Vegetarian diet for health problems
In this fact sheet: Coronary heart disease, Stroke, Cholesterol, Hypertension, Cancer, Diabetes, Osteoperosis
We all know that fruit and vegetables are good for health and that vegetarians typically consume more than average but what effect can a vegetarian or vegan diet have in helping us to avoid or reduce the impact of health problems and serious medical conditions? What is the scientific evidence for the health benefits of a vegetarian diet?
Diet influences most aspects of health and dietary factors clearly contribute to the major degenerative diseases such as heart disease, stroke and diabetes. Obesity and high cholesterol are major contributory factors linked to these diseases and also to meat consumption and low intakes of fruit and vegetables. A high BMI (body mass index) is the result of a number of factors including food choices. Compared with omnivorous diets a varied vegetarian diet contains less saturated fatty acids, cholesterol and more folate, fibre, antioxidants, phytochemicals and carotenoids all of which are associated with specific health benefits.
Vegetarians have lower BMIs on average than the general population and scientists now have a detailed understanding of why dietary factors are important in maintaining a healthy heart, vascular system, bowel and other areas. Antioxidants and other beneficial phytochemicals have been demonstrated to enhance cell function and be protective against cancer. Fibre contributes to bowel health and to lower cholesterol levels whilst an absence of saturated fat from meat benefits the heart.
Current evidence for the effectiveness of vegetarian diets in preventing certain health conditions, including coronary heart disease (CHD), cancer, hypertension and high cholesterol has been provided by studies in UK, USA and Germany. The most comprehensive and well known studies focusing on the health of vegetarians have been the large cohort studies: Oxford Vegetarian Study, the Oxford EPIC study of cancer and nutrition and the long term American study of vegetarian Seventh Day Adventists.
There is now evidence that vegetarians have lower rates of mortality than the general population. Evidence from cohort studies suggests that vegetarians have lower overall standardised all cause mortality ratios than the general population (BNF 2005).
One challenge medical researchers find is the variation in vegetarian diets. Dietary differences between vegetarians and meat-eaters are characterised not only by meat and fish being excluded from the diet, but by the foods which are eaten in greater amounts by vegetarians. There is no single dietary pattern that characterises vegetarianism; several dietary patterns have been identified. A range of lifestyle differences have also been identified. It has been suggested that these lifestyle factors may account for some of the differences in health outcomes that have been reported between vegetarians and meat-eaters. Though conclusions are positive in identifying balanced vegetarian diets as healthy.
Vegan and vegetarian diets can be nutritionally adequate, provided they are carefully planned; both the British Dietetic Association and the American Dietetic Association provide guidelines for a healthy vegetarian diet. For vegetarians and meat eaters alike, the key to a nutritionally adequate diet is balance and ensuring that, where foods are specifically omitted, suitable alternatives are included so that dietary quality is not compromised. (BNF 2005)
Coronary heart disease (CHD)
Vegetarian diets have been associated with a reduction in several of the established risk factors for Coronary Heart Disease (CHD) these include more favourable lipid profiles, lower BMI and lower systolic and diastolic blood pressures. [British Nutrition Foundation 2005].
Phillips et al in 2004 found that changing to a vegetarian diet may lead to a change in body composition. This study found that vegetarians typically have a BMI approximately 1-2 kg/m2 less than non-vegetarians.
Pooled analysis of all 5 vegetarian cohort studies (Key et al 1998) showed that compared with regular meat-eaters vegetarians had a 34% reduced risk of death from CHD. This was in-line with the rate for individuals who consumed fish but no meat.
Specific analyses of mortality rates from CHD have shown that there is at least a moderate reduction in mortality from CHD among vegetarians compared with meat-eaters, in general. However, meat-eaters who follow a healthy lifestyle also have favourable mortality rates compared with the general population.
In addition to the lower risk of factors for CHD above, vegetarians have been proven to have reduced risk of haemostatic factors such as raised plasma levels of fibrinogen and platelet adhesiveness.
