Health Benefits of Complex Carbohydrates

David Kritchevsky

The Wistar Institute, Philadelphia, Pennsylvania

The modern fiber era was launched with a paper by Cleave (1) that attributed many of the diseases of developed countries to consumption of refined sugar and refined flour. Burkitt, Trowell and Walker, all of whom worked in Africa, noticed the low incidence of large bowel cancer, ischemic heart disease, diabetes, and gallstones in rural areas. g

Burkitt and Trowell expanded these observations and brought them to the attention of the nutrition and medical communities (2-6). Hipsley (7) coined the t?

term ''dietary fiber.'' Dietary fiber may be defined as plant material that resists ^

digestion by human alimentary enzymes. It includes substances of unique chemical structure, distinct physical properties, and individual physiological effects. | Except for lignin, all are carbohydrate in nature. They are degraded by enzymes of gastrointestinal bacteria to give by fermentation hydrogen, methane, carbon dioxide and short chain fatty acids (8). The chemical structure of fiber offers no clues to physiological effects but can be grouped into soluble (viscous, easily fermentable) and insoluble (non-viscous, slowly fermentable) fibers. These general types of fiber exhibit different physiological effects; the former may be useful

Kritchevsky in the treatment of diabetes and hyperlipidemia and the latter in the treatment of gastrointestinal problems such as constipation, diverticulitis and may even be protective against bowel cancer. The fiber era in the United States began with a paper by Burkitt, Walker and Painter (9) which summarized the view that a high fiber diet prevented or protected against the disease conditions most prevalent in this country.

There are many data relating to the beneficial effects of fiber but we lack an intellectually satisfying structure/function relationship and while dietary fiber is ingested in the form and structure native to individual plant foods much of our data are derived from experiments using a purified form of a specific fiber whose only relationship to its native state lies in similarity of structure.

For more than two decades evidence has been accumulating regarding the hypoglycemic action of soluble fiber. Jenkins et al. (10) reported that the addition of soluble fiber to a test meal lowered blood glucose and insulin response. Kiehm et al. (11) make a similar observation—soluble fibers such as guar or pectin were effective (12) but insoluble fibers such as cellulose or wheat bran were not (13). The soluble fibers increased viscosity of the food bolus and it was thought that this reduced the rate of gastric emptying (14). Wolever (15) has reviewed the role of dietary fiber in the management of diabetes: fasting serum glucose is reduced, on average, by 16% and plasma cholesterol and triglycerides by 18% and 10% respectively.

Dietary soluble fiber also tends to reduce serum or plasma lipid levels. In what may be regarded as the ''Medieval Age'' of fiber, Ershoff and Wells (16, 17) found that feeding several fibers could lower slightly the serum cholesterol and lower markedly the liver cholesterol compared to levels in rats fed 1% cholesterol in a fiber-free diet. Pectin, guar, or locust bean gum, and carrageenan were effective in this regard but cellulose and alginic acid were not. It has been demonstrated that certain types of fiber can reduce the severity of atherosclerosis in rabbits fed a semi-purified, cholesterol-free diet (18-20). Pectin will reduce the severity of atherosclerosis in cholesterol-fed chickens (21). "S

The possible influence of dietary fiber on heart disease in man cannot be studied directly but there have been numerous studies of fiber effects on hyper-

cholesterolemia, which is one of the major risk factors for development of cardio- t?

vascular disease. A review by Truswell and Beynen (22) tabulated data on a number of different fibers that are effective in this regard. In most of the studies ^

the fiber was used in a pharmacological sense as a lipid lowering agent. An earlier review (23) found that wheat bran was almost totally ineffective in lowering cholesterol levels. Cellulose is also without effect (24). Pectin, guar gum, and oat fiber are generally effective cholesterol lowering agents and Truswell and Beynen (22) cite single studies which showed that other soluble fibers namely gum arabic, xanthan gum, gum acacia, karaya gum, and locust bean gum were hypocholesterolemic. Psyllium has also been shown to lower cholesterol (25-

Health Benefits

27). In general the active fibers will lower total and LDL-cholesterol but do not influence HDL-cholesterol (28). Epidemiological data have recently become available relating to the effects of high fiber diets on coronary disease mortality and these have been reviewed by Humble (29). Khaw et al. (30) studied a group of 356 men and 503 women aged 50-79 years over a 12 year period: they observed reduced mortality from coronary disease with increasing fiber intake. Humble et al. (31) studied 1801 men aged 45-59 years over a 9 year period and Rimm et al. (32) reported data from 43,757 men aged 40-75 years obtained over a 6 year period. In both cases a higher fiber diet led to significant reduction in death from coronary disease. Dietary fiber intake was judged to be more significant indicator of risk than fat intake.

