Effects of a 24-h carbohydrate-poor diet on metabolic and hormonal responses during prolonged glucose-infused leg exercise

Summary

Extant literature dealing with metabolic and hormonal adaptations to exercise following carbohydrate (CHO) reduced diets is not sufficiently precise to allow researchers to partial out the effects of reduced blood glucose levels from other general effects produced by low CHO diets. In order to shed light on this issue, a study was conducted to examine the effects of a 24-h CHO-poor diet on substrate and endocrine responses during prolonged (75 min; 60%$\dot V_{O_{2max} }$) glucose-infused leg exercise. Eight subjects exercised on a cycle ergometer in the two following conditions: 1) after a normal diet (CHO_N), and 2) after a 24-h low CHO diet (CHO_L). In both conditions, glucose was constantly infused intravenously (2.2 mg · kg⁻¹ · min⁻¹) from the 10th to the 75th min of exercise in relatively small amounts (10.4±0.8 g). No significant differences in blood glucose concentrations were found between the two conditions at rest and during exercise although a significant increase (p<0.01) in glucose level was observed in both conditions after 40 min of exercise. The CHO_L as compared to the CHO_N condition, was associated with significantly (p<0.05) lower resting concentrations of insulin, muscle glycogen (8.7 vs 10.6 g · kg⁻¹), and triacylglycerol, and greater concentrations of Β-hydroxybutyrate (0.5 vs 0.2 mmol · L⁻¹), and free fatty acids. During exercise, the CHO_L condition as compared to the CHO_N condition, was associated with significantly (p<0.05) lower insulin and R values, as well as greater free fatty acid (from min 20 to 60) and epinephrine (min 60 to 75) concentrations. Norepinephrine and glucagon concentrations also showed a net tendency (p<0.06) to be higher in the CHO_L condition. There were no significant differences at rest and during exercise in blood lactate and cortisol concentrations between the two conditions. These results demonstrate that blood glucose is not the sole determinant of the metabolic and hormonal responses during prolonged exercise following a low CHO intake and indicate that other factors may be involved in the regulatory mechanism.