Classification

The major fibre components are polysaccharides other than starch that include cellulose, beta-glucans, hemicelluloses, pectins, and gums in addition to the nonpolysaccharide component, lignin. These polysaccharides are defined by their sugar residues and links between them. Cellulose and beta-glucans are glucose polymers with beta bonds. In the beta-glucans bonds are interspersed with beta bonds, and this structure makes the molecule less linear than cellulose.

In pectins the backbone is predominantly galacturonic acid residues, rhamnose units are inserted at intervals, and side chains contain predominantly arabinose and galactose.

Lignin is composed of a mixture of phenolic compounds resulting in a highly complex molecule. Although most foods contain only small amounts of lignin, its presence can greatly affect the digestibility of the cell wall structure, and considerable interest exists in the potential carcinogenic and anticarcinogenic effects of phenolic compounds derived from plant foods.

Nutritional importance

Several clinical and experimental studies have been conducted on human patients and on animal models to demonstrate the need of fibre in the diet. These studies have shown the potential importance of certain sources of dietary fibre for normal gastrointestinal function, lowering plasma cholesterol, blunting glycemic response and insulin release.

Gastrointestinal response

For healthy individuals, dietary fibre is clearly important for normal gastrointestinal function, as summarized in table 1. Several investigators have proposed that the adequacy of fibre intake can be determined by estimating the amount of fibre or non-starch polysaccharides (NSP) needed to maintain an adequate stool weight and transit time.

Table 1. Importance of dietary fibre for gastrointestinal function

Gastrointestinal response Property of fibre Implication Stomach Gastric emptying Water-holding capacity; viscosity Slower delivery of nutrients Small intestine Lower bile acid reabsorption Bile acid binding capacity Bile acid and cholesterol metabolism Digestion and absorption of nutrients Water holding capacity; binding capacity Slow fat and carbohydrates absorption Large intestine Bulk and transit time Fermentability of the polysaccharides; water-holding capacity Stool weight; concentration in stool Microbial growth Fermentability; water- holding capacity Short-chain fatty acid production; microbial metabolism

Gastrointestinal response Property of fibre Implication Stomach Gastric emptying Water-holding capacity; viscosity Slower delivery of nutrients Small intestine Lower bile acid reabsorption Bile acid binding capacity Bile acid and cholesterol metabolism Digestion and absorption of nutrients Water holding capacity; binding capacity Slow fat and carbohydrates absorption Large intestine Bulk and transit time Fermentability of the polysaccharides; water-holding capacity Stool weight; concentration in stool Microbial growth Fermentability; water- holding capacity Short-chain fatty acid production; microbial metabolism

Prevention of diverticulosis

Increased fibre intake has been suggested for disorders involving the large intestine such as constipation, diverticulosis, and irritable bowel syndrome. In all these disorders, numerous factors other than diet can contribute to the development of the disorder.
In particular, in irritable bowel syndrome, personality and anxiety as well as dietary factors can be contributing causes. Among identifiable dietary factors, fibre is the only constituent that appears to affect stool weight.

Hence, in cases of constipation or irritable bowel syndrom in which a low fibre intake is associated with low stool weight, increasing fibre intake by recommending dietary modifications may be beneficial.

Prevention of colon cancer

Epidemiologic and experimental evidence has suggested that a diet high in fibre may be associated with a reduced risk of colon cancer. Although several methodologic difficulties have complicated the interpretation of epidemiologic studies, the majority of these studies support the view that fibre-rich diets and consumption of vegetables are associated with a protective effect against colon cancer. With regard to vegetables, however, one cannot discriminate between effects related to fibre and those related to nonfibre constituents.

Several plausible mechanisms have been formulated by which fibre may provide protection against colon cancer. These include dilution of contents, absorption of potential carcinogens, more rapid turnover of contents (decreased transit time), alteration of large intestinal contents by microbial action, and alteration of bile acid metabolism. Our current knowledge indicates that we are still far from establishing a clear cause-and-effect relationship between fibre intake and protection from colon cancer. However, this area of investigation has renewed interest in the nutritional importance of plant-derived foods and has stimulated research activity on the various constituents of plants that may be anticarcinogenic.

