INTRODUCTORY FOOD SCIENCE
DIETARY FIBRE : SIGNIFICANT TO HUMAN HEALTH OR NOT?
In scientific terms, dietary fibre is a mixture of components derived from plant cell wall material and non-structural polysaccharides, as well as non-starch polysaccharides added to foods. It includes non-digestible polysaccharides such as cellulose, hemi-celluloses, gums, pectins, mucilages and lignin. From a nutrition point of view, some authorities also include ‘resistant starch’ (i.e. starch that is resistant to enzymic degradation, usually as a result of processing).
This dissertation deals with the physical and chemical properties of dietary fibre and discusses its significance with respect to disease prevention e.g. cancer and cardiovascular diseases, and more ultimately with respect to human health as a whole. Emphasis is given to the structure of the different forms of dietary fibre and how this affects the physical and chemical properties of each type resulting in different advantages and disadvantages to human health. The way in which fibre helps prevent disease is also discussed extensively in order to prove that clinical nutritionists are right to encourage intake of dietary fibre in increased amounts with caution to the fact that overindulgence may cause adverse effects.
Lastly the importance of high fibre foods in conjunction with a healthy lifestyle is presented as the ‘formula’ for a healthy body and avoidance of fibre depleted foods, a fact which is sadly abundant nowadays, is also mentioned.
Since ancient times, foods containing complex carbohydrates have been considered to be the ones with most nutritional value. Even the Bible speaks of how the complex carbohydrate manna miraculously sustained the children of Israel during their progress through the wilderness. Bread, of course, became known as the staff of life, and potatoes, a New World food, were grown on the private lands of Frederick the Great in 1774 to set an example to the German people as an inexpensive and easy way to avoid starvation.
Potatoes, breads, cereals, and other foods high in complex carbohydrates have always been regarded as cheap but essential, a staple for the poor. Now, however, as diets high in fats are being linked to heart disease and some cancers, as protein-rich meals are no longer the breakfast of champions, as high dietary fibre (found mostly in complex carbohydrates) is reported to possibly reduce the risk of colon cancer and heart disease, and as Americans learn that a 5-ounce steak has more calories than an equal amount of bread, pasta or potatoes (hold the butter), complex carbohydrates are becoming the chosen food of health and figure-conscious diners in the 1990’s.
U.S. Surgeon General C. Everett Koop in his 1988 Report on Nutrition and Health recommended reduction in four areas (fats and cholesterol, weight, sodium and alcohol). Only one area received a positive recommendation and this was complex carbohydrates and fibre. The report emphasises an increase in consumption of whole grain foods, cereal products, vegetables (including dried beans and peas), and fruits.
According to the director of the division of nutrition at FDA’s Centre for Food Safety and Applied Nutrition, complex carbohydrates should constitute about 55 percent of the calories in our daily diet, with fat making up 30 percent or less and protein the remainder. In an 1,800-calorie daily diet, that 55 percent represents about 250 grams of complex carbohydrates which is the equivalent of nearly 1,000 calories.
Complex carbohydrates, or polysaccharides, are made mostly of long strands of simple sugars for example glucose. They can found in seeds, fruits, legumes (peas and beans) along with a broad range of other vegetables. Complex carbohydrates include three types of dietary fibre – cellulose, hemi-cellulose and gums – and starches.
Dietary fibre (found mostly in plant foods) is the part of food that is not digested by the human body, such as the skin of an apple and the husk of a wheat kernel. The normal functioning of the intestinal tract depends upon the presence of adequate fibre. There are two broad classes of dietary fibre, water-soluble and insoluble.
