Dietary Energy Restriction and Metabolic Aging in Humans

This study is a 3-phase investigation designed to test two caloric restriction (CR) regimens for participant compliance and variability in metabolic and physiologic effects. Initially, and through phase 1 (1 week pre-phase, and 6 weeks phase 1), participants will maintain normal habits at home. Then for phase 2 (24 weeks), the 44 participants will be randomized to one of two diets (Diet HG or Diet LG), which they will consume at either 70% or 90% of baseline energy intake. The participants randomized to 90% CR (both diets) will form the control group. All food will be provided during this phase, and participants will be required to attend weekly group or individual meetings with the behavioral counselor and dietitian, and perform regular self-monitoring. During phase 3 (24 weeks), participants will prepare their own food at home according to their CR regimen and will eat to maintain the CR specified for phase 2. They will continue to attend the weekly meetings and to perform self-monitoring. Diet HG: a diet based on the USDA Food Guide Pyramid, but with additional emphasis on adequate fiber and low energy density foods, including use of whole grains rather than refined carbohydrates, limited liquid calories, high variety of low energy items such as fruits and vegetables and low variety of high-energy discretionary foods. Diet LG: a modification of the HG diet in which low energy density is maintained even though the percent of energy from fat and protein is increased (by increased use of high-protein foods and foods with high water content) and use of different carbohydrate sources and physically intact food items (whole grains, raw fruits and vegetables) to further lower glycemic load through reducing the glycemic index (GI). High fiber intakes, low energy density, limited use of liquid calories, high variety of low energy items such as fruits and vegetables and low variety of high-energy discretionary foods will be the same as for the HG diet.