TOTAL-BODY LIPID AND TRIGLYCERIDE RESPONSE TO ENERGY DEFICIT - RELEVANCE TO BODY-COMPOSITION MODELS

Although the study of human body composition is advancing rapidly, con fusion still prevails regarding the molecular-level lipid component. M ost molecular-level body composition models are presently based on the overall hypothesis that nontriglyceride lipids constitute an insignif icant proportion of total body lipid. A single lipid or ''fat'' compon ent consisting of triglycerides is thus assumed in most molecular-leve l 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 thi s proportion constant or does it change with negative energy balance a nd weight loss produced by calorie restriction and increased exercise? Results indicated that with negative energy balance and weight loss t here mere 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 a t 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 r atios expressed as percentages of the lipid-free body mass (LFM) and t riglyceride-free body mass (TGFM); (e.g., total body water/LFM and TGF M in controls = 72.7 +/- 0.7 and 70.4 +/- 2.21 respectively; P = 0.02) . These observations strongly support the existence and importance of nontriglyceride lipids as a body composition component that responds i ndependently from storage triglycerides, with negative energy balance produced by food restriction and exercise.