Due to selective isotopic effects occurring during photosynthesis, certain natural sugars are enriched in 13C Using such “naturally labeled 13C-glucose,” we studied glucose oxidation during exercise in seven normal volunteers and in six insulin-dependent diabetics after an overnight fast. In the diabetics, blood glucose was monitored the night before the test and adjusted to about 100 mg/dl by intravenous insulin infusion. The insulin infusion was withheld 15 min before exercise in four diabetics and maintained at 0.9 U/h for 2 h; then it was maintained at 0.6 U/h for 2 h in five diabetics. Three patients underwent both tests. All subjects exercised on a treadmill for 4 h at about 45% of their max. After 15 min adaptation, all received 100 g 13C-labeled glucose orally. Total glucose oxidation was derived from non-protein RQ and exogenous glucose oxidation evaluated as previously described. The diabetics had no residual B-cell function as indicated by negligible plasma C-peptide values and a lack of Cpeptide response to the oral glucose challenge. Total glucose oxidation averaged 230 ± 14 g/4 h in the normal subjects. It was similar (238 ± 19 g/4 h) in the diabetics receiving an intravenous insulin infusion, but decreased to 176 ± 14 g/4 h when no insulin was infused. Exogenous glucose oxidation was 92 ± 3 g/4 h and 84 ± 8 g/4 h (not statistically different) in the controls and in the insulin-infused diabetics, respectively. It was 43 ± 11 g/4 h in the diabetics exercising without being infused with insulin. We conclude that (1) in well-insulinized diabetic patients, prolonged muscular exercise can be performed under metabolic conditions which are basically similar to those of normal subjects; (2) during prolonged exercise, well-insulinized diabetic patients are able to oxidize up to 85–90% of a 100-g exogenous glucose load given orally and oral glucose can thus be ingested during prolonged exercise in well-controlled juvenile insulintreated diabetics; (3) even in the absence of insulin administration during exercise, juvenile diabetics who start exercising when blood glucose is near normal are able to perform a 4-h exercise at 45–50% of their max. Under these conditions, however, they are unable to utilize more than 40–45% of a 100-g glucose load given orally. They rely more upon lipid stores than the normal subjects or the well-insulinized diabetics.

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