Effect of dietary sodium on insulin sensitivity in older, obese, sedentary hypertensives

Increased dietary sodium intake has been associated with an increase in blood pressure as well as a decrease in insulin-mediated glucose disposal in young healthy adults. The purpose of this study was to determine whether dietary sodium intake is associated with changes in oral glucose tolerance, insulin sensitivity, and blood pressure in older, sedentary, overweight hypertensives. Eight older (70.0 +/- 1.4 years, mean +/- SEM), overweight (40.2 +/- 3.1% body fat), mildly hypertensive (151 +/- 8/82 +/- 2 mm Hg) patients with a fasting plasma glucose < 7.8 mmol/L were studied after 2 weeks on low (3 g/day) and 2 weeks on high (10 g/day) sodium diets. To examine carbohydrate metabolism we performed a 2 h oral glucose tolerance test and a two-dose (240 and 600 pmol/m2/min) hyperinsulinemic-euglycemic clamp at the end of each sodium diet. High sodium intake was associated with a significantly greater urinary sodium excretion (364 +/- 45 mmol/day v 112 +/- 21 mmol/day; P < .0001). The increase in dietary sodium from low to high did not result in significant differences in fasting plasma glucose (6.0 +/- 0.2 v 5.8 +/- 0.1 mmol/L, P = .20) or insulin (72.5 +/- 7.8 v 69.9 +/- 12.4 pmol/L, P = 0.71) levels or in the glucose (374.0 +/- 50.8 v 493.2 +/- 45.0 mmol/min/L, P = .12) and insulin (43,783 +/- 10,278 v 44,110 +/- 12,392 pmol/min/L, P = .96) areas determined during the oral glucose tolerance test. Similarly, there was no effect of dietary sodium on insulin-mediated glucose disposal at low (5.87 +/- 1.02 v 5.60 +/- 0.94 mg/kgLBM/min, P = .36) or high (12.15 +/- 1.49 v 11.91 +/- 1.49 mg/kgLBM/min, P = .64) insulin infusion rates. Our findings suggest that, in insulin resistant hypertensives, increased dietary sodium does not affect either glucose or insulin responses during an oral glucose tolerance test or glucose disposal during a hyperinsulinemic euglycemic clamp.