Heaton (33) has pointed out that dietary fiber exerts its effects throughout the length of the gastrointestinal tract. In the mouth it stimulates salivary flow and in the stomach it dilutes the contents and prolongs storage. In the small intestine fiber is a diluent of the contents and delays absorption and in the large intestine it acts as a diluent, bacterial substrate and traps water. Finally, dietary fiber softens and enlarges the stool. To expand on some of the above, constipation is relieved by the addition of fiber to the diet, the most effective fiber being wheat bran (34). Fiber may also restore normal bowel function in individuals who have become laxative-dependent (35). Paradoxically, fiber may also reduce diarrhea (36). Patients with irritable bowel syndrome also respond to a high fiber diet (37) as do subjects with diverticular disease (38). Heaton (39) has also reviewed the role of dietary fiber in the prevention and treatment of gastrointestinal disorders. In addition to the conditions discussed above, he marshals data which suggest that dietary fiber can reduce risk of gallstones and possibly appendicitis.

The findings regarding the role of fiber in treatment, or possibly prevention, of obesity are moot (40). Probably the greatest emphasis on a particular health aspect of dietary fiber has been on its possible protective role against colon or colorectal cancer. The data are not unanimous but there is enough encouragement to stimulate interest in continuing trials. A thorough review, which covered the "S

literature through 1986 (41), reported that among 22 ecological (population) stud- g ies there were 15 negative correlations, one positive correlation, and 6 in which no effect was seen. Among 22 case control studies, there were 8 negative correlations, 6 six positive correlations, and 8 in which there was no effect. Subsequently ^ published reviews show a similar scatter of results (42-44). Meta-analysis of 13

studies leads to a conclusion that fiber is protective (45). Two Australian studies |

yielded opposite results. Potter and McMichael (46) in Adelaide found increasing risk with increasing fiber intake whereas Kune et al. (47) in Melbourne found risk to be reduced dramatically with increasing fiber intake. This dichotomy illustrates the importance of other factors, physiologic and dietary.

Jensen et al. (48) compared diet and incidence of large bowel cancer in Finland and Denmark. Total fiber intake, in men aged 50-59 years, was higher

Kritchevsky in the Finnish population. After analyzing 39 variables only two, alcohol intake and concentration of fecal bile acids, were correlated positively with risk. There were significant negative correlations with saturated fatty acids, carbohydrate, starch, protein, cereals and total dietary fiber. Hill et al. (49) found the average fecal bile acid concentration in three countries with low incidence of colon cancer (Uganda, Japan, India) to be 0.61 ±0.13 mg/g feces, whereas for England, Scotland and the United States the average concentration was 6.15±0.66 mg/g feces. Finns and Americans excrete about the same amount of bile acid daily (275 mg) but the concentration is 4.6 mg/g feces in the Finns and 12.3 mg/g feces in Americans (50).

Walker et al. (51) compared fiber intake with colon cancer risk in five groups of South Africans. These were rural and urban blacks (very low risk), Indians (low risk) and white (very high risk). Fiber intake was similar in the five groups, however one striking difference was in fecal pH which ranged from 6.12 in rural blacks to 6.88 in whites. This finding suggests a difference in colonic acidity which could be due, in part, to the short chain fatty acids produced by fermentation of dietary fiber by the colonic flora. Since the total fiber intake of the various groups was similar the differences may be due to different types of dietary fiber or different strains of resident microorganisms. The principal short chain fatty acids formed by fermentation in the colon are acetic, propionic, and butyric. Butyric acid has been shown to suppress cellular proliferation (52) and to alter properties of human colorectal cells grown in vitro (53).

Interaction between dietary fiber and bile acids may underlie its modes of action in reducing hyperlipidemic and cancer risk. Portman and Murphy (54) showed that substitution of a semi-purified diet for commercial ration increased bile acid turnover in rats. It has been demonstrated amply that bile acids and bile salts are bound to the fiber (52-58) or to fiber containing foods (59, 60). The degree of binding is a function of both the fiber and the bile acid or salt. Alberts et al. (61) have shown that a diet high in wheat bran and calcium carbonate leads to significant reduction in bile acid concentrations and excretion rates in subjects "S

with resected colon adenomas. Two studies relating to the mechanisms by which psyllium or oat bran lower cholesterol have also shown effects on bile acid com- j position, pool size, and turnover (62,63).

The fiber hypothesis rests on data derived from observations of populations ^

eating a diet high in fiber. Most chemical and physiological studies, however, are conducted using single substances. Fiber in the diet can be obtained from |

four sources (64), namely:

1. Whole food high in fiber

2. A high fiber fraction (such as wheat bran) which can be produced without affecting the structure or composition of the material as present in food

Health Benefits

3. Concentrated fiber such as cellulose or pectin which has been altered in the course of extraction and purification

4. Fiber-enriched foods

There is no evidence to show that purified pectin, for instance, is biologically equivalent to a pectin-rich food.

Fiber-rich foods may also be sources of carotenoids, antioxidant vitamins (C and E), selenium, dithiolthiones, indoles and glucosinolates, flavonoids, cou-marins, isothiocyanates and thiocyanates, saponins, phenols (such as ferulic or ellagic acid), sterols, inositol hexaphosphate, and allium compounds, all of which have demonstrated biological activity. Most of these compounds have inhibited tumor growth to some extent and some also have hypolipidemic properties. Proper understanding of the mode of action of fiber will involve elucidation of the individual roles played by all the plant components and their interactions.

Recommendations for daily intake of fiber are all in the range of 20-40 g or 13-25 g per 1000 kilocalories. This range represents a level of intake that poses no risk.

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