Changes in body composition in response to challenges during aging in rats.

Body composition changes over the lifespan of Brown Norway rats, in patterns similar to those of humans. Young adults are lean, with little fat, much of which is intra-abdominal. As they age, rats exhibit linear growth, and both lean and fat mass increase until late middle to early old age. Fat mass continues to accumulate throughout the lifespan, both viscerally and subcutaneously; aging animals carry a higher proportion of their fat mass peripherally. After middle age, skeletal muscle mass begins to decline, and sarcopenia develops when animals reach senescence. Finally, in late old age, or senescence, body weights begin to decline, and both fat and lean mass are lost. Healthy aged rats generally respond to negative energy balance challenges less robustly than younger adult animals, although they do appropriately regulate adipose tissue stores and preserve lean mass. The response to a positive energy balance challenge (high fat feeding) is less well regulated in aging animals, and dietary-induced obesity develops rapidly in aged animals. Here we present a summary of several studies of body composition in response to challenges of energy balance in aging male Brown Norway rats, with special emphasis on adipose tissue partitioning.