Chemical Basis for a Vegan Diet --Fibre

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Chemical Basis for a Vegan Diet --Fibre

 

The importance of fibre in the diet is being recognized more and more. The

concept that fibre could play a preventive role in colon cancer was first

proposed in 1971 when Burkitt put forth the proposal that fibre could prevent

colon cancer by regulating the speed and bulk of the food that passes through

the intestines. Since that time many studies have shown this hypothesis to be

true.[ii]

Whole grain consumption in particular was found to be inversely correlated to

the prevalence of colon cancer.[iii] Vegetable fibre was also shown to offer

protection, and the NCI (National Cancer Institute of the US) has therefore

recommended that a variety of foods such as whole grains, vegetables and fruits

be eaten rather than fibre supplements, and that dietary fibre levels should at

least be doubled over current consumption levels of 10-15g fibre/day.[iv]

Fibre consumption is so low in industrialized countries, because of the

excessive consumption of refined foods.

Modern grain mills, for example, separate the natural fibres and wheatgerm from

the wheat, thus stripping the wheat of its natural fibre.

The bran of wheat is rich in fibre which provides bulk and aids intestinal

motility. Furthermore, these outer layers of wheat include the aleurone layers

which contain the B-complex vitamins, phosphorus, iron, and proteins in balanced

proportions.

The wheatgerm is rich in thiamin (vitamin B1) which is essential for

carbohydrate metabolism and the natural antioxidant vitamin E. Vitamin E comes

prepacked with the polyunsaturated oils in the wheat germ and thus offers a

natural protection against the formation of free radicals when these essential

oils are ingested.

Whole grains or unrefined grain products such as stone-ground flour, are thus

vastly superior to refined products in terms of their overall composition.

The addition of unground wheat berries or broken pieces of grain to the flour is

also a poor way of adding fibre to grain products.

These hard pieces pass through the intestines largely undigested, add little

bulk and can even damage the delicate epithelia on their way.

They do not aid in water retention in the stools and do not provide a large

enough surface area for the elimination of wastes.

Flour used for bread-making should be thoroughly ground and if whole grains are

used for the preparation of porridges, then they must be well soaked or/and

thoroughly cooked, as raw grains contain enzyme suppressants which interfere

with the digestive process.

Cooking or sprouting destroys these suppressants and thus allows for maximal

utilization of the nutrients in the grains.

As fibre content in the diet increases, bowel transit time decreases and faeces

weight and the number of defecations increase.[v]

Furthermore, the consistency of the stool is far softer on a natural high-fibre

diet than on a low-fibre diet thus eliminating constipation and its secondary

effects such as colitis, appendicitis, diverticulosis, and hiatus hernia.

Oats and bean products contain large quantities of water-soluble fibre and are

particularly efficient in reducing blood cholesterol levels, particularly

LDL-cholesterol, which is the variety which tends to clog blood vessels.[vi]

Added bran is not nearly as efficient in eliminating cholesterol as is the fibre

in whole organic cereals,[vii] thus emphasizing the importance of the

consumption of whole, unrefined foods as the means of controlling cholesterol

levels.

By binding cholesterol and bile acids, fibre not only reduces cholesterol levels

but also protects against colon cancer, as secondary bile acids, which are

formed by bacterial conversion of bile acids, are carcinogenic.[viii]

Wheat-bran was also found to lower the concentrations of oestrongens in women,

particularly serum oestrogen and oestradiol.[ix]

The presence of free and albumen-bound oestradiol has been associated with an

increased risk of breast cancer,[x] and the bran can thus offer protection

against this form of cancer.

Precisely how fibre affords protection against this form of cancer is uncertain.

When dealing with fibre, it is important to note that there are two categories

of fibre, and these two differ in their function.

Water-Insoluble fibres

Water-insoluble fibres include such fibres as cellulose, lignin and certain

hemicelluloses.

These fibres have a considerable effect on stool size and the time that the

ingested food stays in the digestive tract, but they have little or no effect on

intermediary metabolism,[xi] or the growth of the bacterial population in the

colon.[xii]

The bulking effect of water-insoluble fibre on the intestinal contents, however,

dilutes the concentration of substances that can cause cancer and this type of

fibre also ensures rapid elimination of harmful cancer promoters in the colon.

