Removal of glucose-U-C-14 from plasma was studied in normal subjects and in patients with maturity-onset diabetes mellitus. All measurements were made after an overnight fast, during a period in which plasma glucose and immunoreactive insulin concentrations were not changing. The mean irreversible loss rate (ILR) of glucose from the plasma of normal subjects was not different from that of patients with diabetes: mean ILR (± the coefficient of variation) of all patients was 1.36 (± 14 per cent) mg. glucose/kg, body weight per min. This narrow range attests to the similarity of ILR in the patients studied. There was no correlation between glucose ILR and plasma glucose concentration, plasma immunoreactive insulin concentration, or ponderal index. In contrast, fractional glucose irreversible loss rate varied considerably from patient to patient, with a coefficient of variation of 35 per cent. Furthermore, there was a strong negative correlation between plasma glucose concentration and fractional glucose irreversible loss rate (r = −0.82, p < .01). This decrease in efficiency of glucose removal from plasma of hyperglycemic patients was not related to an absolute lack of insulin. It is concluded that net glucose utilization in the fasting state is similar in normal and mildly diabetic patients, and that fasting hyperglycemia is due to a decrease in efficiency of glucose removal which cannot be attributed to absolute insulin deficiency.

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