PLASMA-GLUCOSE TURNOVER DURING COLD STRESS IN HUMANS

To clarify the source of increased carbohydrate oxidation during cold stress, six males rested for 3 h at 29 and 10 degrees C dressed only i n shorts. After priming the blood glucose and bicarbonate pools, [U-C- 13(6)]glucose was infused for 3 h in each condition to determine the p lasma glucose rate of appearance (R(a)) or turnover under relative ste ady-state conditions. Plasma enrichment (mel %excess) was determined b y selective ion-monitoring gas chromatography-mass spectrometry. Cold exposure decreased rectal temperature and mean skin temperature and in creased heat debt, metabolic rate, and whole body lipid and carbohydra te oxidation (CHOox) compared with the same subjects at thermal neutra lity (P < 0.05). Cold exposure significantly increased R(a) from 13.18 +/- 0.70 to 16.22 +/- 0.43 mu mol . kg(-1). min(-1) (P < 0.05). Plasm a glucose clearance was elevated commensurately by the cold (from 2.68 +/- 0.16 to 3.55 +/- 0.14 ml . kg(-1). min(-1); P < 0.05). If we assu me that R(a) is completely oxidized (thus equivalent to maximum rates of plasma glucose oxidation) [J. A. Romijn, E. F. Coyle, L. S. Sidossi s, A. Gastaldelli, J. F. Horowitz, E. Endert, and R. R. Wolfe. Am. J. Physiol. 265 (Endocrinol. Metab. 28): E380-E391, 1993], the minimum ra tes of glycogen and lactate oxidation in the cold would be the differe nce between CHOox and glucose oxidation (similar to 14.0 +/- 3.0 mu mo l . kg(-1). min(-1)). Therefore, under the present laboratory conditio ns, 54% of CHOox would be fueled by plasma glucose oxidation, whereas the remaining 46% would be derived fr om the combination of glycogen a nd lactate oxidation. The results of the present study demonstrate tha t cold exposure in humans enhances the turnover and metabolic clearanc e of plasma glucose. The results also suggest that carbohydrate oxidat ion during cold stress appears to be about equally divided between the oxidation of plasma glucose and the combination of lactate and intram uscular glycogen.