In addition to its established role in the large intestine, dietary fibre regulates the rate and site of nutrient absorption in the upper gastrointestinal tract. For example, viscous polysaccharides promote nutrient absorption along a greater length of the small intestine. Although the clinical implications of this effect have not been fully explored, nutrient absorption from the ileum delays gastric emptying, induces satiety, and alters postprandial lipoprotein composition.

Prevention of cardiovascular disease, hypocholesterolemic effects of fibre

Several long-term epidemiologic studies indicated a positive association between increased fibre intake and a decreased risk of coronary heart disease. In most of these studies, this association was no longer significant when controlling for other factors, such as total calorie or fat intake. Thus, evidence for a unique protective role of dietary fibre based on population studies is inconclusive. Nonetheless, this potential association has resulted in many clinical and animal studies conducted to investigate the ability of sources of dietary fibre to lower plasma cholesterol.

In reviewing the large number of clinical and animal studies conducted to test the hypocholesterolemic effects of various sources of dietary fibre, several conclusions can be drawn. Wheat bran and cellulose, both sources of nonviscous, insoluble polysaccharides, do not lower plasma cholesterol levels. In contrast, pectin, guar gum, oat bran, psyllium husk, beans (legumes), and fruits and vegetables have been reported to lower plasma cholesterol and specifically LDL-cholesterol levels. The extent of the hypocholesterolemic response to sources of fibre fed, by the initial plasma lipid values of the subjects studied, by the whether the total diet is self-selected or clinically controlled, and by other dietary variables that influence plasma cholesterol. In studies in which oat products have been added to diets that had already been modified to lower total fat, saturated fatty acids, and cholesterol intake, and additional 3 to 4 % reduction in plasma cholesterol level was observed. Thus, dietary fibre's effect on plasma lipids seems to be independent of the effect of dietary fats.

Because fibre alone only affects plasma cholesterol to a small extent, its effect may not be evident in small clinical trials, in free-living subjects which are not confined to a metabolic unit, or in individuals whose plasma cholesterol is already in the low range. For example, in free-living subjects a minimum of 40 participants may be needed to detect a difference of 5% in plasma cholesterol levels, given the normal variability that occurs in a population. In evaluating the actions of dietary fibre in therapeutic diets designed to lower plasma lipid levels, it is important to recognize that certain types of fibre, especially those containing soluble, viscous polysaccharides, make a specific contribution to lowering plasma cholesterol.

Several mechanisms have been proposed whereby sources of fibre affect cholesterol metabolism. These include increasing of fecal excretion of bile acids, slowing the rate of lipid absorption, and enhancing the production of SCFA by polysaccharide fermentation in the large bowel. Each of these factors undoubtedly contributes to the hypocholesterolemic effects of fibre; however, the relative importance of each is not well understood at this time.

Prevention of breast cancer

A pooled analysis of 12 case-control studies of dietary factors and risk of breast cancer found that high dietary fibre intake was associated with reduced risk of breast cancer. Dietary fibre intake also has been linked to lower risk of benign proliferative epithelial disorders of the breast.

Plasma glucose and insulin response

The ability of soluble, viscous polysaccharides to blunt the increase in plasma glucose and insulin following a glucose load has been related to a delay in gastric emptying and an increased viscosity of the gastrointestinal contents.

In unrefined foods, the presence of fibre is likely to slow carbohydrate absorption with the digestion of starch or other saccharides. In unrefined foods, plant cell walls or bran layers in cereal grains can serve as barrier to the penetration of digestive enzymes. For example, in rice kernels with bran layer intact, amylase digestion is relatively low, whereas grinding the whole rice kernel to a fine powder increases starch digestion substantially. In evaluating the potential importance of fibre on carbohydrate utilization, it is important to recognize that the presence of fibre may serve as a marker for the structure of plant foods because cell walls are rich in unavailable polysaccharides. Hence the potential benefit of fibre which is slowing carbohydrate utilization may be achieved by consuming foods with intact cell walls, not isolated fibre supplements.

