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Nitrogen and Protein Losses in Rats during Weight Reduction with a High Protein, Very Low Energy Diet or Fasting¹

Department of Food Science and Human Nutrition, Michigan State University, East Lansing, MI 48824

Male Osborne-Mendel (OM) 220- to 240-day-old rats weighing 701 ± 19 g (relative weight 146%) that had been fed a high fat diet were reduced for 12 days by feeding either a high protein, very low energy (HPLE) diet or by fasting. The amount of HPLE diet fed to each rat was limited so that the daily food allotment provided only 19% of the energy of the high fat diet but the same quantity of nitrogen, minerals and vitamins. Control rats were fed the high fat diet ad libitum. In the 12 days, the HPLE group and fasted rats lost 16 and 18% of their initial body weight, respectively. All groups of rats were in negative nitrogen balance on the first day of the experiment. However, fasted rats lost 8–11 times more nitrogen the first day than HPLE or control rats, respectively. After the first day, controls and HPLE rats approached nitrogen balance. However, cumulative nitrogen loss for the fasted rats was a mean and SE of 808 ± 87 mg nitrogen over the 12 days. Livers of fasted rats weighed significantly less than livers for the other 2 groups and contained 58 and 67% as much nitrogen as controls and HPLE rats, respectively. Livers from fasted rats contained approximately 50% more lipid than controls and HPLE rats. Values for hearts including heart weights, total protein and RNA, were similar for HPLE and fasted rats, and values for all three parameters for both groups were significantly lower than for controls. The kidneys of fasted rats weighed 15% less (P < 0.01) than those of controls or HPLE rats. Compared to fasting, use of the HPLE diet for short-term weight reduction corrected nitrogen, liver weight and protein and kidney weight losses but did not significantly alter body weight or heart weight, protein and RNA losses.

KEY WORDS: • weight reduction • very low energy diet • fasting • rats • protein loss

¹ Michigan Agricultural Experiment Station Journal Article No. 10336. Supported in part by Biomedical Research Support Grant RR 07049-17, National Institutes of Health.

² Current address: Eskaton Monterey Hospital, Monterey, CA 93940.

Manuscript received 23 December 1982.