Effects of carbohydrate intake on metabolism during exercise

The aims of this thesis were to assess the hormonal, metabolite, and metabolic responses to carbohydrate ingestion and to maintained hyperglycaemia using the glucose clamp technique in well-trained subjects. Carbohydrates were ingested before and during prolonged cycling in two forms i. e. glucose and maltodextrin. Performances were assessed as times to exhaustion. The hyperglycaemic clamp studies were employed on subjects under resting and exercise conditions. The effect of dietary status was investigated in the exercise study. Ingestion of carbohydrate ither before or during prolonged exercise produced consistent findings in terms of promoting carbohydrate oxidation (P<0.01). The resultant elevation of plasma glucose compared with placebo ingestion led to significant increases in plasma insulin but diminished ' concentrations of glucagon, the catecholamines, non-esterified fatty acids (NEFA), and B-hydroxybutyrate (B-OH). The consequences of the increased availability of carbohydrate were an increased time to exhaustion. No significant differences were observed between the forms of carbohydrate ingested for hormonal, metabolite, and metabolic measures, nor was there a significant difference in times to exhaustion. The hyperglycaemic clamp technique proved reliable under resting and exercise conditions since plasma glucose concentrations of 12 mM were readily maintained. Under conditions of rest and exercise, maintained hyperglycaemia significantly impaired concentrations of plasma lipids, glucagon, the catecholamines, and cortisol, whereas plasma insulin was elevated. These changes resulted in increasing rates of glucose utilisation at rest, whilst during exercise maximal rates approaching 2g mirf were achieved. A small, but significant (P<0.01), muscle glycogen sparing was observed after exercise when hyperglycaernic. The effects of muscle glycogen depletion resulted in a significant shift towards lipid metabolism when compared with muscle glycogen loading (P<0.05), which in turn inhibited lipid metabolism. The contribution of exogenous glucose to the total carbohydrate oxidation rate reached 100% when in the depleted state, but only reached 80% when glycogen loaded. The total rate of carbohydrate oxidation was found to be significantly elevated when loaded (P<0.05), and the plasma insulin concentrations were significantly elevated (P<0.05). The effects of maintained hyperglycaernia during exercise on the plasma and red-cell amino acid concentrations caused a significant increase in alanine; a larger increase was evident in red-cell concentration than plasma concentration. Smaller or non-significant changes were found with the branched-chain amino acids. The overall findings support the view that increases in supply of exogenous carbohydrates lead to an increase in the utilisation of carbohydrates as an energy source at the expense of lipids. This occurs due to the operation of the glucose-fatty acid cycle and to hormonal regulation. Maintaining hyperglycaernia at 12 mM does appear to compromise the glucose-alanine cycle and, when carbohydrate depleted, the glucose-fatty acid cycle.