When members of indigenous groups move into a more urban setting they develop high blood pressure. Maybe there is something about urban life besides diet that leads to hypertension. In the 1980s a study was done of two different groups of urban dwellers. This study showed that the critical difference for better blood pressure is the potassium to sodium ratio of the diet.
Urban Vegetarians With A Better Potassium Sodium Ratio Have Better Blood Pressure
Vegetarians in Tel Aviv were compared to non-vegetarians in Tel Aviv (1). The vegetarians had consistently lower blood pressure. In the Tel Aviv study, the researchers compared multiple dietary factors to try to explain the reason there was a difference. They showed no difference in the amount of saturated fat versus unsaturated fat in the diet of the two groups. There was no difference in protein intake between the two groups. The amount of sodium in the diet was the same. The only difference in nutrients was a much higher amount of potassium in the diet of the vegetarians. This resulted in a higher potassium to sodium ratio.
EPIC is a large ongoing study of modern vegetarians that has produced multiple publications since the 1980s. An interesting pair of studies came out of this group that we discussed at this post. A group of 76,000 was divided into vegan and nonvegan which showed lower mortality from ischemic heart disease among vegans. One year later these same researchers reported on the same identical group of 76,000 subjects, but divided them into lactoovovegetarians and pescatarians, as well as pure vegans and meat eaters. In the second study, the pescatarians and lactoovovegetarians had better mortality than the pure vegans. In the first study, the lactoovovegetarians and pescatarians had been lumped in with the meat eaters, so their improved mortality did not show.
Ethnicity Does Not Affect Improved Vegetarian Blood Pressure
Many other studies of urban vegetarians show lower blood pressure in vegetarians. Most vegetarian studies do not include a variety of ethnicities. However, one study from the 1980s (2) looked specifically at ethnicity. It showed lower blood pressure in both urban African-American vegetarians and urban Caucasian vegetarians. Although the African-American vegetarians had a higher blood pressure than the Caucasian vegetarians, they also had a lower potassium to sodium ratio. So the lower blood pressure in vegetarians does not appear to be based on ethnicity.
Vegetarians Are More Health Conscious About Diet Than Average Person
Several studies suggest that vegetarians have lower mortality compared to the normal population because they are more health conscious. An EPIC-Oxford study (3) compared vegetarians to health conscious nonvegetarians. In this case, both groups had similar mortality. And both had better mortality than the general British population.
A recent study found that vegetarians frequently made food choices based on maintaining or improving health (4). The vegetarians used this basis for choosing food far more frequently than the general public.
This same study also found that the majority of those identifying themselves as vegetarian consumed some animal products. They were not strict vegans. But, because of their increased intake of fruits and vegetables, these vegetarians had a higher ratio of potassium to sodium than nonvegetarians. In this study the nonvegetarian men had a potassium to sodium intake ratio of 0.97. The vegetarian men had a ratio of 1.30. Comparable ratios for the women were 1.08 and 1.39.
Causation Versus Association
Yet, as previously discussed, these population based studies, and the previously discussed studies of indigenous peoples, only show an association. Although there is a strong association of the potassium to sodium ratio and lower blood pressure, it does not necessarily show causation. Only because of the basic underlying science that shows how this ratio affects blood pressure, as discussed here, can we be confident that the potassium to sodium ratio is the critical factor in reducing blood pressure through diet.
Sir Austin Bradford Hill, in the 1960s, discussed how to distinguish association from causation epidemiologically. Next post we will tie together the basic science studies with these population studies using his approach to show causation.
1. Low blood pressure in vegetarians: the possible role of potassium. Ophir O, Peer G, Gilad J, Blum M, Aviram A. Am J Clin Nutr. 1983 May;37(5):755-62.
2. Relation between vegetarian/nonvegetarian diets and blood pressure in black and white adults. Melby CL, Goldflies DG, Hyner GC, Lyle RM. Am J Public Health. 1989 Sep;79(9):1283-8.
3. Mortality in British vegetarians: review and preliminary results from EPIC-Oxford. Key TJ, Appleby PN, Davey GK, Allen NE, Spencer EA, Travis RC. Am J Clin Nutr. 2003 Sep;78(3 Suppl):533S-538S.
4. Diets and selected lifestyle practices of self-defined adult vegetarians from a population-based sample suggest they are more ‘health conscious' Bedford JL, Barr SI. Int J Behav Nutr Phys Act. 2005 Apr 13;2(1):4.