High blood pressure (B.P.) or hypertension (HT) has been described as a silent "killer." Apart from its own morbidity, HT facilitates and accelerates another killer viz. atherosclerosis.Naturally there has been world-wide concern about prevention of these diseases by dietary alterations or drugs. Currently there is an increasing scientific interest in vegetarian diets in the prevention of several diseases. The questions to be tackled here are:

1. Do vegetarians have significantly lower levels of blood pressure than appropriately matched omnivorous controls? ("Non-vegetarian" is not an appropriate term because those who eat fish, chicken and meat also eat vegetables, fruits and cereals).
2. If so, can such differences be attributed to the vegetarian diet per se or to other confounding factors such as abstinence from alcohol and tobacco, tea and coffee, deep religious commitment or other life style factors ?
3. Is it possible to use vegetarian diet as a therapeutic strategy, to reduce blood pressure in hypertensive patients on a long term basis ?
4. If a significant reduction in the level of blood pressure is indeed brought by vegetarian diet, what is the physiological and biochemical basis?

EPIDEMIOLOGICAL STUDIES

A number of vegetarian populations have been reported with lower blood pressure, serum cholesterol and body weight than omnivores, but there is a strong possibility that an effect of diet may be confounded by other lifestyle factors. A study in Western Australia was particularly commendable since it attempted to isolate the influence of vegetarian diet from other potentially confounding life style factors associated with hypertension and cardiovascular disease.

Dietary Intervention Study

In one of the studies 59 healthy omnivores were allocated to a control group (which ate an omnivorous diet for 14 weeks) or to one of two intervention groups whose members ate an omnivorous diet for the first two weeks and a lacto-ovo-vegetarian diet for one of two six-week experimental periods. Home, clinic, and laboratory blood pressures, dietary intake, body weight and lifestyle factors were carefully month bred throughout the project. Urine and blood collections were made for each experimental period.

There were no appreciable differences between vegetarians and omnivores with respect to mean age and height. For weight and Quetelets' index, however, vegetarians of both sexes were lower than omnivores. Mean blood pressures adjusted for age, height and weight were significantly lower in vegetarians than omnivores, and were not related to past or present use of alcohol, tobacco, tea and coffee, physical activity, personality or religious observance.

The prevalence of mild hypertension ( ' 140 mmHg systolic and/or 90 mmHg diastolic) was 10% in omnivores and 1% in vegetarians.

Analysis of the diet records showed that the vegetarians ate significantly more dietary fibre, polyunsaturated fats, magnesium and potassium, and significantly less total fat, saturated fat and cholesterol than did the omnivores.

There was a significant fall in mean systolic and diastolic pressures in both experimental groups during the period on vegetarian diet. Mean blood pressure in experimental group I rose after resumption of the omnivore diet (period 2) to the level which preceded the vegetarian diet. Considering both experimental groups together, the mean fall in blood pressure associated with a vegetarian diet was 6.8 mm Hg systolic (SD 8.8) and 2.7 mm Hg diastolic (SD 6.3). Multiple regression analysis showed that the change in B.P. was associated with eating a vegetarian diet independent of age, sex, Quetelet's index, blood pressure before dietary modification or change in body weight.

Analysis of diet records indicated that intake of several nutrients changed with change to the vegetarian diet, particularly significant increases in polyunsaturated fat ( + 96%) dietary fibre (+75%), vitamin C (+80%), vitamin E (+ 85%) magnesium ( + 34%), calcium ( + 36%) and potassium ( + 18%) significant decreases were in protein (-27%), saturated fat (-16%), monounsaturated fat (-19%) and vitamin B12 (- 61%). The P:S ratio of the diet changed from 0.29 to 0.68, intake of sodium, calories and total fat did not change with change to the vegetarian diet. Factor analysis suggested that changes in polyunsaturated fat, fibre or protein were most likely to have mediated the observed changes in blood pressure.

It would now be worthwhile to examine the following nutrients in relation to blood pressure—sodium, potassium, calcium and magnesium, dietary fibre, polyunsaturated fats and low proteins

Sodium

The recognition that modifying renal excretory capacity for sodium and thus changing sodium balance can induce high B.P. in experimental animals focussed attention on sodium as the principle nutritional factor in the development of high B.P. Epidemiological surveys are often cited as proof that excessive sodium intake increases the prevalence of high B.P. However, sodium does not meet a number of criteria listed above. Intrasocietal studies have not shown a difference in the sodium chloride consumption between normal and hypertensive subjects. Animal studies have not used levels of intake that reflect reasonable variations in the human diet. Adverse effects of excess sodium have not been shown in normal persons. We still do not understand how dietary sodium exerts its pressor effects, when it does so and why it does so only in certain people. Whether the action of sodium is a direct effect or an indirect effect through changing the activity of other ions such as potassium, magnesium and calcium is also not clear. The available scientific evidence does not allow a blanket recommendation of restriction of salt intake to 5 gm/day for the entire population.

Potassium

Clinical, experimental and epidemiological evidence suggests that a high dietary intake of potassium is associated with lower B.P. It is often overlooked that the Kempner rice fruit diet is not only a low sodium diet but.also a high Potassium diet. The vegetarian diet is significantly higher in potassium content. Low salt consuming populations also have high potassium intake. In Japanese villages, populations with similar sodium intake but different blood pressure levels have different potassium intake. It may therefore be a good idea to express the Na / K ratio in the diet as a major controlling factor in hypertension. Students of biology have long observed the reciprocity of function of Na and K on the tissues of animals in vitro. This reciprocity may also play an important role in the development and maintenance of high B.P. Increased consumption of fruits and vegetables as a rich source of potassium can be recommended as a public health measure in the prevention of high B.P. It is interesting to note that the protective effect of potassium in strokes may be mediated by mechanisms other than lowering B.P. A 10 mmol increase in the dietary potassium is associated with 40% reduction in risk.

