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Experimental Obesity and Weight Reduction in Young Female Rats: Development of Procedures and Calcium and Phosphorus Balance Studies¹

Graduate School of Nutrition, Cornell University, Ithaca, New York

A study of the effect of the production of obesity, weight reduction, and post-reduction weight maintenance on the calcium and phosphorus metabolism of young adult female Sprague-Dawley rats was made by means of 7 balance studies (4 days each) spaced throughout the various phases of the experiment, and by means of left femur bone density determinations of the 27 paired control and experimental animals (three initial control animals plus 24 experimental and control animals paired by initial weight). The intakes of nitrogen, calcium and phosphorus were calculated to be constant for all animals throughout the experiment; calories were varied with the phase of the experiment. A tube-feeding technique was used with a synthetic diet to insure quantitative control of intake.

Weights of the approximately 240 gm females were maintained on a daily intake of 43 Cal. An average weight gain of 75 gm, which represented an increase of 30% over the initial pre-obesity weight, was produced in 62 days on an intake of 61 Cal./day, the average daily weight gain being 1.2 gm. Fed an average daily intake of 29 Cal. over a 34-day period, the obese animals were reduced at the rate of approximately 2 gm/day to their pre-obesity weights. The rate of weight loss decreased with the prolongation of reduction so that by mid-reduction, caloric intake had had to be reduced from 32 to 26 Cal./day to maintain a satisfactory weight loss.

In the post-reduction weight maintenance period, the animals in which weight reduction had been effected required fewer calories (38) for weight maintenance than controls of similar weight and age and than these experimental rats had required at the same weight prior to the production and reduction of obesity.

In general, during the pre-obesity and obesity periods, the majority of the animals retained calcium. Some were in equilibrium and, just prior to the onset of weight reduction, two were in calcium deficit. In the early phase of the reduction period in the majority of the animals calcium retention or equilibrium tended to shift into equilibrium or deficit. For some unexplained reason, calcium retention improved somewhat in the mid-retention phase. However, late in the reduction period there was a return to the poorer calcium retention found in the early reduction phase. With the elimination of caloric restriction in the post-reduction maintenance phase, calcium retention again improved. These results are similar to those obtained in young female human subjects, although data were not available for the early reduction phase for the human subjects.

No adequate explanation could be offered for the negative calcium balances observed in some of both the rat and human subjects during weight reduction.

Phosphorus retention was noted in practically all of the animals until caloric restriction in the reduction phase, at which time retention decreased. By late reduction over 80% of the animals were in phosphorus deficit; none were retaining phosphorus. With the restoration of calories for weight maintenance in the post-reduction period, all remaining animals returned to a state of phosphorus retention.

Control animals maintained at the relatively constant initial weight of 240 gm were less healthy throughout and several died before the time scheduled for sacrifice with initially paired experimental rats.

Bone densities of the left femur of paired control and experimental rats showed no consistent pattern with phases of the experiment which might be attributed to the production and reduction of initial obesity or to the calcium balance status in experimental animals. For all animal groups, bone density appeared to decrease as the experiment was prolonged and as the animals progressed from 32 to 48 weeks of age.

Manuscript received 10 August 1959.