Dietary salt restriction increases vascular insulin resistance

Recent studies have shown that insulin has a direct vasodilator effect and that vascular sensitivity to insulin is impaired in hypertension. How the vasodilator effect of insulin is regulated physiologically is unknown. It has been appreciated that salt restriction may have adverse effects on glucose and lipid metabolism--processes regulated by insulin. To determine whether dietary salt restriction might affect vascular sensitivity to insulin, we studied 13 subjects (including eight borderline hypertensive subjects and five normotensive subjects) after 1 week of a normal sodium diet (240 mEq/day) and after 1 week of a low-sodium diet (20 mEq/day) with a randomized, double-blind crossover design.

Vascular sensitivity to insulin was assessed with the dorsal hand vein linear variable differential transformer technique. When the "normal" salt diet was given, vascular sensitivity for insulin was significantly less (i.e., dose that produced the half-maximal response [ED50] insulin was higher) in hypertensive subjects (ED50 insulin for hypertensive subjects, 5.75 milliunits (mU)/min; ED50 insulin for normotensive subjects, 0.23 mU/min; p < 0.05). Vascular sensitivity to insulin was inversely correlated with mean arterial pressure and plasma norepinephrine concentration. When the low salt diet was given, vascular sensitivity to insulin decreased in both the normotensive and hypertensive groups, paralleling an increase in plasma norepinephrine. Blood pressure was not significantly decreased by reducing salt intake.

In these younger normotensive and hypertensive subjects, dietary salt restriction increases resistance to the vasodilating effects of insulin.