Nutritional and health benefits of dietary fibre
(By Dr. Paul Nanna)
As recent as the 1970s, certain diseases that were common among the Caucasians and Americans were almost non-existent in Africa. These diseases include intestinal disorders such as constipation, haemorrhoids, appendicitis, polyps, diverticulitis, and diverticulosis and colon cancer. Others are cardiovascular diseases like hypertension, heart attack, stroke and metabolic conditions such as diabetes.
A group of researchers led by Dr. Denis Buckitt found that the reason this was so had to do with the diet of the African. The diet of the African was in the main made up of unrefined carbohydrates and other high-fibre foods, such as raw vegetables, as against the high level of refined, processed and canned foods that the Caucasians ate. They also discovered that the advent of these diseases in Europe and the Americas coincided with the introduction of techniques of converting whole wheat to white flour in the late 1800s. This involved the removal of fibre from the wheat to produce white flour. The attraction was the whiteness of the flour, not thinking about the hazards of such a product to our health. Not only that, rice is polished by eliminating the fibre, also to make it look whiter. In the last 40 years or so, not only have these processing plants been established all over Africa, the finished products, such as white flour and rice, are imported into our continent in large quantities.
What is dietary fibre?
The first impression you get when you hear the name “dietary fibre” is that it must be a fibrous kind of substance. The name is actually a misnomer in that this fibre has nothing to do with fibrous tissue.
According to the Food and Nutrition Board of the Institute of Medicine in Washington DC, dietary fibre consists of non-digestible carbohydrates and lignin that are intrinsic and intact in plants. Functional fibre consists of isolated, non-digestible carbohydrates that have beneficial physiological effects in humans.
Total fibre is the sum of both dietary and functional fibres. The dietary fibre includes non-starch polysaccharides found in plants, such as cellulose, pectin, gum and hemicellulose. Others are fibres contained in oats and wheat bran, oligosaccharides, lignin and some resistant starch.
There are two main components of fibre – soluble and insoluble. Soluble fibre can dissolve in water and it is readily fermented in the colon into gases and physiologically active by-products. This type of fibre slows down the passage of food in the digestive tract. Insoluble fibre, on the other hand, does not dissolve in water. It is metabolically inert and provides what is known as bulking. In bulking, this fibre absorbs water throughout the length of the digestive tract. This produces two distinct and important effects. Firstly, it increases the bulk of stools, and secondly, it speeds up the passage time of food through the intestines. The end result is that passing out stools becomes easier and more frequent. This is very important, in that it is the mechanism by which insoluble fibre cleanses, detoxifies the colon and reduces the acid waste load in the colon. This helps to prevent certain diseases as we shall see later.
Plant sources of dietary fibre
Dietary fibres are predominantly found in plants. Some plants contain both types of fibre but the decision, whether or not to eat some fibre, should not be based on eating one or the other. In other words, let your decision be based on the fact that the body needs fibre and you are going to supply enough fibreto it on a daily basis.
Common sources of both soluble and insoluble fibre include: fruits such as avocado, bananas, pears, apples, prunes, plums, skin of kiwifruit and grapes; vegetables like broccoli, celery, carrots, green beans, cauliflower etc. Other sources are whole grains, wheat and corn bran, oats, rye and barley. A variety of legumes are also high in fibre and in this category are black beans, white beans, kidney beans and lentils. The rest are almonds, flaxseed, sweet potato, onions, brown rice and Ofada rice.
Recommended daily intake of fibre for the American adult is 20-35 grams but research shows that they have fallen short of this because of the typical American diet. Going by diet, one can safely say that an adult Nigerian should be doing more than 50 per cent of this daily requirement. I am certain that we can do even better than this if we add a few of those foods that have a high content of fibre to our meals daily.
