Estimation of the Sympathetic and Thyroidal Partitions to Diet Induced Thermognesis in the Rat

The physiologic mechanism of diet induced thermogenesis (DIT) during overfeeding implicates multiple hormonally mediated biochemical pathways of energy metabolism. The effects of overfeeding highly palatable diets via the cafeteria feeding diet method (café) is a reliable and reproducible experimental model to induce DIT in normally lean rats of several strains. Groups of adult lean LANtul//-cp rats were offered a Purina chow (CHOW) diet or the same diet plus a daily café supplement from 10 until 24 weeks of age. The noradrenergic responses enable an organism to elicit a rapid response to accommodate short term thermogenic demands of diet and environment, and act via activation of thermogenesis in BAT in man and animals. The sympathoplegic drug -methylparatyrosine ( MPT) was administered groups (250 mg -MPT/kg BW, i.p.) to ablate sympathetic (SNS) activity or a sham injection of physiologic saline given, and measures of resting thermogenesis obtained at thermal neutrality (30°C) before and after the -MPT or sham administration. Following café overfeeding, both groups experienced a 67% increase in body weight (BW) and a 25% increase in VO2, whereas normally CHOW fed rats experienced only a 40% increase in BW and no additional improve in VO2, and the sham injection had no effect on normal physiological reactions as predicted. Sympathetic blockade with -MPT was associated with modest decreases in body temperature and an average 15% decrease in VO2 in both -MPT treated groups, but when the RMR data were arithmetically corrected to isothermal conditions only a net 3% decrease occurred in CHOW fed rats, while the -MPT treatment resulted in a 12 % decrease in both café groups. Serum T3 concentrations increased by 86 % and urinary catecholamine excretion of VMA >100% following the café diet, but the VMA excretion was virtually nil following the sympathetic blockade. These findings suggest that the SNS-mediated contribution component to DIT following prolonged café overfeeding under conditions of thermal neutrality contribute approximately 50% of the thermic response, compared to as little as 3% of the thermic response in normally fed rats, and that the total thermic response of DIT likely indicates a mix of short acting SNS and longer acting non-SNS mechanisms, including a likely important thyroidal component in normally lean animals.