However, studies suggest that some vegetarians and vegans may be at greater risk of raised plasma homocysteine levels, an emerging risk factor for CVD, perhaps in association with a low vitamin B12 intake.
Robinson et al 2002 found in the Oxford Vegetarian Study that on average total cholesterol was 0.43mmol/L lower in vegetarians than in meat eaters and that this is explained by lower low density lipoprotein (LDL) and is related to the lower BMI of the vegetarians in the study. Sacks et al 1975 however found that even when vegetarians were heavier than non-vegetarians they had lower cholesterol levels than non-vegetarians. More recent studies have suggested that a diet high plant protein is more effective than a diet high in animal protein in reducing cholesterol levels - leading to the promotion of the idea of the vegetarian 'Atkins diet'.
Hypertension (high blood pressure)
Raised blood pressure is another major contributory factor for heart disease and stroke. A 5 mmHg increase in diastolic blood pressure increases stroke risk by 34% and heart disease by 21%. (MacMahon et al 1990). Several studies have reported lower systolic and diastolic blood pressure, of the order of 5–10 mmHg, in vegetarians compared with non-vegetarians (Ophir et al. 1983; Melby et al. 1985). The EPIC-Oxford study reported lower systolic (4.2 mmHg and 2.6 mmHg lower in men and women, respectively) and diastolic blood pressures (2.8 mmHg and 1.7 mmHg lower in men and women, respectively) and a lower prevalence of hypertension among vegans compared to meat-eaters, but this was largely attributable to differences in BMI between the groups (Appleby et al. 2002b).
Trials have shown that changing to a prescribed vegetarian diet was associated with small, but clinically useful, reductions in blood pressure (Margetts et al. 1987; Sciarrone et al. 1993; Beilin 1994) and a recent international study of macronutrients and blood pressure (INTERMAP Stamler et al. 2003) reported that lower intake of vegetable protein was one of the dietary factors associated with adverse blood pressure levels, although this study did not differentiate between vegetarians and meat eaters.
Second to smoking, diet is probably the most important modifiable risk factor for cancer. A high intake of plant derived foods has been linked with a reduced risk of certain cancers, although the mechanisms are not clear (see BNF 2003 for a detailed review). Vegetarians and vegans would be expected to consume more plant-derived foods than meat-eaters and it has been suggested by some, but not all studies, that cancer mortality patterns differ between vegetarians and meat-eaters. In a review of the epidemiological evidence of the protective effect of fruit and vegetables on cancer risk, Riboli and Norat (2003) reported that case–control studies show a significant reduction in the risk of cancers of the oesophagus, lung, stomach and colorectum with increased consumption of fruit and vegetables.
Breast cancer risk was lower with increased consumption of vegetables, but not fruit. Prospective studies, which are generally regarded as more robust, have shown weaker evidence than case–control studies (see Riboli & Lambert 2002, for a review of nutrition and lifestyle factors involved in cancer prevention). As a consequence of higher intakes of plant-derived foods, vegetarians are also likely to have higher intakes of phytochemicals, which may have protective effects and work via a range of mechanisms (see BNF 2003). In general, studies examining differences between vegetarians and non-vegetarians, in breast cancer incidence and mortality, are inconsistent in their conclusions, indicating that any dietary association is likely to be weak. For example, pooled data from five prospective studies of vegetarians (Key et al. 1998) showed that breast cancer mortality was significantly lower among vegetarians in the Adventist Health Study, but overall the pooled data showed no significant difference for breast cancer.
