SUBSTRATE UTILIZATION DURING EXERCISE AND SHIVERING

It is generally assumed that exercise and shivering are analogous proc esses with regard to substrate utilisation and that, as a consequence, exercise can be used as a model for shivering. In the present study, substrate utilisation during exercise and shivering at the same oxygen consumption ((V) over dotO(2)) were compared. Following an overnight fast, eight male subjects undertook a 2-h immersion in cold water, des igned to evoke three different intensities of shivering. At least 1 we ek later they undertook a 2-h period of bicycle ergometry during which the exercise intensity was varied to match the (V) over dotO(2) recor ded during shivering. During both activities hepatic glucose output (H GO), the rate of glucose utilisation (Rd), blood glucose, plasma insul in, free fatty acid (FFA) and beta-hydroxybutyrate (B-HBA) concentrati ons were measured. The (V) over dotO(2) measured during the different levels of shivering averaged 0.49 l . min(-1) (level 1: low), 0.6 l mi n(-1) (level 2: low-moderate), and 0.9 l min(-1) (level 3: moderate), and corresponded closely to the levels measured during exercise. HGO a nd Rd were greater (P < 0.05) during exercise than during shivering at the same (V) over dotO(2) (9.5% and 14.7%, respectively). The average (SD) HGO during level 3 exercise was 3.0 (0.91) mg . k(-1) . min(-1) compared to 2.76 (1.0) mg . kg(-1) . min(-1) during shivering. The val ues for Rd were 3.06 (0.98) mg . kg(-1) . min(-1) during level 3 exerc ise and 2.68 (0.82) mg . kg(-1) . min(-1) during shivering. Blood gluc ose levels did not differ between conditions, averaging 5.4 (0.3) mmol . l(-1) over all levels of shivering and 5.2 (0.3) mmol . l(-1) durin g exercise. Plasma FFA and B-HBA were higher (P < 0.01) during shiveri ng than during corresponding exercise (12.3% and 33.3%, respectively). FFA averaged 0.61 (0.2) mmol . l(-1) over all levels of shivering and 0.47 (0.16) mmol . I-1 during exercise. The figures for B-HBA were 0. 44 (0.13) mmol . l(-1) during all levels of shivering and 0.32 (0.1) m mol . l(-1) during exercise. Plasma insulin was higher (P < 0.05) duri ng level 2 and 3 shivering compared to corresponding exercise; at thes e levels the average value for plasma insulin was 95.9 (21.9) pmol . l (-1) during shivering and 80.6 (16.1) pmol . l(-1) during exercise. On the basis of the present findings it is concluded that, with regard t o substrate utilisation, shivering and exercise of up to 2 h duration should not be regarded as analogous processes.