Insoluble fibre, more broadly known as roughage, is associated with hastening food through the digestive tract and has a well deserved reputation for promoting ‘regular’ bowel movements. Soluble fibre, though also contributing to regularity in some ways aiding insoluble fibre, is more often referred to in the context of reducing the risks of heart disease and diabetes. Soluble fibre has a more subtle impact on the body’s metabolism of sugars and fats. It is implied, by leading scientists in the field of clinical nutrition that insoluble fibre may help prevent digestive problems such as diverticulosis, constipation and colon cancer. They also believe that soluble fibre works against high blood pressure, glucose intolerance and high blood cholesterol. Therefore, it is suggested that humans consume both types of fibre in order to receive the all round benefits that can be provided. The best sources of dietary fibre as a whole, include fresh fruits, vegetables, nuts, seeds, whole-grain products, potatoes. Cruciferous vegetables (broccoli, brussels sprouts, cabbage and cauliflower) are excellent sources of dietary fibre. It is also available commercially in capsules, tablets, chewable tablets, oral suspension and flakes or wafers. Most fibre-containing foods feature both, but one or the other type often more abundant depending on specific parts of a food and this fact determines the characteristic texture of that portion of the food. For example, insoluble fibres produce the tough, chewy feel of wheat kernels, popcorn, apple skin and nuts. Essential to the cellular structure of plants, insoluble fibres include cellulose, hemi-celluloses and lignin. They do not dissolve in water. Soluble fibres include pectin, gums, mucilages and algal polysaccharides. Although pectin is part of cell walls, most soluble fibres are found within plant cells. The gummy essence of oat bran and the mushy centre of a cooked kidney bean reflect both the soluble fibre content of those foods and the ability of soluble fibres to absorb water. The fibre content of a food varies according to the species of the plant and stage of maturation, but seeds, berries, fruit skins and the bran layers of cereal grains generally contain larger amounts of a plant’s fibre.
How much fibre should be consumed? Current nutritional guidelines recommend that adults get 25 grams of fibre each day from a diet rich in cereals, fruits and vegetables. The new guidelines recommend 6-11 daily servings of cereal and grain, 2-4 servings of fruit and 3-5 servings of vegetables daily. Not only will one get the fibre one needs but should also meet one’s daily requirements for vitamins and minerals. While Americans currently consume an average of 11 grams of dietary fibre daily, the National Cancer Institute advises an increase to 20 to 35 grams a day. Although soluble fibres have received much attention lately, Bruce Trock, cancer epidemiologist at the Fox Chase Cancer Centre in Philadelphia, cautions against the particular emphasis on any one type of fibre when planning a daily diet. Trock said that people should increase the level of fibre by increasing foods from all the vegetable, grain and fruit sources. The American Dietetic Association recommends 20-35 grams of dietary fibre. Studies on the American population state that when Americans started going from 17 grams per day to 46 grams per day of fibre, their increased faecal weight went from 79 to 228 grams. In 1988, C. Everett Koop (surgeon general) said that 35 grams was sufficient to produce the kinds of results that were desired to enhance health. It is important to have larger, healthier stools (softer and laden with water) because they are easier to pass and they have a shorter transit time. Thus, toxic substances such as cancer producing agents are excreted in faeces without lingering in the colon and hence decrease the risk of contracting colorectal cancer. Dietary fibre also excretes bile which is the substance the liver uses to produce cholesterol.
It has been shown though, that over-consumption of dietary fibre may have adverse effects on human health. A large increase in fibre intake over a short period of time can cause bloating, constipation or diarrhoea. One tablespoon of fibre will give you approximately 10 grams of additional dietary fibre. Insoluble fibre is found mostly in whole grains (i.e. wheat bran and oat bran) it is also found in raw blackberries, lima beans and peas. Soluble fibre can be found in psyllium, beans, barley and oats. Most complex carbohydrates are a mixture of both types of fibre.
There are six major components of dietary fibre :
1. CELLULOSE and HEMICELLULOSE which are insoluble and absorb water, a process known as the laxative effect.
2. LIGNIN also insoluble, absorbs little water and lowers cholesterol levels.
3. GUMS and PECTIN are soluble and are involved in the decrease of fat absorption, the decrease of cholesterol levels. They also function to decrease the rate of sugar absorption.
4. MUCILAGES which are soluble components.
Celluloses and Hemicelluloses show a relative resistance to breakdown. This property accounts for the rigid cell structure in plant cells (since they are abundant in the plant kingdom) providing support to structures in the plant tissues. They are also insoluble in both hot and cold water. Since they cannot be digested by the human body, energy cannot be derived from them in human nutrition. However, in herbivores, especially in ruminants whose diet consists solely of plant material (mainly cellulose), micro-organisms that secrete enzymes which break down the cellulose into glucose units used to provide energy, are present in the rumen of the animal. This is a situation known as mutualism. Lastly the fibre in food which provides the roughage needed is cellulose.
