Dietary patterns and chronic disease

Thompson Valley Farms
12029 92nd Street
Edmonton, AB T5G 1A8
Canada
E-mail: lance{at}thompsonvalley.com

Dear Sir:

The September 2003 issue of the Journal featured a series of articles on vegetarian diets, health, and prevention. This issue as well as the September 1999 and January 2000 issues discuss vegetarian diets, essential fatty acids, and optimal ratios of n-6 to n-3 essential fatty acids in the diet.

Simopoulos (1) wrote, "Studies indicate that a high intake of n-6 fatty acids shifts the physiologic state to one that is prothrombotic and proaggregatory, characterized by increases in blood viscosity, vasospasm, and vasoconstriction and decreases in bleeding time. n-3 Fatty acids, however, have antiinflammatory, antithrombotic, antiarrhythmic, hypolipidemic, and vasodilatory properties."

A few recommended optimal ratios of n-6 to n-3 fatty acids are given in the article by Davis and Kris-Etherton (2). They wrote, "The World Health Organization/Food and Agriculture Organization suggests a ratio of 5:1-10:1, Sweden recommends a ratio of 5:1, Canada recommends 4:1-10:1, and Japan recently changed its recommendation from 4:1 to 2:1. Another research group suggested that the optimal ratio to maximize the conversion of -linolenic acid to docosahexaenoic acid is 2.3:1."

I cannot help but wonder about the relevance of the dietary ratio of n-6 to n-3 fatty acids in other recently published studies. Key et al (3) wrote, "Further categorization of diets showed that, in comparison with regular meat eaters, mortality from ischemic heart disease was 20% lower in occasional meat eaters, 34% lower in people who ate fish but not meat, 34% lower in lactoovovegetarians, and 26% lower in vegans." In that study, vegans were grouped together seemingly regardless of the ratio of n-6 to n-3 fatty acids in their diets. How would the results of Key et al have changed if the vegans had been categorized on the basis of whether they had a high or moderate ratio? Furthermore, how would the results have changed if the vegans with a moderate dietary ratio had been compared with the meat-reduced and lactoovovegetarian groups?

In another study by Key et al, published in the September 2003 issue of the Journal (4), the subjects were classified as either vegetarian or otherwise; the vegans were dropped as a group entirely, and I found no mention of n-6:n-3 ratios. In my opinion, findings that exclude any given factor are confounded by that factor’s actual relevance. Whatever the relevance of the factor to the hypothesis, the results will be "off" by that factor’s relative effectiveness. In this study (4), whatever the relevance of the dietary ratio of n-6 to n-3 fatty acids is to mortality in British vegetarians, the findings of Key et al are probably off by that amount. In future reports of dietary patterns and their relation to chronic disease, the ratios of dietary n-6 to n-3 fatty acids should be taken into account.

REFERENCES

1. Simopoulos AP. Essential fatty acids in health and chronic disease. Am J Clin Nutr 1999;70(suppl):560S-9S.
2. Davis BC, Kris-Etherton PM. Achieving optimal essential fatty acid status in vegetarians: current knowledge and practical implications. Am J Clin Nutr 2003;78(suppl):640S-6S.
3. Key TJ, Fraser GE, Thorogood M, et al. Mortality in vegetarians and nonvegetarians: detailed findings from a collaborative analysis of 5 prospective studies. Am J Clin Nutr 1999;70(suppl):516S-24S.
4. Key TJ, Appleby PN, Davey GK, et al. Mortality in British vegetarians: review and preliminary results from EPIC-Oxford. Am J Clin Nutr 2003;78(suppl):533S-8S.