In this non-black population we report significant differences in both systolic and diastolic BP and the odds of hypertension, depending on vegetarian dietary pattern. These results are from a population that includes a wide variety of age, socio-economic status and both genders, and they represent dietary habits stable over many years for the most part. Subjects summarized their diets over the previous year. Few other studies have been able to compare habitual non-vegetarians with both habitual vegans and lacto-ovo vegetarians. The effect appears to be moderately stronger in vegans as there were fewer vegans taking antihypertensive medications and those not taking such medications had BP as low (approximately) as the lacto-ovo vegetarians. Effects appear to be partially explained by dietary effects on BMI, which are strong in this population(36)
. Dietary effects were still evident in those with normal BMI.
The percentage overweight or obese in this population was lower than in the general US population(36)
, though still considerable. Our results are adjusted for these factors but it seems likely that the effect of a vegetarian diet to reduce body weight is one mechanism partially responsible for the BP effect. However, some additional effect probably still remains. A partial mediating effect of BMI is consistent with the results of some other observational studies(21,27)
. Interestingly, in previous short-term feeding studies there was typically no weight loss during the vegetarian feeding period, despite the well-known long-term large differences in BMI between vegetarians and others. It does seem possible that although no weight changes were seen over a few weeks of the feeding studies, physiological processes (e.g. insulin/glucose metabolism)(19)
resulting in or associated with weight loss over a longer period may have begun and may already have started to affect BP.
As with most other similar studies, we found effects on both systolic and diastolic BP. While a few reported studies found changes in systolic(9,20,22)
pressures only, this may have been due to limited statistical power.
Beyond BMI, which dietary factors in the vegetarian diet may account for effects on BP levels is not well understood. Vegetarians have higher fibre and K intakes as a result of their greater intakes of fruits, vegetables, fruits, nuts and wholegrain products(37–39)
. Recent meta-analyses of randomized trials(16,17)
demonstrate small but significant effects of fibre, particularly in those with higher baseline pressures.
Consumption of a K-rich diet has a natriuretic effect and diets that are high in K usually are low in Na, as long as unprocessed foods are consumed(5)
. Low Ca or dairy intake has also been associated with higher BP(40–42)
. Inconsistent with this, our results and those from the European Prospective Investigation into Cancer and Nutrition (EPIC)–Oxford study(27)
show that vegans who avoid dairy products have the least hypertension. However, Adventist vegans do not have particularly low Ca intakes, perhaps due in part to supplementation but also because of vegetable sources of Ca.
Proposed physiological mechanisms that may mediate the effect of a vegetarian diet include modulation of baroreceptor sensitivity, direct vasodilatory effects, changes in catecholamine and renin–angiotensin–aldosterone metabolism, improvement of glucose tolerance with lower insulin levels(5,19)
, and lower blood viscosity in vegetarians(43)
Strengths and weaknesses
We measured BP using an automated sphygmomanometer which provides BP with acceptable validity(30)
. A large amount of apparently random error is associated with BP measurement, despite taking measures to counteract the known influencing factors. In addition to using the digital machine we standardized other environmental factors that may influence BP. Together with the relatively large number of study subjects these factors should reduce the effects of random errors. Most study subjects had been church members for decades, further suggesting stability of dietary habits. Differences in duration of church membership by dietary category seem unlikely to affect results given that only 8–21% had been members for <25 years across the dietary groups.
It was necessary to assign vegetarian dietary pattern based on the results of an FFQ rather than the repeated recalls, as a small number of recalls will easily miss less frequent consumption of animal products. There is inevitably recall and reporting error in these data. However, compared with the average of six 24 h recalls, correlation coefficients (r) corrected for deattenuation are as follows: red meat (r = 0·76); poultry (r = 0·76); fish (r = 0·53); eggs (r = 0·64); dairy protein (r = 0·77). Thus by usual standards the validity of food frequency questions used in the algorithm assigning vegetarian status is excellent.
Inevitably there were small amounts of missing dietary data (<1% for most composite variables in this study). However, 9·4 % of subjects were missing at least one of the longer lists of individual dairy items where we needed to assume missing at random for imputation and this may have resulted in a small amount of misclassification.
The proportion of this older study population who were taking antihypertensive medication was 25·3 %, compared with 21·3 % in the US population aged 18 years and above(44)
. It is interesting that 10·0% of vegans but 28·8% of non-vegetarians are taking antihypertensive medications. This could mean that the vegan population is less willing to take medications or that BP is indeed lower in this subgroup. Given the relatively low BP among vegans not taking antihypertensive medications, their need for medications is probably less. The much lower odds of hypertension in this group is thus due both to the lower proportion taking anti-hypertensive medication and the low BP in those not taking medication.
As this is a cross-sectional study we do not know the stability of dietary patterns over time, meaning that it is not possible to exclude a reverse causation in that some may have changed their diet after they received a diagnosis of elevated BP. However, if this occurred it would likely work against reverse causation as most Adventists with a health problem would actually move towards a plant-based diet.