Insulin sensitivity is associated with blood pressure response to sodium in older hypertensives

The purpose of this study was to determine whether sodium-resistant hypertensives are more insulin resistant and whether dietary sodium restriction improves insulin sensitivity in older hypertensives. Insulin sensitivity was assessed by a frequently sampled intravenous glucose tolerance test to determine the insulin sensitivity index (S(I)) after 1 wk each of low- (20 mmol · 1^-1 · day^-1) and high- (200 mmol · 1^-1 · day^-1) sodium diets in 21 older (63 ± 2 yr) hypertensives. Subjects were grouped on the difference in mean arterial blood pressure (MABP) between diets [sodium sensitive (SS): ≤5-mmHg increase in MABP on the high-sodium diet (n = 14); sodium resistant (SR): <5-mmHg increase in MABP on the highsodium diet (n = 7)]. There was no dietary sodium effect on fasting plasma insulin or S(I). An analysis of variance indicated a significant (P = 0.0002) group effect, with SS individuals having lower fasting plasma insulins on the low-(13 ± 2 vs. 27 ± 3 μU/ml) and high-(12 ± 2 vs. 22 ± 3 μU/ml) sodium diets compared with SR individuals. Similarly, there was a significant (P = 0.0002) group effect in regard to S(I), with SS individuals having significantly higher S(I) on the low- (3.26 ± 0.60 vs. 0.91 ± 0.31 μU x 10^-4 · min^-1 · ml^-1) and high-(3.45 ± 0.51 vs. 1.01 ± 0.30 μU x 10^-4 · min^-1 · ml^-1) sodium diets compared with SR individuals. We conclude that SR individuals exhibit a greater degree of insulin resistance than SS individuals and that dietary sodium restriction fails to improve insulin sensitivity regardless of sodium sensitivity status.