Popular Diets Study

For most of the last half century, reduction in fat intake has been the primary nutritional approach for the prevention and treatment of obesity and cardiovascular disease (CVD). Over the last few years, very low carbohydrate (Atkins-type) diets have achieved great popularity, with publication of several studies suggesting greater weight loss and improvements in CVD risk factors over 3 to 6 months. Recently, a third dietary approach focused on glycemic index (GI) has generated interest. However, few studies have compared the effects of these diets on body weight regulation and risk for CVD. The primary hypotheses of this study are that any diet that lowers the postprandial rise in blood glucose (very-low-carbohydrate or low-GI) will have beneficial effects on the physiological adaptations to weight loss and on some CVD risk factors. However, other CVD risk factors will be adversely affected by a very-low-carbohydrate vs. a low-GI diet. Preliminary data provide strong support for these hypotheses, by showing that resting energy expenditure declines less and CVD risk factors improve more with weight loss on a low-glycemic load diet compared to a conventional low-fat diet. This application proposes a cross-over feeding design to study the effects of three diets following 12.5% weight loss in obese young adult subjects (n = 24, age 18 to 40 years). The diets are: 1) conventional low-fat, with 60% carb, 20% fat, 20% protein; 2) low-GI with 40% carb, 40% fat, 20% protein; and 3) very-low-carbohydrate with 10% carb, 60% fat, 30% protein. The primary outcome is resting energy expenditure (indirect calorimetry). Secondary outcomes include total energy expenditure (doubly labeled water), thermic effect of food (indirect calorimetry), physical activity (accelerometry), insulin resistance and B-cell function (frequently-sampled OGTT), blood lipids, blood pressure and measures of systemic inflammation and coagulopathy. This study should have major public health implications to the millions of Americans currently following diets to decrease body weight and risk for heart disease.

Inclusion criteria

• BMI ≥ 27 kg/m2
• Willing and able to come to the GCRC 5 days per week to consume a supervised meal and pick-up food for all other meals
• Available for scheduled hospital admissions
• Willing to abstain from alcohol consumption for the duration of the study
• If female, regular menstrual cycles (defined as 26 to 30 days between cycles; no more than one day variation in the duration of menstrual flow)

Exclusion criteria

• Weight > 350 lbs
• Change in body weight (± 10%) over preceding year
• Taking any medications or dietary supplements that might affect body weight, appetite, or energy expenditure
• Smoking (1 cigarette in the last week)
• High levels of physical activity
• Currently following a special diet
• Abnormal laboratory screening tests
• Type 2 diabetes mellitus
• Allergies or aversions to foods on the study menu
• Previous diagnosis of an eating disorder or any other mental health disorder
• If female, pregnant in the past 12 months or planning to become pregnant during the study period
• If female, lactating in the preceding 12 months
• If taking birth control medication, change in medication in previous 3 months or plans to change medication during the study period