the bloodstream and transported to the cells of the body.
Rapidly Digestible Starches have a high GI (glycaemic index) and have been linked to high blood glucose and insulin levels, metabolic syndrome, heart disease, diabetes, cancer and ageing. Consequently RDS should not be eaten in large quantities unless you need a fast release of energy, for example when you are doing intensive physical activity.
Slowly Digestible Starches are digested slowly and keep blood sugar and insulin levels steady for longer. These are the most desirable forms of starch and usually have a low GI (Garrow et al, 2000).
Finally there is the concept of "Resistant Starch" (RS), which is formed when Rapidly Digestible Starch is cooked and then cooled down (Garrow et al, 2000). RS has a lower GI than RDS and will have a less diabetogenic effect. Examples of RS are mealiemeal porridge that has been cooked and cooled down, and potatoes that have been cooked and cooled down (potato salad).
Examples of Different Carbohydrate Foods
1) Sugars
Sources of sugars are: Glucose, fructose, cane sugar, sucrose (table sugar), golden syrup, honey, sugar beet, all sweets and candies, all chocolates, lactose in milk, all sugar-sweetened cold drinks, fresh and dried fruit and fruit juices, all foods that taste sweet which are not made with artificial sweeteners. Cakes, tarts, biscuits and breakfast cereals all contain added sugar.
2) Starches
Sources of starch are: All cereals (wheat, rye, barley, oats, rice, sorghum, maize, spelt, titricale, quinoa), all flours made by processing such cereals (wheat, rye, rice, or sorghum flour; maize meal and oat bran), all food products made from such cereals and cereal flours (bread, rolls, porridges, biscuits, crackers, breakfast cereals, pasta, thickened sauces, pastry, cakes, tarts, pies, semolina, maizena, etc). Tapioca, sago and cassava.
All starchy vegetables including potatoes, sweet potatoes, corn on the cob, sweetcorn, turnips, Swedish turnips, and kohlrabi. Legumes (dry beans, peas, lentils and soya) also have a high starch content (e.g. on average dry beans contain about 45% carbohydrate, which is reduced to about 20% after cooking or canning).
Always keep in mind that most foods are a mixture of carbohydrates, proteins and fats, although certain foods such as plant fats and oils, and meat, fish, eggs, cheeses, unsweetened yoghurt and lard, either contain no carbohydrates or just a trace.
Nutrient content
Energy
Carbohydrates are our best source of rapidly available energy, so if you do a lot of exercise and are very active, then you require a lot of carbs to provide sufficient energy to fuel your activity and to replenish the glycogen stores in your muscles and the liver. Top athletes need readily available energy both before, during and after strenuous exercise. Avoiding carbs and concentrating on high-protein foods, can leave you feeling exhausted and lacking in energy.
A fact that is often forgotten is that compared to fats and oils which provide 37 kJ of energy per gram, carbohydrates only provide 16 kJ of energy per gram and are, therefore, much less obesogenic than fats and oils. The higher the dietary fibre content of a carbohydrate, the lower the energy value.
Dietary fibre
Most starch-rich foods, such as unsifted grains and products made with unsifted grains, as well as legumes, vegetables and dried fruit, have a relatively high dietary fibre content. If you decide to cut all carbohydrates out of your diet, then one of the most direct consequences is usually constipation. Individuals using high-protein diets often complain that they suffer from severe constipation. The best way to rectify this problem without resorting to harsh laxatives is to increase your intake of high-fibre carbohydrates.
B vitamins
Most unprocessed or unsifted starches are rich sources of B complex vitamins (thiamine, riboflavin, niacin, pyridoxine, folic acid, vitamin B6, pantothenic acid, biotin). The process of grinding or milling starches like wheat, rice or maize and then sifting them to make flour, removes large amounts of the B vitamins and other nutrients such as minerals. In some cases as in breakfast cereals, the manufacturers add back some of the vitamins and minerals that are removed during processing and breakfast cereals are among the main sources of certain nutrients in western countries. In South Africa all wheat flour and maize meal are fortified with vitamins A, B1 (thiamine), B2 (riboflavin), niacin, folic acid, pyridoxine, as well as iron and zinc, to ensure that our population can obtain these nutrients from their staple foods.
Minerals
Unprocessed and unsifted starches contain considerable amounts of minerals such as iron, zinc, and calcium. Unfortunately unprocessed grains also contain chemical compounds which are called phytates, which bind minerals and make them less available for absorption. It is, therefore, a good idea to combine staple carbohydrates with other foods rich in minerals, like dairy products (calcium) or meat (iron) to ensure a balanced diet.
The basis of most meals
According to the Food-Based Dietary Guidelines for South Africa (Vorster & Nell, 2001), we should make starchy foods the basis for most meals. This means that we should use starches, particularly unprocessed starches, as our staple foods and add more expensive proteins and fats in smaller quantities, plus fruits and vegetables, so that everyone can achieve a balanced diet.
Bad press
No macronutrient has been given as much bad press as the humble carbohydrate and most fad slimming diets banish carbs ruthlessly, but without carbohydrates the human race would probably have died out long ago. They are our staple foods providing the basis of the diet of millions of people around the world and if you eat unprocessed carbohydrates such as brown rice, unsifted maize meal, wholewheat flour, crushed wheat, high fibre cereals, as well as fruits and vegetables, as part of a balanced diet, then carbohydrates really are “the staff of life”.
- (Dr IV van Heerden, DietDoc, October 2011)
References
(Garrow JS, James WPT, Ralph A, 2000. Human Nutrition & Dietetics. 10th Ed Churchill Livingstone. Edinburgh; Mahan LK, Escott-Stump S, 2000. Krause’s Food, Nutrition & Diet Therapy. 10th Ed. W. B. Saunders Co, Philadelphia, USA; Vorster HH, Nell TA 2001. Make starchy foods the basis of most meals. SA J Clin Nutr, Vol 14(3) (Suppl) S17-S24.)
(Image: iStock)
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Read more:
The carbohydrate debate
The A-Z of sugar intake
Carbo facts for sport fanatics
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