Pectins and Gums are sugar derivatives and are present in smaller amounts in plants than the previously mentioned forms of fibre. They are made up of chains of repeating units of sugar acids unlike cellulose chains that are made of simple sugars. Pectins are more abundant in fruits and vegetables and have a gum-like structure due to their function. They are found between the cells and help hold them together. Industrially, pectins in solution form gels and this is the basis of jelly formation. Similarly, in colloidal solution they contribute to the viscosity of tomato paste. Another use of pectins and gums in industry, is that in addition to their natural occurrence, they are added to foods as stabilisers and thickeners.
There are potential problems with increased fibre consumption. Foods containing fibre are known to decrease the bio-availability of minerals. This is partly because of the ability of certain fibres to bind minerals and increase their faecal excretion. The fact that certain phytates and oxalates bind trace minerals, is also thought to contribute to a lower availability of minerals. Minerals are more likely to be compromised in those who do not supplement their mineral intake while consuming high levels of fibre supplements. Intestinal obstructions may occur in people with already existing gastrointestinal problems and in those who do not consume adequate water. It is therefore advisable to check with a physician before self-diagnosing and administering supplemental fibre. A number of physiological effects of foods have been connected with dietary fibre such as an increased faecal bulk, lowered plasma cholesterol levels and an improved glucose tolerance. However, various types of fibre have different effects and therefore a balanced diet containing most or all types of dietary fibre should be preferred. Most studies so far have been focused on cereals and purified polysaccharides, whereas vegetables and fruits were, and still are, less investigated. The observed differences in physiological effects between various foods are related to differences in chemical ( e.g. solubility, monomeric composition and molecular weight of the fibre) as well as in physical (e.g. particle size, viscosity and water-holding capacity) properties of the fibre. Another factor of importance is that foods rich in dietary fibre, often are processed before consumption. This may change the physical and chemical properties of the fibre, and therefore also be of nutritional significance. Most of the fibre has been removed from our foods by modern processing methods. For example, when whole wheat flour is “refined” to white flour, nearly all of the fibre is removed (along with many other nutrients including most of the B vitamins). Humans also tend to eat too many foods that are poor sources of fibre including meats, fats, sugars, and overcooked foods.
A recent study has found dietary fibre – particularly cereal fibre – to significantly reduce the risk of diabetes, while high glycaemic index foods increased it in a group of women aged 40+ years. Fruit and vegetable fibre were not associated with a reduction in risk. It is suggested that soluble fibres help control the rise in blood sugar following a meal and reduce insulin requirements in some patients with diabetes mellitus. By increasing the viscosity of gastrointestinal contents, soluble fibres retard gastric emptying, slowing the absorption of glucose in the process.
The authors suggest that grains should be consumed in a minimally refined form to reduce the incidence of diabetes. The combination of a low cereal fibre intake together with a high glycaemic load was associated with a twofold increase in risk compared with a diet high in cereal fibre and low in glycaemic load. Intake of dietary magnesium was also associated with a reduction in risk of diabetes. However, cereals are one of the main sources of magnesium in the diet (provide 33% of U.K dietary magnesium, Lewis 1988), so it is difficult to separate the effect of magnesium from that of cereal fibre.
Cell walls of plants are made of fibre that give a plant structure and stability. Fibre cannot be broken down by enzymes in the digestive tract, so fibre passes through without being absorbed. The chemicals this supplement contains are in structured and non-structured substances in plant carbohydrate (starches).
Associated with the decrease of cholesterol levels, a reduction of the risk of heart disease is also statistically implied. As noted earlier dietary fibre also reduces the risk of contracting cancer of colon and rectum but intake of excessive amounts of fibre may decrease absorption of minerals, especially calcium, iron, zinc. At present no “safe” dosage has been established. Most experts feel that increasing fibre is healthful, but no one knows for sure the optimal amount.
