The facilitation of glucose disposal (Staub-Traugott effect) and potentiation of serum insulin (IRI) concentration normally occurring after closely spaced intravenous glucose loads, are known to disappear after prolonged starvation. To study the effects of minimal amounts of glucose during fasting upon the insulin response and disposal of repeated intravenous glucose tolerance tests, obese volunteers were fasted for a mean of 25 +/- 2 days, while receiving either 8 or 16 gm of oral glucose every 6 hr, and compared to totally fasted subjects without glucose supplementation. Weight loss rate and the fall in basal IRI and glucose levels were similar to those of totally fasted subjects. However, the Staub-Traugott effect and insulin secretory dynamics after stimulation by repetitive intravenous glucose loading were preserved by this glucose modified fast, while baseline serum glucagon levels (IRG) were significantly lower, and the basal IRI/IRG ratios were thus unchanged from the fed state. IRG and free fatty acid suppression were similar in the fed and glucose modified fasted states. Lactic acid levels increased as expected after the repeated glucose injections in the fed state, but failed to do so after the prolonged modified fast until the second and third repetitive glucose loads, in which a significant rise coincided with accelerated glucose disposal. It is suggested that minimal amounts of carbohydrate during fasting preserve the insulin potentiating action of glucose, preferentially sparing a delayed releasable pool of insulin, while protecting the glucose utilization mechanisms, including increased glycolysis, responsible for the Staub-Traugott effect.