Nutritional interventions are important for reducing obesity and related conditions. Soy is a good source of protein and also contains isoflavones that may affect plasma lipids, body weight, and insulin action. Described here are data from a monkey breeding colony in which monkeys were initially fed a standard chow diet that is low fat with protein derived from soy. Monkeys were then randomized to a defined diet with a fat content similar to the typical American diet (TAD) containing either protein derived from soy (TAD soy) or casein-lactalbumin (TAD casein). The colony was followed for over two years to assess body weight, and carbohydrate and lipid measures in adult females (n=19) and their offspring (n=25). Serum isoflavone concentrations were higher with TAD soy than TAD casein, but not as high as when monkey chow was fed. Offspring consuming TAD soy had higher serum isoflavone concentrations than adults consuming TAD soy. Female monkeys consuming TAD soy had better glycemic control, as determined by fructosamine concentrations, but no differences in lipids or body weight compared with those consuming diets with TAD casein. Offspring born to dams consuming TAD soy had similar body weights at birth but over a two-year period weighed significantly less, had significantly lower triglyceride concentrations, and like adult females, had significantly lower fructosamine concentrations compared to TAD casein. Glucose tolerance tests in adult females were not significantly different with diet, but offspring eating TAD soy had increased glucose disappearance with overall lower glucose and insulin responses to the glucose challenge compared with TAD casein. Potential reasons for the additional benefits of TAD soy observed in offspring but not in adults may be related to higher serum isoflavone concentrations in offspring, presence of the diet differences throughout more of their lifespan (including gestation), or different tissue susceptibilities in younger animals.