IS ENERGY SUBSTRATE MOBILIZATION A LIMITING FACTOR FOR COLD THERMOGENESIS

Energy substrate mobilization has been suggested as being a limiting f actor for the rate of cold-induced thermogenesis (M), and consequently in delaying hypothermia. The evidence supporting this hypothesis in h umans, however, is not convincing and the hypothesis has yet to be tes ted in a rigorous manner using a full heat balance analysis (partition al calorimetry). The goal of this study was therefore to re-investigat e whether enhancing energy substrate mobilization by feeding cold-expo sed subjects would improve M and affect heat debt (S; the minute-by-mi nute balance of M and heat losses) as well as rectal (T(re)) and mean skin temperatures (T(sk)BAR). Nine healthy semi-nude fasted subjects w ere exposed to 5-degrees-C (3 h at rest, 1 m.s-1 wind) on three occasi ons following the ingestion at min 0 and 90 of either: (1) a placebo, (2) 710 kJ of pure carbohydrates (100%-CHO), or (3) 710 kJ of a high-c arbohydrate bar (High-CHO). As expected in the cold, T(re) and T(sk)BA R decreased whereas M, S and heat losses increased (P<0.01). However, there were no differences between treatments, including the final T(re ) [mean (SEM); 36.4 (0.2); 36.5 (0.3) and 36.5 (0.2)degrees-C for the placebo, 100%-CHO and High-CHO tests, respectively]. During the 100%-C HO treatment, rates of carbohydrate oxidation were the highest and fat oxidation the lowest (P<0.05), whereas the High-CHO treatment caused smaller changes. The results demonstrate that in the cold, enhancing e nergy substrate mobilization by ingesting substrates in the form of a supplement containing either mainly or only CHO does not cause detecta ble changes in M, heat loss, S or body temperatures, compared to the i ngestion of a placebo. Under the present conditions, the results do no t support the theory that energy substrate mobilization is a limiting factor for cold-induced thermogenesis in humans.