The mechanism of the dawn phenomenon was studied in 12 C-peptide-negative type I diabetic patients (age 30 ± 2 yr) treated with continuous subcutaneous insulin infusion. During constant basal infusion, nocturnal glycemia remained constant until 4 a.m., but began to rise thereafter in 10/12 patients, with the mean rise from 4.6 ± 0.4 mmol/L to 6.1 ± 0.7 mmol/L (P < 0.01) by 8 a.m. In these patients the rate of glucose production (Ra, 2.14 ± 0.04 mg/kg/min, 3-H3-glucose infusion) exceeded the rate of utilization (Rd, 1.89 ± 0.03 mg/kg/ min, P < 0.02). When the patients were restudied after the infusion rate was increased by 49 ± 7%, Ra fell to 1.75 ± 0.03 mg/kg/min (P < 0.01) and the dawn phenomenon was abolished. However, both Ra and Rd remained higher in the diabetic subjects (P / 0.05) than in eight healthy control subjects, in whom Ra (1.66 ± 0.02 mg/kg/min) was equal to Rd with glycemia remaining unchanged. Peripheral free insulin levels in the diabetic patients were similar during constant (12.3 ± 0.5 mU/L) and increased infusion rate (11.3 ± 0.4 mU/L), and higher than those of the control subjects (5.2 ± 0.2 mU/L, P < 0.05). A diurnal rise in serum cortisol levels occurred 1 h earlier in the diabetic than in the control subjects, and Ra was directly proportional to serum cortisol concentration (r = 0.61; P < 0.01). Serum growth hormone levels were also slightly higher in the diabetic than the control subjects.

In conclusion: (1) A dawn phenomenon is associated with an excessive rate of glucose production, rather than impaired utilization; (2) this may be explained, at least in part, by elevated counterregulatory hormone levels; and (3) a step-up in the overnight insulin delivery reduces hepatic glucose production and so prevents the dawn phenomenon.

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