Magnesium and human health

What is magnesium?

Magnesium is one of the minerals that we require in relatively large amounts. It is particularly abundant in green vegetables, and it is also available in natural supplements.

What does magnesium do?

Magnesium plays many roles in the body. It promotes absorption and use of other minerals such as calcium, helps to move sodium and potassium across the cell membranes; it is involved in the metabolism of proteins, and turns an essential enzymes.

Why do you need magnesium?

Magnesium helps bones to grow and teeth to remain strong. It enables nerve impulses to travel through the body, keeps the body's metabolism in balance, and helps the muscles — including the heart — to work properly. Small amounts of magnesium work as an antacid; large amounts of magnesium work as a laxative.

Up to three-quarters of the US population get less than recommended amount of magnesium (400 milligrams a day). Those most likely to be deficient in magnesium include the elderly, diabetics, moderate or heavy drinkers, and people taking diuretics.

Doctors use magnesium to treat heart rhythm abnormalities and it may help prevent clogging of the arteries.

Green tea and cancer prevention

The human body constantly produces unstable molecules called oxidants, also commonly referred to as free radicals. To become stable, oxidants steal electrons from other molecules and, in the process, damage cell proteins and genetic material. This damage may leave the cell vulnerable to cancer. Antioxidants are substances that allow the human body to scavenge and seize oxidants. Like other antioxidants, the catechins found in tea selectively inhibit specific enzyme activities that lead to cancer. They may also target and repair DNA aberrations caused by oxidants.

All varieties of tea come from the leaves of a single evergreen plant, Camellia sinensis. All tea leaves are picked, rolled, dried, and heated. With the additional process of allowing the leaves to ferment and oxidize, black tea is produced. Possibly because it is less processed, green tea contains higher levels of antioxidants than black tea.

Although tea is consumed in a variety of ways and varies in its chemical makeup, one study showed that steeping either green or black tea for about five minutes releases over 80 percent of its catechins. Instant iced tea, on the other hand, contains negligible amounts of catechins.

In the laboratory, studies have shown tea catechins act as powerful inhibitors of cancer growth in several ways: They scavenge oxidants before cell injuries occur, reduce the incidence and size of chemically induced tumors, and inhibit the growth of tumor cells. In studies of liver, skin and stomach cancer, chemically induced tumors were shown to decrease in size in mice that were fed green and black tea.

Two studies in China, where green tea is a mainstay of the diet, resulted in promising findings. One study involving over 18,000 men found tea drinkers were about half as likely to develop stomach or esophageal cancer as men who drunk little of tea, even after adjusting for smoking and other health and diet factors. A second study at the Beijing Dental Hospital found that consuming 3 grams of tea a day, or about 2 cups, along with the application of a tea extract reduces the size and proliferation of leukoplakia, a precancerous oral plaque.

National Cancer Institute (of U.S.A.) studies are testing green tea as a preventive agent against skin cancer. For example, one is investigating the protective effects of a pill form of green tea against sun-induced skin damage while another explores the topical application of green tea in shrinking precancerous skin changes.

The cancer-fighting prowess of these two staples of oriental diet derives from isoflavones, a form of flavinoids that occurs in a variety of vegetables, but is particularly concentrated in soybeans and green tea.

In green tea they can account for about 30 percent of dry weight of the tea leaves. Isoflavones work in two ways against cancer: as antioxidants and antimutagens, preventing cell mutations which can become malignancies. In animal studies, some isoflavones - called phytoestrols - have also been shown to inhibit production of the estrogen that is thought to stimulate breast cancer.

Green tea, the type usually served free of charge in Chinese and Japanese restaurants, has been linked to lower risk of esophageal cancer. Other studies have suggested that this kind of tea may also block the growth of other kinds of tumors. Researches attribute the tea's affects to its antioxidant chemicals.