Effect of age on protein conservation during very-low-energy diet in obese Sprague-Dawley rats

To examine the effect of age on body protein losses occurring during severe energy restriction in obesity.

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.

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 VLED (70.6+/-30.4 vs. 32.6+/-29.1 g in 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.

Dietary obese young rats appear better able than aged rats to conserve body protein while losing body fat during severe energy restriction.