Cross-sectional and longitudinal analysis of age-associated changes in body composition of male Brown Norway rats: association of serum leptin levels with peripheral adiposity. 
Aging-associated alterations in body composition are accompanied by changes in the endocrine system.  We evaluated, in male Brown Norway rats, the effects of aging on body composition and the association with serum levels of leptin, insulin, and testosterone.  Body composition was assessed cross-sectionally in male rats (3, 8, 17, and 29 months) by a combination of dual energy x-ray absorptiometry (DEXA) and dissection of specific muscles and adipose depots.  Longitudinal changes in body composition were quantified by DEXA before and after 3 months of ad-libitum feeding.  Body weight, lean mass, absolute and percentage fat increased with age, whereas percentage of lean mass decreased.  Leptin and insulin levels increased with age in proportion to adiposity; the increase in leptin with age was related to increased total and peripheral, but not visceral, fat.  Testosterone decreased with age, and was associated with decreased lean and skeletal muscle mass.  These findings suggest that alterations in body composition with age may be due to decreased trophic and increased lipogenic hormones.  Relative to other rodent models, Brown Norway rats undergo shifts in body composition and in the hormonal milieu that are consistent with changes seen in aging humans.