JOHNSON, JULIA A, CHOON-HIE YU, MEI-UlH YANG, F. XAVIER PI-SUNYER. Effect of age on protein conservation during very-low-energy diet in obese SpragueDawley rats. Obes. Res. 1998;6:448--457. Objective: To examine the effect of age on body protein losses occurring during severe energy restriction in obesity. Research Methods and Procedures: Weanling (young) Sprague-Dawley rats (YR) were fed a high fat (35% energy) diet (HFD) until mean body weight approached that of a group of chow-fed retired breeder (aged) rats (AR). Both groups were then fed HFD for an additional 2 weeks, after which selected controls from YR and AR groups were killed for baseline carcass analysis. Remaining rats were fed a very-low-energy diet (VLED, 33% kcal of HFD) for 3 weeks and then killed for carcass analysis. Results: YR had greater fat stores before VLED, and lost proportionately more fat and less protein during VLED than did AR. Weight loss composition during VLED was 66.7% fat, 11.1% protein, and 22.2% water in YR, and 39.4% fat, 26.2% protein, and 34.3% water in AR. Greater YR fat loss during VLE]j (70.6 ± 30.4 vs. 32.6 ± 29.1 gin AR; mean ± SD) was paralleled by significantly larger decreases in epididymal and retroperitoneal fat pad weights, mean adipocyte size, and lipoprotein lipase activity. Greater protein loss in AR (21.6 ± 13.9 g vs. 11.8 ± 10.7 g in YR) coincided with larger decreases in visceral organ weights and serum thyroxine and triiodothyronine. Energy expenditure changes during VLED were similar between groups. Discussion: Dietary obese young rats appear better able 448 OBESITY RESEARCH Vol. 6 No.6 Nov. 1998 than aged rats to conserve body protein while losing body fat during severe energy restriction.