Although it wasn’t called “fibre” until the 1950s, Hippocrates realized the laxative effects of dietary fibre in 430 B.C.. Not until the 1960s, however, did scientists seriously begin to investigate the role of fibre in health. At that time, studies showed rural Africans, with diets higher in fibre than the typical American or European diet, had a lower incidence of colon cancer, diverticulosis, haemorrhoids, gallstones, appendicitis, diabetes and some forms of heart disease than Americans or Europeans. Today, heart disease, cancer, and diabetes are nearly non-existent in rural African societies. Studies since, have generally indicated a positive relationship between a high-fibre diet and good health, although it has been difficult to separate the effects of fibre from other dietary and lifestyle factors that may play a role in health. Again Trock said that it is the total dietary pattern that has been linked to a reduced risk of disease and by looking at all the studies together, what one will find is that a diet that is high in grains, vegetables and fruits, which also is a diet that is high in fibre, is clearly protective against colon cancer and possible cardiovascular diseases.
Heart disease, cancer and adult onset diabetes are the three most common causes of death (in that order) in industrialised nations for example the United States. Medical research has shown that all three diseases are to a large extent preventable. Research has confirmed that insufficient dietary fibre is a major contributor to all three diseases. A recent medical study suggested that if the Americans could increase the per capita consumption of fibre by 13 grams, the risk of colorectal cancer in the U.S. would decrease by 31%. That translates to two lives saved every hour. Other forms of cancer have also been shown to be related to a low intake of dietary fibre, including breast cancer, which strikes one out of every nine women in the U.S.
A high-fibre diet appears to reduce disease risk by increasing faecal bulk, decreasing the transit time of food through the gastrointestinal tract, reducing blood cholesterol levels and helping to control blood sugar levels. With their distinct physical characteristics, insoluble and soluble fibres work differently to produce these results.
Insoluble fibres seem to have their greatest impact on the health of the colon or large intestine. Large amounts of insoluble fibres increase faecal bulk and draw water into the large intestine. The result is a larger, softer stool that exerts less pressure on the colon walls and is eliminated more quickly. Indeed, the most well-established benefit of a high-fibre diet is in the treatment and prevention of constipation. The reduced pressure also may help prevent diverticulosis (small herniations in the colon wall that may become inflamed). In addition, large amounts of insoluble fibres dilute the concentration of potential carcinogens that may be present in the stool, and the decreased transit time reduces the exposure of the intestinal wall to those substances. Furthermore, insoluble fibres alter the pH of the large intestine, interfering with microbial activity that produces carcinogens. The combined effect may be a reduced risk of colon cancer.
Complementing the action of insoluble fibres, some soluble fibres also add to faecal bulk and increase its water content. But soluble fibre’s potential for reducing blood cholesterol levels has recently been considered of greater significance. Studies have shown that diets rich in soluble fibres such as oat bran may help reduce total cholesterol and low density lipoprotein (LDL) cholesterol in people with both high and normal blood cholesterol levels. Margo Denke, M.D., a nutrition research scientist at the University of Texas South-western Medical Centre, estimates that a diet low in fat and high in soluble fibre may reduce an individual’s blood cholesterol by 3 percent to 6 percent.
Soluble fibres appear to reduce blood cholesterol in two ways. Firstly, they prevent the reabsorption of vital bile acids from the small intestine. To replace the lost bile acids, cholesterol is drawn from the body, hence reducing its cholesterol supply. Second, the fermentation of soluble fibres in the intestine produces short-chain fatty acids which block the synthesis of cholesterol.
As it is known high cholesterol levels in the blood is a major player in the development of coronary heart disease. But just having extra cholesterol floating around your bloodstream is not enough to cause coronary plaques to form. The mechanism for plague formation is a very complex one involving the interplay between the cholesterol molecule, the body’s clotting mechanisms (which include platelets) and blood sugar level. Cholesterol might be thought of as the bricks with platelets and other clotting factors as the cement and the wall of the artery as the surface to be covered. If the surface of the arteries is very smooth, then it is difficult for the cholesterol to attach to it, but as the arterial wall becomes less smooth, which occurs naturally with age it can activate the clotting factors more easily. These clotting factors allow the cholesterol to stick to the wall of the artery. The rougher the wall gets, the easier it is for the cholesterol to attach itself. For reasons that are still unclear, elevated blood sugar levels seem to enhance the whole plague formation process. It has been known for years that diabetics have much higher rate of coronary heart disease at a younger age. A blood sugar less than 115 was considered normal and levels greater than 140 to be abnormal with 115-140 an intermediate range. We always assumed that if the fasting blood sugar was less than 140 and certainly less than 115, the body didn’t activate the same mechanism we see in diabetics that lead to premature heart disease. But recently the Framingham study found that blood sugars over 90 increase the likelihood of plague formation. Again fibre is the substance that helps prevent this ( the mechanism linking stress to coronary heart disease is that it increases your cholesterol level, makes your platelets more sticky, and therefore more likely to clot. It also increases your blood sugar level – all the factors that lead to coronary heart disease.)