Refined food leads to constipation, but on a high natural fibre diet stool will

not only be softer, but will be passed more frequently. Two to three stools a

day are consistent with healthy bowel movements, whereas one stool or even fewer

per day may be categorized as constipation. This might seem excessive to those

who pass stools only once a day or even less frequently.

If we, however, consider that most people consume two to three meals per day,

then it is only logical to eliminate the waste material more than once a day.

Waste products that remain in the colon for lengthy periods of time will be

acted upon by the colonic bacteria and converted to potential carcinogens.

Furthermore, the compaction and pressure required to eliminate fibre-poor stools

can produce diverticulosis of the intestinal tract (small pouches), and hardened

portions of faeces can become trapped, thus eventually causing inflammation as

in the case of appendicitis.

Moreover, the increased abdominal pressure required to eliminate hard stools can

also produce hernias and will force more blood from the large abdominal vessels

into the femoral vessels thus causing varicose veins.

Considering the high fibre content of whole-plant foods, it is thus not

surprising that diverticulosis is less common in vegetarians than in

omnivores.[xiii]

As there is a linear relationship between stool weight and fibre consumption (5

g increase in stool weight for every 1 g fibre consumed), both the WHO and the

UK department of health have recommended an average daily intake of 18 g

fibre.[xiv]

Water-soluble fibres and resistant starch

In contrast to the insoluble fibres, water-soluble fibres have little effect on

stool size, transit time and mineral absorption, but they do have an important

effect on secondary metabolism.

As noted previously, resistant starch is starch that resists digestion in the

small intestine because of the different stereochemistry of these starches.

All starchy foods such as grains, legumes and starchy vegetables such as

potatoes contain resistant starch. Even fruits, such as bananas that are still

slightly green, contain resistant starch. When soluble fibre and resistant

starch enter the colon, they are fermented anaerobically to produce short chain

fatty acids (SCFA), acetate, propionate, and butyrate as well as gas. Moreover,

bacterial growth is enhanced which, together with water binding to residual

unfermented fibre, leads to increased stool weight, dilution of colonic contents

and faster transit time through the large gut.[xv] This in turn reduces the time

that potentially harmful substances can come into contact with mucosal cells.

Consumption of whole foods rich in complex carbohydrates thus offer protection

against cancer in more ways than one. Diets rich in grains, legumes, fruits and

vegetables are thus optimal for maintaining a healthy digestive system.

The water-soluble fibres include such fibres as pectin, hemicelluloses, and

storage polysaccharides that are found mainly in fruits and vegetables, and gums

which are found mainly in organic cereals.

As the bacteria in the colon break down a large proportion of these

water-soluble fibres, they do not contribute to faecal bulk as do no-soluble

fibres.

Besides contributing fermentation products, for use by the mucosal cell of the

colon, soluble fibres are known to lower cholesterol level and to prevent to

postprandial (after meal) glucose surge and subsequent hypoglycaemia associated

with a refined food diet.

Once again, it is the fruits, vegetables, legumes and grains which contain

soluble fibres which can retard the rate of glucose absorption.

In the case of fruits, the soluble fibre which retards the rate of glucose

absorption is pectin, and it prevents the glucose surge and subsequent

hypoglycaemia associated with a refined food diet devoid of natural fibre.[xvi]

The same effect is achieved by soluble fibres found in oat bran.

An argument which is often leveled against a fibre-rich diet, is that it will

inhibit the uptake of minerals and other essential nutrients.

In a recent study it was, however, found that the higher fibre intake of

vegetarians did not affect mineral utilization adversely, and depending on the

variety in the diet, the uptake of magnesium, iron, copper and manganese could

actually be enhanced.[xvii]

A varied whole food diet rich in carbohydrates will therefore afford the highest

protection against degenerate diseases, as well as supplying all the body’s

energy, mineral and vitamin needs.

--

References:

Burkitt, D.P. 1971. The Epidemiology of cancer of the colon and rectum.

