The effect of age upon the influx of glucose into the brain.

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Abstract
Rats aged from 2-116 wk were studied. Influx of glucose into the brain was low in suckling rats but rose after weaning, to reach its highest level in the young adult, thenceforward declining slowly as age increased. The blood-brain barrier for glucose was fully developed in the rat by the age of 18 days and glucose entered the brain, at this stage, by carrier-mediated transport, as in the adult. The low influx of glucose into the brain of the suckling animal was due to a low maximum rate of transport of glucose rather than to a low affinity of the carrier-molecule for glucose. In the young adult rat, efflux of glucose back from the brain into the blood was greater than in either the suckling or the old animals. Thus the margin of safety, i.e., the extent to which the blood glucose can be reduced without affecting the utilization of glucose by the brain, was highest in the young adult. The lower margin of safety in the suckling animals was compensated for by the high influx of the ketone bodies which provide an alternative source of energy at this age. In the old animals there was no alternative source of energy, so that the older brain was at greatest risk in hypoglycemia.

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