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Chronic Protein Restriction Does Not Alter Energetic Efficiency or Brown Adipose Tissue Thermogenic Capacity in Genetically Obese (fa/fa) Zucker Rats^1,2,

S. E. Specter^*, J. S. Hamilton, J. S. Stern^*,#, and B. A. Horwitz^,3

^* Department of Nutrition Section of Neurobiology, Physiology, and Behavior, Division of Biological Sciences ^# Department of Internal Medicine, School of Medicine, University of California, Davis, CA 95616

Attenuated regulatory thermogenesis in genetically obese (fa/fa) Zucker rats involves an impaired capacity to increase sympathetic drive to brown adipose tissue in response to dietary stimuli. Young, growing lean rats fed a low protein diet reduce energetic efficiency to compensate for elevated energy intake; however, it is not known if obese rats adapt similarly to chronic protein restriction by decreasing energetic efficiency and whether this would be accompanied by increased brown adipose tissue thermogenic capacity. Lean (Fa/Fa) and obese Zucker rats were either protein-restricted (protein 8% of total energy) or fed a control diet (21% protein) starting at age 5 wk. At 9 wk, oxygen consumption (VO₂) was measured in response to an intubated meal of mixed macronutrient composition. Mass-adjusted food intake (i.e., food intake/body weight^0.67) was greater in protein-restricted than in control lean rats, but not different due to diet in obese rats. Mass-adjusted brown adipose tissue uncoupling protein levels were more than 100% greater in protein-restricted vs. control lean rats, but not different between protein-restricted and control obese rats. The uncoupling protein level was not significantly different in control lean vs. obese rats. Energetic efficiency was 40% lower in protein-restricted vs. control lean, but not different in obese rats; however, the efficiency of protein utilization was significantly greater in obese protein-restricted than in obese control rats. Meal-induced energy expenditure (VO₂) did not differ significantly due to diet or genotype. In conclusion, protein restriction led to overfeeding in lean rats which appeared to enhance brown adipose tissue thermogenic capacity and decrease energetic efficiency. Protein efficiency increased to more than two times its original value in obese (fa/fa) rats, but otherwise no metabolic accommodation in the capacity for regulatory thermogenesis was observed in this genotype.

KEY WORDS: • protein restriction • thermogenesis • obesity • Zucker rats • brown adipose tissue

¹ Supported in part by National Institutes of Health grants DK 32907, DK18899, and DK35747 and the Nestlé Foundation.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence and reprint requests should be addressed.

Manuscript received 5 July 1994. Revision accepted 22 February 1995.