Breast Cancer Cases Among Vegetarians

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 so on. Antioxidants and other beneficial photochemicals have been demonstrated to enhance cell function and be protective against cancer. Fiber 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 standardized 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 characterized 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 characterizes 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.

Cancer

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 fiber and starch intakes of which differ between vegetarians, vegans and meat-eaters. Fecal 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 faucal 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 faucal 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 quin tiles 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 non vegetarians 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

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Tuesday, July 19, 2011

Cancer

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