To determine the etiology of euglycemic ketoacidosis, the effect of a 32-h fast on the rate of metabolic deterioration was examined in a group of 10 healthy subjects with type I diabetes mellitus. Patients were studied during 5 h of insulin withdrawal after 8 h (postprandial) and 32 h (fasted) of food deprivation. Study parameters included substrate levels, electrolytes, counterregulatory hormone levels, and rates of glucose and glycerol turnover. In the fasted state, mean peak plasma glucose concentrations were significantly lower than those in the 8-h postprandial state (13.3 +/- 1.6 vs. 17.4 +/- 1.4 mmol/L, respectively; P < 0.05), and mean rates of glucose production were also significantly lower at all time points in the fasting state. The rate of development of ketosis was significantly more rapid during insulin deficiency after a fast (8.82 +/- 0.63 vs. 6.23 +/- 0.30 micro/L.min; P < 0.05), while plasma nonesterified fatty acids and glycerol turnover showed a biphasic response to insulin withdrawal, which was also more robust after a fast. Metabolic acidosis, as reflected in the rate of decrease in serum bicarbonate concentration, was more severe after 32 h of fasting than in the postprandial state (mean nadir, 15.4 +/- 0.9 vs. 18.6 +/- 0.5 mmol/L; P < 0.001). In contrast to values in the postprandial state, serum glucagon levels rose during insulin withdrawal in the fasting state, and plasma norepinephrine levels also correlated positively with the ongoing metabolic decompensation. Other counterregulatory hormones did not differ significantly in the fasted vs. postprandial states in these short term metabolic studies. We conclude that a fast of moderate duration, such as might be expected to occur during the development of diabetic ketoacidosis, predisposes patients with type I diabetes to euglycemic ketoacidosis during periods of insulin deficiency. Furthermore, decreased rates of hepatic glucose production are responsible for the lower plasma glucose values observed during a fast. The development of ketosis continued progressively in both conditions, but the rate of rise of plasma ketones was increased in the fasted state. This accelerated development of ketosis may be attributable to the effects of elevated levels of glucagon and/or catecholamines on lipolysis.