Dietary protein and blood pressure in monozygotic twins

Cross-sectional studies relating blood pressure to dietary intake have shown equivocal results, in part due to the inability to take into account the strong genetic component of blood pressure. Intervention studies, using the same subject as his own control, often encounter additional problems when subjects are asked to adhere to an alternate diet. The National Heart, Lung, and Blood Institute Twin Study of middle-aged men provided information concerning the possible relationship of food-frequency-estimated nutrient intake to blood pressure while controlling for genetic effects in a free-living group of subjects. Using differences in monozygotic twins, a direct association of dietary protein intake and diastolic blood pressure was identified and persisted after adjustment for known covariates of blood pressure. Adjusting for known covariates and holding total calories constant, a 9-g difference in daily protein intake was directly associated with a 1 mm Hg difference in diastolic blood pressure. For protein intake as a percentage of total calories, a 2.18% difference was directly associated with a 1 mm Hg difference in diastolic blood pressure. The co-twin-control method provides a powerful design to address the interrelationships between nutrients and blood pressure in an observational as well as an experimental setting.