Mental alertness in response to hypoglycaemia in normal man: the effect of 12 hours and 72 hours of fasting.

To study the influence of hypoglycaemia and starvation on mental functions eight healthy male students age 25-34 years with an ideal body mass of 99.9% +/- 2.5% (mean +/- SEM) were recruited. Hypoglycaemia was induced in random order by an insulin-glucose clamp technique (insulin: 2.4 mU/kg/min + glucose at variable rate) keeping the venous blood glucose at 2.2 mmol/l both after an overnight fast and after 72 h fasting. Mental alertness was assessed by measuring the recognition time, moving time and total reaction time to a visual signal and by a verbal mental clearness test and a synonym learning test during normo- as well as hypoglycaemia. Hypoglycaemia prolonged the total reaction time (p less than 0.05) and the time required for the mental clearness test (p less than 0.05). Compared with a control study performed at normoglycaemia the learning effect of the synonym test was reduced by hypoglycaemia. Fasting, which resulted in a body weight reduction of 2.6 +/- 0.3 kg and ketonuria prolonged the total reaction time (p less than 0.005) by increasing the moving time but did not affect the mental clearness test. When hypoglycaemia was preceded by 72 h fasting it did not increase the total reaction time, nor did it modify the mental clearness test. Moreover, the learning effect of the synonym test was less impaired. In conclusion, mental alertness was reduced by moderate hypoglycaemia after an overnight fast while similar hypoglycaemia did not reduce mental alertness after prolonged fasting. This may illustrate a decrease of the glucose dependency of the central nervous system during prolonged fasting.