The colonic environment of vegetarians and vegans is different to that of meat-eaters. Vegans have considerably lower levels of potentially carcinogenic secondary bile acids compared to vegetarians, who, in turn, have lower levels than meat-eaters (van Faassen et al. 1993). Vegetarians also have fewer intestinal bacteria able to convert the primary bile acids into secondary bile acids Vegetarian nutrition 157 © 2005 British Nutrition Foundation Nutrition Bulletin, 30, 132–167 (Finegold et al. 1977). Secondary bile acids have been positively associated with dietary intakes of saturates and negatively associated with intake of fibre and starch (Reddy et al. 1998), intakes of which differ between vegetarians, vegans and meat-eaters. Faecal variables associated with colon cancer risk have been examined, before and after changing to a Scandinavian lactoovo- vegetarian diet (Johansson 1990)\. Twenty subjects participated for 12 months and, after 3 months, significant reductions were observed in the faecal content of deoxycholic acid and bacterial enzymes, and a significant increase was observed in faecal weight. The increase in faecal weight was explained by a higher water content, which diluted the faecal bile acids and enzymes and appeared to result from a significantly higher fibre intake. The EPIC study has also shown that dietary fibre is inversely related to large bowel cancer (Bingham et al. 2003). The adjusted relative risk was 0.75 [95% CI 0.59, 0.95] for the highest vs. the lowest quintiles of intakes and the authors suggest that in populations who consume a low fibre diet (approximately 12 g/day or less), doubling fibre intake could reduce the risk of olorectal cancer by as much as 40%. The Adventists Health Study reported that, after controlling for age, sex and smoking, non-vegetarians had an 88% increased risk for colorectal cancer (Fraser 1999). Allen et al. (2000) reported lower levels of serum insulin-like growth factor-1, which is thought to be involved in the aetiology of several cancers, including colorectal cancer, in vegans, compared with non-vegetarians and lactoovo- vegetarians.
In terms of meat consumption, there is some suggestion that a high consumption of processed meats (e.g. bacon, salami, sausage) increases the risk of colorectal cancer. No consistent associations have been made with red meat per se (Hill 1999). High temperature cooking (e.g. barbecuing, grilling and frying) has also been associated with raised cancer risk (Knize et al. 1999) because these cooking methods are thought to produce potentially carcinogenic substances (such as heterocyclic amines). In spite of all of these potentially protective dietary factors, major studies of vegetarians have failed to show a consistent effect on colorectal cancer. Key et al. (1998) reported that mortality from colorectal cancer was almost identical in vegetarians and nonvegetarians in the pooled analysis of five prospective studies (death rate ratio = 0.99 [95% CI 0.77, 1.27]) regardless of the length of time for which people had been vegetarian. Results from EPIC are likely to provide some of the answers.
In summary, although some studies have reported lower rates of cancers in vegetarians compared with the general population, these differences are not so apparent when vegetarians are compared with similar non-vegetarians. Nevertheless, a diet based on a high intake of plant-derived foods, whether meat is included or not, seems to be associated with reduced risk of several types of cancer, although more research is necessary to understand the mechanisms involved.
Snowdon (1985) found type II diabetes to be only half as common as a cause of death amongst the largely vegetarian Seventh Day Adventist population as in the general population.
An average vegetarian diet closely matches the British Diabetic Association's recommendations for diabetic patients. Vegetarian diets tend to be high in complex carbohydrates and dietary fibre, which has a beneficial effect on carbohydrate metabolism, lowering blood sugar levels. The leanness of vegetarians also contributes to reduced incidence of diabetes. Diabetes is often associated with raised blood cholesterol levels and a vegetarian diet confers protection against this.
Vegetarianism has been associated with some factors that result in lower bone density and, consequently, osteoporosis, but studies examining vegetarianism and bone density have found conflicting results. Overall,there is little evidence to suggest that bone mineral density differs markedly between Western vegetarians and meat-eaters.
Osteoporosis is a complex disease that is characterised by low bone mass and deterioration of bone tissue, leading to increased bone fragility and greater risk of fracture. Studies examining the association between vegetarianism and bone density have found conflicting results. Several studies conducted prior to 1990 (Marsh et al. 1980, 1988; Tylavsky & Anderson 1988; Hunt et al.1989) found bone mineral density to be higher among vegetarians than meat-eaters, but confounding lifestyle factors were apparent for many of these. For example, differences in caffeine and alcohol consumption, smoking habits and activity levels were found, all of which independently affect bone mineral density. Subsequent studies have shown no difference in bone mineral density between meat-eaters and vegetarians (Lloyd et al. 1991; Tesar et al. 1992; Lau et al. 1998).