In some parts of the world, food science is giving “nature’s most complete food” a nutritional boost. In Japan, for example, the country’s largest dairy, Show Brand, has introduced a milk fortified with dietary fibre and Vitamin E. Packaged in the country’s popular 300ml-sized carton, the milk called “Seni-i-Jijutsu” (which translates to – “plenty of dietary fibre”), contains 5.3 grams of fibre in the form of Litesse, a bulking agent developed by Cultor Food Science. That’s about the amount of fibre found in six sticks of celery. The fibre fortification is a revolutionary idea since due to the over- consumption of processed foods in their country, the Japanese do not eat as much dietary fibre as they should. The new milk also supplies 8 mg of Vitamin E which is enough to provide the recommended daily requirement for both men and women.
Most food composition data bases today reflect the crude fibre content of food, which is determined by subjecting food to a chemical treatment that destroys large amounts of its insoluble fibres and almost all of its soluble fibres. The resulting value seriously underestimates the true dietary fibre content of food, which has been judged to be three to five times higher. This created difficulties when attempting to evaluate fibre intake.
Methods that provide a more accurate reading are now in limited use in the United States. The neutral detergent fibre method is primarily useful in estimating insoluble fibre content. The total dietary fibre (TDF) method is the only process that estimates total fibre content. The Association of Official Analytical Chemists, which establishes all accepted U.S. methods for determining nutrients in foods, advocates use of the TDF method. Modification of the TDF method also allows determination of soluble and insoluble fractions, but provisional data bases using figures derived from this method currently do not reflect the type of fibre in a food. According to the U.S. Department of Agriculture (USDA) Human Nutrition Information Service, an update of the information is expected in the near future. Currently, there is no universally accepted method for determination of dietary fibre. For some years the UK Ministry of Agriculture, Fisheries and Food (MAFF) has adopted the definition, for the purposes of label declaration, that dietary fibre is non-starch polysaccharides as determined by the Englyst method; but in Guidelines issued in March 1994, MAFF indicated that analysts may use any other methods which give similar results. The Englyst method excludes resistant starch. Most EU countries and the USA use the AOAC Prosky method. This method includes resistant starch and the value for dietary fibre obtained is therefore invariably higher than that by the Englyst method. It should be noted, however, that no recognised analytical method fully corresponds to biological performance.
Indeed, dietary guidelines issued by the USDA, the U.S. Department of Health and Human Services, and the Surgeon General’s office emphasize an increased intake of fibre-rich foods in general. Fibre supplements are not recommended as a way to meet dietary guidelines.
The Diet and Health report of the National Academy of Sciences has gone one step further by specifying recommended amounts of foods high in fibre. It advises a daily intake of five or more servings of fruits and vegetables and six or more servings of whole grain breads and cereals and legumes. Health professionals caution against making an immediate leap from a low-fibre intake to recommended levels. As noted earlier increasing fibre consumption too rapidly can result in flatulence, cramping and intestinal distension. Undesirable side effects may be avoided through the gradual addition of fibre to the diet along with an adequate fluid intake. Although concerns that fibre may interfere with the absorption of trace minerals have been voiced, studies show people consuming well-balanced and varied diets high in fibre are unlikely to experience mineral deficiencies.
While fibre does seem significant to human health, scientists stress its connection with other factors must not be ignored. The effects of heredity, the quality of the overall diet and habits such as smoking and exercise can easily outweigh any single dietary modification. The best guarantee is an integrated effort that includes a high-fibre diet as part of a healthy lifestyle. Fibre’s significance in maintaining a healthy body and regulating sugar levels within it, together with the prevention of coronary heart disease and cancer, is too important to be overlooked. Research in this field must continue in order to enhance our knowledge of the advantages and disadvantages of dietary fibre since it is a very crucial part of the human diet.