Dietary Factors with Limited or Uncertain Effect on Blood Pressure

Fiber

Evidence from observational studies and several clinical trials suggests that increased fiber intake may reduce blood pressure. A meta-analysis documented that supplemental fiber (average increase of 14g/day) was associated with net systo-lic/diastolic reductions of 1.6/2.0 mmHg, respectively. Still, high-quality epidemiologic studies and clinical trials are needed before one can recommend increased fiber intake as a means to lower blood pressure.

Calcium and Magnesium

Evidence that increased calcium intake might lower blood pressure comes from a variety of sources, including animal studies, observational studies, clinical trials, and meta-analyses. Meta-analyses of trials documented modest reductions in systolic and diastolic blood pressure of 0.89-1.44 and 0.180.84 mmHg, respectively, with calcium supplementation (400-2000 mg/day). There is also evidence that calcium intake may affect the blood pressure response to salt. Overall, data are insufficient to recommend supplemental calcium alone as a means to lower blood pressure.

The body of evidence implicating magnesium as a major determinant of blood pressure is inconsistent. In observational studies, often cross-sectional in design, a common finding is an inverse association of dietary magnesium with blood pressure. However, in pooled analyses of clinical trials, there is no clear effect of magnesium intake on blood pressure. Hence, data are insufficient to recommend increased magnesium intake alone as a means to lower blood pressure.

Fats (Other Than Fish Oil) and Cholesterol

Numerous studies, including both observational studies and clinical trials, have examined the effects of fat intake on blood pressure. Overall, there is no apparent effect of saturated fat and n-6 polyunsatu-rated fat intake on blood pressure. Although a few trials suggest that an increased intake of monounsa-turated fat may lower blood pressure, evidence is insufficient to make recommendations. Likewise, few studies have examined the effect of dietary cholesterol intake on blood pressure. Hence, although modification of dietary fat and cholesterol intake can be recommended as a means to prevent and treat hyperlipidemia and dyslipidemia, evidence is insufficient to recommend these changes alone as a means to lower blood pressure.

Protein Intake

A large and generally consistent body of evidence from observational studies has documented that higher protein intake, particularly protein from plant-based sources, is associated with lower blood pressure. In contrast to the large volume of evidence from observational studies, comparatively few trials have examined the effects of protein intake on blood pressure. Recent trials have tested the effects of soy-based interventions on blood pressure. In several but not all of these trials, soy supplementation reduced blood pressure. Although it is reasonable to speculate that an increased intake of protein from plant sources can lower blood pressure, this hypothesis has not been adequately tested in a clinical trial of sufficient size and rigor.

Vitamin C

Laboratory studies, depletion-repletion studies, and epidemiological studies suggest that increased vitamin C intake or status is associated with lower blood pressure. However, few trials have addressed this issue, and results of these trials have been inconsistent. Overall, it remains unclear whether an increased intake of vitamin C lowers blood pressure.

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