Calcium

It has been observed in epidemiological studies that both potassium and calcium intake are significantly reduced in both white and non-white hypertensive subjects (27% less Ca and 17% less K in whites; 42% less Ca and 34% less K in nonwhites). It is postulated that an inadequate calcium intake may contribute to elevate B.P. Milk, peas, beans and cereal grains are a good source of calcium in the vegetarian diet. Drinking water can provide significant amount ranging from 75 mg to over 200 mg per day in water obtained from wells sunk in chalk or limestone.

Magnesium

Vegetarian diet is rich in magnesium, which could affect plasma and intracellular magnesium and hence influence cardiac or vascular smooth muscle contraction.

Dietary Fibre

The vegetarian diet is rich in fibre, which is not digested by the human digestive enzymes. Some types of dietary fibres, notably hemicellulose of wheat, increase the water-holding capacity of colonic contents and the bulk of the stools, thus relieving constipation. Other viscous indigestible polysaccharides such as pectin and gum guar slow gastric emptying, contribute to satiety, retard the absorption of glucose and cholesterol and reduce plasma cholesterol. Some dietary intervention studies in humans have been compatible with effects of dietary fibre on blood pressure but they do not have an entirely satisfactory experimental design.

Polyunsaturated Fats

Humans are unable to svnthetize fatty acids with double bonds more distal to the carboxyl end of the fatty acid than the 9th carbon atom. Thus linoleic acid (C18: 2 ~- 6) is an essential fatty acid which must be provided in the diet. It is also called X 6 or omega 6 fatty acid. It is the principle polyunsaturated fatty acid in oil from plant seeds (e.g. corn oil, safflower oil). Another essential fatty acid alpha linolenic acid, is present in green leaves, and some plant oils, notably linseed, rapeseed and soyabean oil. Elongation and further desaturation of alpha linolenic acid (C 18:3co3) occurs in animals and (slowly) in humans to yield eicosapentaenoic acid (EA) and docosahexaenoic acid (DA). These fatty acids enter the food chain with marine phytoplankton, which are eaten by fish, which are in turn eaten by seals, walruses and whales—the principle component of Eskimo diet in Greenland. The low prevalence of atherosclerosis and myocardial infarction in Eskirnos had been attributed to their daily dietary consumption of 5-10 g of the long chain n-3 polyunsaturated fatty acids EA (C20: Sco3) and DA (C22: 6a3). When s 3 fatty acids are introduced in the diet, their derivatives EA and DA compete with arachidonic acid in several ways. The net result is a change in the homeostatic balance towards a more vasodilatory state, with less platelet aggregation. They reduce the viscosity of whole blood by increasing the deformability of red blood cells. They cause a moderate reduction of blood pressure both in normal and mild hypertensive subjects. Further they reduce the vasospastic response to catecholamines and possibly to angiotensin. The anti-atherogenic properties of Go3 fatty acids have also been described.

Vit. C and E

The higher content of these vitamins in vegetarian diet is a great advantage in protecting against high concentrations of co3 fatty acids, which can increase the likelihood of lipid peroxidation, with its toxic effects on the cell. Those who ingest large amounts of fish oil would need added supplements of vit C and E for their anti-oxidant effects.

It seems clear that vegetarians tend to have lower blood pressure than omnivores and that a shift in dietary pattern towards a lacto-ovo-vegetarian diet would result in reduced incidence of hypertension, strokes and cardiovascular disease in the community. The identification of specific nutrients responsible for this benefit is still not precise although a high potassium polyunsaturated fat and fbre content of the vegetarian diet seem to be significantly associated. The subject should be approached with an open mind with the realisation that newer knowledge will emerge with continued research.

REFERENCES

1. Sacks F.M., Rosner B., Kass E.H. Blood pressure in vegetarians. Amer. J. Epidemiol. 1974, 100, 873-77 (390-98).
2. Anholm A.C. The relationship of a vegetarian diet and blood pressure. Prev. Med. 1975, 4, 35.
3. Sacks F.M., Castelli W.P., Donner A., Kass E.H. Plasma lipids and lipoproteins in vegetarians and controls. N. Eng. J. Med. 1975, 292, 1148-51.
4. Haines A.P., Chakrabarti R., Fisher D. et al Haemostatic variables in vegetarians and nonvegetarians. Thromb. Res. 1980, 19, 139-48.
5. Rouse I.L. Beilin L.J. Armstrong BK, Vandongen R. Vegetarian diet, blood pressure and cardiovascular risk. Aust. NZ J. Med. 1984, 14, 439-443.
6. McCannon DA and Pucak G.J. (Ed.) Symposium on Nutrition and Blood pressure control—Current status of dietary factors and hypertension. Ann. Int. Med. 1983, 98, 701.
7. Dahl L.K. Salt and hypertension Amer. J. Clin. Nutr. 1972, 25, 231-44.
8. Khaw K.T., Barett-connon E. Dietary potassium and stroke-associated mortality—A 12 year prospective population study. NEJM 1987, 316, 235.
9. Altura B.M ., Altura, B.T. Magnesium ions and contraction of vascular stnooth muscle; relationship to vascular disease Fed. Proc. 1981, 40, 2672-79.
10. Wright A., Burstyn, P.G. Gibney M.J. Dietary fibre and blood pressure. BMJ 1979, 2, 1541-43.
11. Leaf A., Weber P.C. Cardiovascular effects of n-3 fatty acids. NEJM 1988, 318 page 549-557.