Although the study of human body composition is advancing rapidly, confusion still prevails regarding the molecular-level lipid component. Most molecular-level body composition models are presently based on the overall hypothesis that nontriglyceride lipids constitute an insignificant proportion of total body lipid. A single lipid or "fat" component consisting of triglycerides is thus assumed in most molecular-level body composition models. To test this hypothesis, the present study, carried out in adult rats, was designed to examine two questions: 1) What is the proportion of total lipids as triglycerides? and 2) Is this proportion constant or does it change with negative energy balance and weight loss produced by calorie restriction and increased exercise? Results indicated that with negative energy balance and weight loss there were progressive losses of total body triglyceride and lipid. The proportion of total lipids as triglyceride was 0.83 ± 0.08 (SD) in control animals, with reductions at 2 and 9 wk of energy restriction [0.82 ± 0.04 (P = NS vs. control) and 0.70 ± 0.15 (P = 0.05)] and at 9 wk for energy restriction plus exercise [0.67 ± 0.09 (P = 0.003)]. Nontriglyceride lipids comprised 2.8% of carcass weight at baseline and decreased to 2.2% by 9 wk of energy restriction and exercise (P = NS). Substantial differences were observed between body composition ratios expressed as percentages of the lipid-free body mass (LFM) and triglyceride-free body mass (TGFM); (e.g., total body water/LFM and TGFM in controls = 72.7 ± 0.7 and 70.4 ± 2.2, respectively; P = 0.02). These observations strongly support the existence and importance of nontriglyceride lipids as a body composition component that responds independently from storage triglycerides, with negative energy balance produced by food restriction and exercise.