Dietary Fibre as a Preventive
Dietary fibre can be used as a preventive or treatment for various kinds of health conditions. These are health conditions that affect the gastrointestinal tract such as constipation, haemorrhoids, diverticulitis, irritable bowel syndrome and gallstones. They also include diseases that affect the cardiovascular system, artheriosclerosis, cardiovascular disease, hypertension etc. The third group of diseases are the metabolic diseases like diabetes and syndrome X. There is also the cancer group – colon cancer and breast cancer especially. When there is a long standing constipation, the colon becomes a hugereservoir of acidic wastes that actually increase the acid that can lead to cancer development in other parts of the body. Finally, we have other health conditions such as obesity and hypercholesterolemia.
Now let us look at the effect of dietary fibre on conditions such as constipation and general health of the colon, diabetes, cardiovascular disease, obesity and blood cholesterol regulation.
Insoluble, fermentable fibres are fermented by the normal bacterial flora found in the colon to produce short-chain fatty acids. These are butyric, propionic and acetic acids. Butyric acid is a primary fuel for the cells of the colon to carry out their functions. It also helps to keep the colon healthy by destroying the unfriendly and destructive bacteria that may accompany food and water into the intestines. By this action, fibres help to boost the immunity of the body. Not only that, insoluble and non-fermentable fibres absorb water in the colon, increase the bulk of the faecal matter and cleanse the colon. As already noted, with this bulk, they increase the transit time of the passage of stools out of the anus. These fibres, by this action, maintain the health and integrity of the colon, prevent constipation and other diseases of the colon and reduce the risk of developing colon cancer.
Management of type 2 diabetes
Soluble, viscous fibres play a significant role in the prevention and treatment of diabetes. In the stomach, these fibres delay the rate at which food is emptied into the small intestine for absorption. This delay, in turn, causes a delay in absorption of glucose by reducing the amount released into the small intestine per unit time. Glucose absorption occurs in little amounts spread over a longer period and eliminates the glucose surge and insulin spikes that lead to insulin resistance. Dietary fibre also increases insulin sensitivity and function.
Prevention of hypercholesterolemia
Soluble, viscous fibres combine with bile salts (acids) to form complexes which prevent the bile salts from being reabsorbed back to the liver. Bile salts function in the proper digestion of fats and they are produced in the liver from cholesterol. Preventing their reabsorption means that the liver will have to continually utilise cholesterol (especially the LDL cholesterol) to produce more bile salts. This lowers the concentration of total and LDL cholesterol in the blood.
One of the fatty acids produced by fermentation of insoluble fibre by the friendly bacteria in the colon is propionic acid, as we found out earlier. This propionic acid inhibits an enzyme known as HMG-CoA reductase in the liver. This enzyme is responsible for cholesterol production in the liver. As this enzyme gets inhibited, cholesterol level in the blood drops. Also, soluble fibre directly reduces the absorption of cholesterol.
Prevention of cardiovascular disease (CVD)
A lot of studies by different researchers all over the world have proved that dietary fibre prevents incidences of cardiovascular disease. Soluble fibre in particular has been found to lower total and LDL cholesterol concentration. The mechanism of action is thought to be by preventing the re-absorption of bile acids that have formed complexes with the fibre. The result of this is that, there is increased uptake of LDL cholesterol for production of bile acids by the liver. This, as I explained earlier, leads to a decrease in blood cholesterol concentration.
With normal levels of cholesterol in circulation, none gets deposited on the walls of the arteries to form artheriosclerotic plaques, which may block the blood vessel and cause cardiovascular disease. Some researchers have said that a diet high in water soluble fibre is inversely associated with the risk of CVD. To prevent cardiovascular disease therefore, a diet high in water soluble fibre is highly recommended.
Prevention of obesity
Fibre, not being digested, does not provide calories to the body. Water soluble fibre in the stomach absorbs water and increases the bulk of the stomach contents. This gives a feeling of fullness and satiety which, together with slowing down the movement of food out of the stomach to the intestines, leads to a reduction of food eaten.
It is recommended that fibre foods be chewed for a long time before swallowing. This gives a signal to the brain that you have eaten enough.