The Metabolic Effects of Different Weight Loss Diets

Overview

Current estimates suggest that 65% of American adults are overweight or obese. Excess body weight has been associated with an increased risk of a number of metabolic abnormalities, including high blood sugar, high blood pressure, high triglyceride levels, and low HDL ("good") cholesterol levels. Insulin resistance (when the body becomes less sensitive to the blood sugar-lowering hormone insulin, and more of the hormone is needed to keep blood sugar levels under control) also frequently occurs as a result of excess body weight. These abnormalities can all increase the risk of heart disease and other serious medical problems. Individuals who are overweight often have a number of these abnormalities at the same time, a situation which is often called "the metabolic syndrome." Dietary changes, physical activity, and weight loss can lead to improvements in each of the metabolic abnormalities described above. However, the best type of diet for people with the metabolic syndrome is not known. This study has been designed to test the effects of several promising dietary patterns, with and without weight loss, in overweight adults with the metabolic syndrome. Most individuals who have the metabolic syndrome do not know they have the condition, so we will be screening many healthy overweight volunteers to see if they may be eligible.

BACKGROUND: Several dietary approaches, including the Dietary Approaches to Stop Hypertension study diet (the DASH diet) and the low glycemic index diet (low GI diet), may be helpful for overweight individuals who have the metabolic syndrome. The DASH diet is rich in fruits, vegetables, whole grains, and low-fat dairy products. It also contains moderate amounts of nuts, beans, fish, and poultry. Consumption of red meat, sweets, and sugary beverages is limited. As a result, the DASH diet contains more potassium, magnesium, calcium, and fiber and less total fat, saturated fat, cholesterol, simple sugars, and sodium than the average American diet. The DASH diet has been shown to lower blood pressure and LDL cholesterol ("bad" cholesterol). It has also been shown to be an effective diet for weight loss. It may also reduce inflammation and insulin resistance, two important features of the metabolic syndrome. Low GI diets are typically lower in carbohydrate content and higher in protein content than the average American diet. Many popular diet books promote the use of low glycemic index diets. In this study, the low GI diet will contain plenty of fruits and vegetables and moderate amounts of nuts, beans, lean meats, fish, and reduced-fat dairy products. Refined grains, potatoes, and sweets will be avoided. The low GI diet can help improve levels of blood sugar and insulin in the blood after meals. It may also help control hunger after meals. Some researchers believe it may promote more weight loss than other diets, especially in people with the metabolic syndrome. The low GI diet may also reduce cholesterol and triglyceride levels. Although both the DASH diet and the low GI diet appear to have beneficial effects, it is not clear whether one diet is better than the other for people with the metabolic syndrome. It is also not clear whether these diets would be better for weight loss than a reduced-calorie average American diet. This weight loss study has been designed to test the metabolic effects of the DASH diet and the low GI diet, as compared to the average American diet. STUDY OVERVIEW: Up to 18 research volunteers will be studied at Rockefeller University Hospital during a multi-phase study of almost 15 weeks duration. This study includes a 2-week run-in period at home, a 19-day inpatient period during which weight will be kept stable, an 8-week outpatient weight loss period, and a 2-week inpatient period of weight stability at a lower body weight. All food will be provided for about 13 weeks (during the inpatient periods and the 8-week outpatient weight loss period). Volunteers will first undergo testing (described below) at baseline, on an average American diet. They will then be randomized to one of three diets (the DASH diet, the low GI diet, or the average American diet). Weight will initially be kept stable for two weeks on the study diet. At the end of the first inpatient period, volunteers will undergo repeat testing to assess the metabolic effects of the diets, in the absence of weight loss. Volunteers will then be discharged to home for 8 weeks, during which they will receive a 50% reduced-calorie study diet. Volunteers will return to Rockefeller University Hospital twice a week during the outpatient phase for weight-management skills training and follow-up with study staff. It is expected that volunteers will lose between 5 and 9% of their starting weights during this time. At the end of the weight loss phase, volunteers will be readmitted to the research hospital for a final 2-week stay. Body weight will be maintained at the new, lower weight, and volunteers will undergo repeat testing on the study diet. At the conclusion of testing, volunteers will be discharged from the hospital and the study will conclude. TESTING: During this study, a number of tests and procedures will be performed at various times. These will include: * Questionnaires about physical activity, eating habits, attitudes about food, quality-of-life, and hunger * A 3-day record of food intake at baseline * Use of a pedometer (step-counter) to assess usual physical activity * Blood tests (including blood sugar, insulin levels, cholesterol levels, fat hormones, and inflammatory markers) * 24-hour ambulatory blood pressure monitoring * Measurement of energy expenditure (the rate at which a body burns calories) * Noninvasive assessment of body composition before and after weight loss * Insulin clamp procedures (during which insulin and sugar are infused through a vein to help determine how sensitive a person's body is to insulin)