During exercise in the cold increased availability of plasma nonesterified fatty acids does not affect the pattern of substrate oxidation

Exercise in the cold was investigated to establish if the relative contribution of fat to energy expenditure is affected by the increased availability of circulating nonesterified fatty acids (NEFA). Seven men after an overnight fast cycled at approximately 70% of peak oxygen uptake for 60 minutes at an ambient temperature of 0.0 degrees C +/- 0.1 degrees C. Fifteen minutes prior to exercise and then throughout the exercise, subjects were infused with either heparin (heparin) or saline (control). Immediately before exercise NEFA concentration (control, 0.27 +/- 0.04 mmol. L(-1); heparin 1.09 +/- 0.13 mmol. l(-1)) was significantly higher (P <.05) in the heparin trial. Pre-exercise concentration of plasma triacylglycerol (TG), blood glycerol, glucose, oxygen consumption (VO(2)) and respiratory exchange ratio (RER) were not significantly different between heparin and control trials. During exercise, plasma NEFA and blood glycerol concentrations were significantly higher (P <.05) in the heparin trial, and levels of plasma TG and glucose were not different between trials. Over the exercise period rectal temperature, mean skin temperature, VO(2), RER, and heart rate (HR) were not different between the 2 trials. Gross energy expenditure of cycling (control, 3.3 +/- 0.1 MJ; heparin 3.3 +/- 0.1 MJ), the oxidation rates of fat (control, 0.67 +/- 0.05 g. min(-1); heparin, 0.71 +/- 0.06 g. min(-1)) and carbohydrate (CHO) (control, 1.68 +/- 0.04 g.min(-1); heparin, 1.62 +/- 0.17 g. min(-1)) and the proportion of energy derived from fat (control, 43 +/- 4%; heparin trial, 44 +/- 9%) and CHO (control, 57 +/- 4%; heparin trial, 56 +/- 4%) were not different between the 2 trials. These findings suggest that despite increased availability of plasma NEFA, the pattern of substrate oxidation during exercise in cold temperatures does not change. This implies that uncoupling between the availability and oxidation of plasma NEFA may be a mechanism involved in the reduced oxidation of fat seen during cold exposure. Further research is needed on the utilization of intramuscular TG and circulating plasma TG-rich lipoproteins in the cold.