A diet based on whole grains selectively reduced diastolic blood pressure by more than threefold relative to a matched refined-grain diet in a study of overweight or obese adults under age 50. The improvement was independent of weight loss or fat loss and suggests that whole grains may be an effective nutritional tool for lowering cardiovascular-related mortality in this population.
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“Because diastolic blood pressure is predictive of mortality in adults under 50, increased intake of whole grains may provide a functional approach to hypertension control that could benefit patients at high risk of vascular-related mortality,” says the study’s lead author, John Kirwan, PhD, Director of the Metabolic Translational Research Center in Cleveland Clinic’s Endocrinology & Metabolism Institute.
The study, conducted by Cleveland Clinic researchers in collaboration with Nestlé Research Center, was published online Oct. 19, 2016, by the Journal of Nutrition.
The researchers used a double-blind, randomized, controlled, crossover study design. Forty overweight or obese adults with no history of a cardiovascular event were enrolled. Complete whole-grain and refined-grain diets were provided to participants for two 8-week periods separated by a 10-week washout period. The diets were identical in macronutrient composition except for their content of whole vs. refined grains (50 g/1,000 kcal in each diet). Blinding was achieved by packaging meals in identical containers and covering grains with sauce when appropriate.
“A lack of adequate control over intervention diets may have limited some past trials of whole-grain diets,” says Dr. Kirwan. “To address this issue, we provided the entire diet for each intervention arm.” To ensure that participants complied with the diet, the researchers measured plasma levels of alkylresorcinols, commonly used biomarkers of whole-grain wheat and rye intake.
At the start and end of each diet period, participants spent three days undergoing metabolic testing in a clinical research unit. Those taking antihypertensive drugs (n = 5) were instructed to maintain their medication use throughout the study.
Outcome measures included blood pressure, body composition, blood lipids and adiponectin, and markers of inflammation and glycemia.
Among the 33 participants who completed the trial (6 men and 27 women; mean age, 39 ± 7 years), the researchers observed the following changes in diastolic blood pressure:
The difference between the diets was statistically significant (P = .01), and reductions in diastolic blood pressure correlated with circulating adiponectin levels.
Substantial decreases in body weight, fat mass, systolic blood pressure, and total and LDL cholesterol were observed after both diet interventions, with no relevant differences between diets.
“Based on published population studies, the degree of incremental reduction in diastolic blood pressure we observed with the whole-grain diet versus the refined-grain diet — 4.2 mm Hg— equates to reductions of about 40 percent in stroke mortality risk and 30 percent in ischemic heart or vascular mortality risk,” says Dr. Kirwan. “Since elevated diastolic blood pressure has been shown to be a primary risk factor for cardiovascular disease in the population we studied — adults under age 50 — these findings have important implications for patient care and public health.”
Dr. Kirwan notes that while these findings build on strong observational evidence that whole grains can be an important dietary adjunct for hypertension treatment and prevention, some previous intervention studies have not supported that evidence. He says study design may help explain the difference.
“Most studies examining whole-grain foods have allowed participants to remain in ‘free-living’ environments where these foods are provided in their usual diets,” he says. “That approach can lead to participants adding, rather than substituting, the intervention foods to their diet, which can cause higher overall food consumption and offset any benefit from whole-grain foods.”
Dr. Kirwan adds that his study’s suggestion that a whole-grain diet may help attenuate hypertension raises a clinically relevant therapeutic option for improving health “over and above improvements from weight loss that can be derived from whole grains versus refined grains.”
Better yet, it may be a dietary approach patients might actually accept. “We observed no differences in compliance or preference for either diet,” he says. “What’s more, the foods we used were based on commercially available ingredients and recipes, and they were well accepted by participants.”