Cancer. 28: 3-13

[ii] Greenwald, P., Lanza, E., Eddy, G. 1978. Dietary fibre in the reduction of

colon cancer risk. J.Am.Diet.Assoc.87:1178-88

[iii] Reddy, B.S. 1982. Dietary fibre and colon carcinogenesis: A critical

review. In Vahoung, G.V.; Kritchesky, D., eds. Dietary fibre in health and

disease. New York: Plenum Press. 265-85

[iv] Butrum, R.R., Clifford, C.K., Lanza, E. 1988. NCI dietary guidelines:

Rationale. Am.J.Clin.Nutr.48:888-95

[v] Beyer, P.L., Flynn, M.A. 1978. Effects of high- and low-fibre diets on human

faeces. J.Am.Diet.Assoc.72;271-7.

Eastwood, M.A., Elton, R.A., Smith, J.H. 1986. Long-term effects of white meal

bread on stool weight, transit time, faecal bile acids, fats and neutral

sterols. Am.J.Clin.Nutr.43:343-9.

[vi] Anderson, J.W., Gustafson, N.J. 1988. Hypocholestrolemic effects of oat and

bean products. Am.J.Clin.Nutr.48:749-53.

Anderson, J.W., Chen, W.J.L. 1986. Plant fibre: diabetes and obesity.

Am.J.Gastroenterol. 81:898-906.

Gold, K.V., Davidson, D.M. 1988. Oat bran as a cholesterol reducing dietary

adjunct in a young healthy population. Western.J.Med.148:299-302.

Keenan, J.M., Wenz, J.B., Huang, Z., Myers, S.R. 1990. A randomized controlled

trial of oat bran cereal for hypercholesterolemia. Arteriosclerosis 10:873a.

Anderson, J.W., Spencer, D.B., Hamilton, C.C. et al. 1990. Oat bran cereal

lowers serum total and LDL cholesterol in hypercholesterolemic men.

Am.J.Clin.Nutr.52:495-499.

[vii] Burkitt, D.P. 1975 Refined Carbohydrate foods and disease. Academic Press

P 341.

[viii] Nair. P.P. 1988. Role of bile acids and neutral sterols in

carcinogenesis. Am.J.Clin.Nutr.48: 768-74

[ix] Rose, D.P., Golman, M., Conolly, J.M. and Strong, L.E. 1991. High-fibre

reduces serum estrogen concentrations in premenopausal women.

Am.J.Clin.Nutr.54(3):

[x] Jones, L.A., Ota, D.M., Jackson, G.A. et al. 1987. Bioavailability of

estradiol as a marker for breast cancer risk assessment. Cancer.Res.47:5224-9.

[xi] Munoz, J. 1984. Fibre and diabetes. Diabetes care. 7:297-8

Crapo, P.A. 1985. Simple versus complex carbohydrate use in the diabetic diet.

Ann.Rev.Nutr.5:95-114

[xii] Mendeloff, A.I. 1987. Dietary fibre and gastrointestinal disease.

Am.J.Clin.Nutr.45 (suppl):1267-70

[xiii] Dwyer, J.T. 1988. Health aspects of vegetarian diets.

Am.J.Clin.Nutr.48:712-28

[xiv] Bingham, S.A. 1996. Epidemiology and mechanisms relating diet to risk of

colorectal cancer. Nutrition Research Reviews. 9:197-239

World Health Organization. 1990. Diet, Nutrition and the Prevention of Chronic

Disease *Technical Report Series no. 797). Geneva: WHO.

[xv]Bingham, S.A. 1996. Epidemiology and mechanisms relating diet to risk of

colorectal cancer. Nutrition Research Reviews. 9:197-239

[xvi] Leeds, A.R., Ralphs, D.N.L., Ebied. F., Metz, G., Dilawari, J.B. 1981.

Pectin in the dumping syndrome and plasma volume changes. Lancet 1:1075-8

[xvii] Kelsay, J.L.; C.W. Frazier; E.S. Prather; J.J. Canary; W.M. Clark and

A.S. Powell. 1988. Impact of variation in carbohydrate intake on mineral

utilization by vegetarians. Am.J.Clin.Nutr.48:875-9.

The above has been excerpted from the book Diet and Health by Professor Walter

J. Veith available through our webstore.

Page updated 10/17/2003

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