Vegetarianism: What the Science Tells Us

Read this in: 简体中文繁體中文

Health writers may denigrate animal foods with insouciance but, in fact, the scientific literature offers very little in the way of long-term studies on the value of a vegetarian diet. Dr. Russell Smith, a statistician, analyzed the existing studies on vegetariansim¹ and discovered that while there have been ample investigations which show, quite unsurprisingly, that vegetarian diets significantly decrease blood cholesterol levels, studies evaluating the effects of vegetarian diets on mortalities continue to be few in number. In fact, Smith speculated that the available data from the many existing prospective studies are being shelved because they reveal no benefits of vegetarianism. For example, mortality statistics are strangely absent from the Tromso Heart Study in Norway which showed that vegetarians had slightly lower blood cholesterol levels than nonvegetarians.²

In a review of some 3,000 articles in the scientific literature, Smith found only two that compared mortality data for vegetarians and nonvegetarians. One was a 1978 study of Seventh Day Adventists (SDAs). Two very poor analyses of the data were published in 1984, one by H. A. Kahn and one by D. A. Snowden.³ The publication by Kahn rather arbitrarily threw out most of the data and considered only subjects who indicated very infrequent or very frequent consumption of the various foods. They then computed “odds ratios” which showed that mortality increased as meat or poultry consumption increased (but not for cheese, eggs, milk or fat attached to meat.)

When Smith analyzed total mortality rates from the study as a function of the frequencies of consuming cheese, meat, milk, eggs and fat attached to meat, he found that the total death rate decreased as the frequencies of consuming cheese, eggs, meat and milk increased. He called the Kahn publication “yet another example of negative results which are massaged and misinterpreted to support the politically correct assertions that vegetarians live longer lives.”

The analysis by Snowden published mortality data for coronary heart disease (CHD), rather than total mortality data, for the 21-year SDA study. Since he did not eliminate the intermediate frequencies of consumption data on meat, but did so with eggs, cheese and milk, this represents further evidence that both Kahn and Snowden based their results on arbitrary, after-the-fact analysis and not on pre-planned analyses contingent on the design of their questionnaire. Snowden computed relative risk ratios and concluded that CHD mortality increased as meat consumption increased. However, the rates of increase were trivial at 0.04 percent and 0.01 percent respectively for males and females. Snowden, like Kahn, also found no relationship between frequency of consumption of eggs, cheese and milk and CHD mortality “risk.”

Citing the SDA study, other writers have claimed that nonvegetarians have higher all-cause mortality rates than vegetarians⁴ and that, “There seems little doubt that SDA men at least experience less total heart disease than do others. . .”⁵ The overpowering motivation to show that a diet low in animal products protects against CHD (and other diseases) is no better exemplified than in the SDA study and its subsequent analysis. While Kahn and Snowden both used the term “substantial” to describe the effects of meat consumption on mortalities, it is more obvious that “trivial” is the appropriate descriptor. It is also interesting that throughout their analyses, they brushed aside their totally negative findings on foods which have much greater quantities of fat, saturated fat and cholesterol.

The second study was published by Burr and Sweetnam in 1982.⁶ It was shown that annual CHD death rate among vegetarians was only 0.01 percent lower than that of nonvegetarians, yet the authors indicated that the difference was “substantial.”

The table below presents the annual death rates for vegetarians and nonvegetarians which Smith derived from the raw data in the seven-year Burr and Sweetnam study. As can be seen, the “marked” difference between vegetarian and nonvegetarian men in Ischemic Heart Disease (IHD) was only .11 percent. The difference in all-cause death rate was in the opposite direction, a fact that Burr and Sweetnam failed to mention. Moreover, the IHD and all-cause death rates among females were actually slightly greater for heart disease and substantially greater for all causes in vegetarians than in nonvegetarians.

These results are absolutely not supportive of the proposition that vegetarianism protects against either heart disease or all-cause mortalities. In fact, they indicate that vegetarianism is more dangerous for women than for men.

The claim that vegetarians have lower rates of cancer compared to nonvegetarians has been squarely contradicted by a 1994 study comparing vegetarians with the general population.⁷ Researchers found that although vegetarian Seventh Day Adventists have the same or slightly lower cancer rates for some sites, for example 91 percent instead of 100 percent for breast cancer, the rates for numerous other cancers are much higher than the general US population standard, especially cancers of the reproductive tract. SDA females had more Hodgkins disease (131 percent), more brain cancer (118 percent), more malignant melanoma (171 percent), more uterine cancer (191 percent), more cervical cancer (180 percent) and more ovarian cancer (129 percent) on average.

References

1. Russell L Smith, Diet, Blood Cholesterol and Coronary Heart Disease: A Critical Review of the Literature, Vol 2, Vector Enterprises, November 1991. The author was a statistician who subjected the many studies on coronary heart disease to appropriate rigorous statistical analysis.
2. V Fonnebo, “The Tromso Heart Study: diet, religion and risk factor for coronary heart disease,” American Journal of Clinical Nutrition, 1988, 48:739
3. H A Kahn et al, “Association between reported diet and all-cause mortality,” American Journal of Epidemiology, 1984, 119:775; D A Snowden et al, “Meat consumption and fatal ischemic heart disease,” Preventive Medicine, 1984, 13:490
4. J T Dwyer, “Health aspects of vegetarian diets,” American Journal of Clinical Nutrition, 1988, 48:712
5. G E Fraser, “Determinants of ischemic heart disease in Seventh-Day Adventists: a review,” American Journal of Clinical Nutrition, 1988, 48:833
6. M L Burr and P M Sweetnam, “Vegetarianism, dietary fiber and mortality,” American Journal of Clinical Nutrition, 1982, 36:873
7. P F Mills, et al, “Cancer incidence among California Seventh-Day Adventists, 1976-1982,” American Journal of Clinical Nutrition, 1994, Vol 59 (Supplement), Pages 1136S-1142S.

Read this in: 简体中文繁體中文