Adipose tissue dynamics during cyclic weight loss and weight gain of ground squirrels

Ad libitum fed golden-mantled ground squirrels undergo marked annual fluctuations in body mass; during the first cycle, peak and trough masses were ~291 and ~192 g, respectively. Peak masses were significantly higher (9%) during the second than the first cycle, reflecting a 15% increase in fat-free dry mass and a 12% increase in lipid reserves. The reduction in body mass during the weight loss phase was almost entirely due to an 84% decrease in total lipid reserves and reflected a decrease in adipocyte size but not number. All measured fat depots decreased at similar rates, and there was no evidence of preferential utilization or sparing of individual depots. Fat cell size decreased progressively in each depot during the transition from peak to trough masses and did not vary among the several depots at any stage of annual cycle. Adipocyte number increased in parametrial and retroperitoneal depots but not in the subcutaneous fat between the first and second body mass peaks; fat cell size in these two depots decreased by 28 and 20%, respectively, from the first to the second peak in body mass. Reduced lipid filling of adipocytes at the second peak mass, concomitant with adipocyte hyperplasia, suggests that total lipid mass, not simply fat cell size, is regulated during the annual body mass cycle. Maturation of the subcutaneous fat depot before the initial entry into hibernation may be adaptive in providing insulation during dormancy.