We have determined total body carbohydrate and lipid oxidation rates in response to a standard breakfast in nine obese patients with non-insulin-dependent diabetes mellitus (NIDDM) and in seven age-and weight-matched controls. The patients with NIDDM were studied twice, once while in poor glycemie control (fasting blood glucose concentration 267 ± 24 mg/dl, urinary glucose excretion 28.9 ± 6.3 g/24 h) and again after modest glycemie improvement following 2 mo of fiber treatment (fasting blood glucose 227 ±19 mg/dl, urinary glucose excretion 10.7 ±1.9 g/24 h). Basal carbohydrate (CHO) oxidation rates were normal in patients with NIDDM before and after fiber treatment. However, in patients before fiber treatment the rise in CHO oxidation rates, the reciprocal fall in lipid oxidation rates, and the rise in serum insulin and C-peptide concentrations after the breakfast were all severely blunted. In addition, storage of ingested CHO was significantly reduced (from 55% to 32%, P < 0.05). After fiber treatment, postbreakfast CHO oxidation rates had improved and were no longer significantly lower than control values. In contrast, CHO storage remained suppressed. We conclude that (1) basal CHO oxidation remained normal but that postbreakfast CHO oxidation was impaired in our obese patients with NIDDM. This impairment, however, appeared to be a relatively late event, occurring only during severely uncontrolled NIDDM. (2) Inability to dispose of CHO by storage appeared to be an earlier defect with a greater impact on glucose tolerance than the impairment of CHO oxidation.

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