The effects of substrate and fluid provision on thermoregulatory, cardiorespiratory and metabolic responses to prolonged exercise in a cold environment in man

Abstract

SUMMARYDuring prolonged exercise in a cold environment, fatigue is generally associated with a depletion of endogenous glycogen stores. This has lead many authors to hypothesize that the carbohydrate (CHO) content of fluids ingested in cool environments should be high, yet this hypothesis has not been specifically examined. In the present study, six healthy males cycled to exhaustion at approximately 80 % of their maximum oxygen consumption (Vo2,max) with either no drink (ND), a 15 % CHO-electrolyte drink (15 % CHO) or a 2 % CHO-electrolyte drink (2 % CHO). Dietary intake and exercise were replicated 2 days prior to each trial. Mean (S.D.) ambient temperature was 10.0 (0-3)°C with a relative humidity of 72 (2) % and an air velocity of approximately 0-7 m s-5 on all trials. Weighted mean skin temperature was calculated, and rectal temperature and heart rate were recorded at rest, during exercise and at exhaustion. Venous samples were drawn before and during exercise and at exhaustion for determination of haemoglobin, haematocrit, blood metabolites and serum electrolytes and osmolality. Expired air was collected for calculation of VO and respiratory exchange ratio which were used to estimate rates of fuel oxidation. Ratings ot perceived exertion (RPE) were also obtained. Exercise capacity was not different (P = 0.49) between trials, with median (range) times to exhaustion of 90 6 (66 8-106 0), 97-7 (60*9-112 0) and 102.0 (77.1-170.7) min for the ND, 15 % CHO and 2 % CHO trials, respectively. The 15 % CHO drink significantly (P < 0.05) elevated blood glucose concentration and total CHO oxidation compared with the ND trial. The 2 % CHO drink significantly (P < 0.05) reduced the estimated change in plasma volume and resulted in a lower serum osmolality than the other two trials. No differences were observed in any thermoregulatory or cardiorespiratory responses between trials. These data suggest that maintenance of blood-borne substrate and/or plasma volume does not influence exercise capacity in the cold, as is commonly reported during this type and intensity of exercise in warmer environments.

Neuromuscular responses to hydration in moderate to warm ambient conditions during self-paced high-intensity exercise

“…Previous studies have shown that differences in environmental temperature result in differences in fixed intensity exercise duration consistent with the development of hyperthermia and fatigue, with alterations to the physiological and subjective responses 1 2. Fluid ingestion is a practical intervention strategy used to improve exercise in the heat by reducing thermoregulatory strain3 by offsetting the effects of dehydration, which at levels of less than 2% reduction in body mass has been shown to compromise exercise performance 4.…”

Modulation of carbohydrate and fat utilization by diet, exercise and environment

“…Studies have generally found increases in the relative contribution of carbohydrate and decreases in the relative contribution of fat [36,37] to energy expenditure although some studies have reported no change with moderate cold exposure [38]. During extreme cold shivering is a mechanism to increase energy expenditure and heat production.…”

Fluid and fuel intake during exercise

“…Discussion of the physiological mechanisms mediating the performance effects will be limited to that which helps elucidate the practical application or refine the recommendation for fluid and fuel intake during exercise. Several excellent scientific review articles have already detailed the physiological mechanisms mediating reduced physiological strain during exercise by ingesting fluids, carbohydrate and electrolytes (Galloway and Maughan, 1998;Sawka and Coyle, 1999;Sawka and Montain, 2000;Cheuvront and Haymes, 2001;Cheuvront, 2001;Sawka et al, 2001). The last 40 years have seen dramatic shifts in both scientific thinking and popular practice regarding water, carbohydrate and salt intake during exercise.…”