This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Background and rationale: The effectiveness of weight loss interventions in African Americans has been disappointing, and to date, there has been no systematic study of the best approach to enhance weight loss for African Americans from a physiologic standpoint. Ideally, a dietary prescription would produce improvements in CVD risk profiles even with modest amounts of weight loss achieved. Achieving this ideal might be important for African American populations for 2 reasons: (1) given the disparities in CVD outcomes such as stroke and death from CVD events, the dietary prescription should improve overall cardiovascular health and not just one risk factor (for example, sodium reduction improves blood pressure, but has no impact on lipids); (2) with lower than average weight loss from various interventions for obese African Americans, the optimal impact for improvement of cardiovascular risk may have to be achieved with lesser amounts of weight loss. The Dietary Approaches to Stop Hypertension (DASH) dietary pattern may serve as an effective weight loss intervention in African Americans; it emphasizes fruits, vegetables, and low-fat dairy products while being lower in total fat, saturated fat, and cholesterol. This dietary pattern is high in potassium, magnesium, calcium, total protein and dietary fiber, while being low in red meat and sugar containing beverages. In feeding studies of the DASH diet, African Americans who consumed this pattern had larger decreases in blood pressure than non-African Americans. This differential effect by race may be mediated by insulin sensitivity, a central factor in modifying vascular tone; in addition, other cardiovascular disease (CVD) risk factors may be positively influenced by such a dietary pattern. Insulin sensitivity and a host of other CVD risk factors, including C - reactive protein, serum lipids, and abdominal fat mass, are also improved by weight loss. The efficacy of a hypocaloric DASH diet and its impact on these CVD risk factors has never been tested, particularly in African Americans.
The aim of this project is to characterize the physiologic effects of the DASH dietary pattern, with and without weight loss, on insulin sensitivity (using the frequently sampled intravenous glucose tolerance test) and selected CVD risk factors in obese African Americans. Study participants (n = 96) will be randomized to one of three, 8- week feeding protocols that include an isocaloric DASH diet, a hypocaloric DASH diet, or a hypocaloric control diet (typical American intake).Study Question: This study design will allow the testing of the following hypotheses: (1) African Americans on an isocaloric DASH diet will have greater improvement in insulin sensitivity compared to African Americans on a reduced calorie, control diet; (2) African Americans on a weight reducing DASH diet will have greater improvement in insulin sensitivity compared to African Americans on a reduced calorie, control diet; and (3) the effects on insulin sensitivity of a weight reducing DASH diet will be sub-additive compared to the combined effects on insulin sensitivity of an isocaloric DASH diet and a reduced calorie control diet.Utilization of GCRC: Outpatient nursing at the GCRC will be utilized for 1 screening visit required to determine eligibility. Bionutrition is another key GCRC resource that has been utilized to provide assistance with creating menus for the 8 weeks and preparation of aliquots of meals for diet validation; in addition we will utilize the kitchen for preparation and distribution of study meals for the 8 week feeding period. Inpatient nursing at the GCRC will be utilized to perform the intravenous glucose tolerance test (IVGTT), completed after overnight stays at baseline and follow-up. The Processing Core Laboratory & the Physiology and Metabolic Core Laboratory will be utilized to process and analyze the blood samples from the screening visit and also from the IVGTT. The Physiology and Metabolic Core Laboratory will also be used for the DEXA scan for body composition analysis and the determination of abdominal fat content by CT. The Biostatistical Core will be used for data analysis.
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