Pre-exercise ingestion of carbohydrate and transient hypoglycemia during exercise

To study the occurrence and contributing factors of transient hypoglycemia after pre-exercise ingestion of glucose after a 4-hour fast, 19 well-traine d cyclists ingested 50 grams of glucose dissolved in water around noon afte r having a normal breakfast. The ingestion of the glucose solution was foll owed by 30 minutes rest after which the subjects cycled for 40 minutes at 6 0% of the predetermined maximal power output. Every 10 minutes blood was sa mpled for determination of glucose, catecholamines, and insulin concentrati ons. In 6 subjects (hypo-group) plasma glucose levels dropped transiently b elow 3.0 mmol/l, while in the other 13 subjects (non-hypo group) plasma glu cose level remained above this level. Although at the onset of exercise the plasma glucose levels were lower in the hypo-group, insulin levels were si milar in both groups, suggesting a higher insulin sensitivity in the hypo-g roup. During exercise, norepinephrine was lower in the hypo-group, indicati ng a lower sympathetic activity in the hypo-group. The lowest plasma glucos e levels in both groups were observed after 20 minutes of exercise, after w hich plasma glucose concentration returned to normal levels. It is conclude d that pre-exercise carbohydrate ingestion after a 4-hour fast is sufficien t to induce a transient hypoglycemia. The data suggest that the occurrence of hypoglycemia is determined by a combination of a high insulin sensitivit y, a small amount of ingested glucose, and a low sympathetic activity.