THE EFFECT OF DIETARY SALT ON INSULIN SENSITIVITY

Acute reduction of salt intake causes an increase in serum lipid and insulin levels in healthy volunteers and patients with essential hypertension, suggesting induction of insulin resistance by salt restriction. Direct measurements of insulin sensitivity using the euglycaemic clamp showed no significant change after 7 days of salt restriction. Our previous study showed a time dependent course of dyslipidaemia after institution of a low salt diet. We therefore assessed insulin sensitivity (M-value) under euglycaemic conditions (clamp technique) at discrete time points using a parallel group design. Two groups of healthy males were examined on high (200 mmol d(-1)) and low (20 mmol d(-1)) salt intake. One group (n = 7, 25 +/- 3 years, BMI 22.4 +/- 2.1 kg m(-2)) received high and low salt diet in random order each for 7 days. The other group (n = 7, 26 +/- 3 years, 22.1 +/- 1.9 kg m(-2)) received the respective diet in random order for 3 days. A significantly (P < 0.01) different mean M-value was noted in the group receiving the diets for 3 days, i.e. after low salt intake it was 7.4 +/- 1.2 mg kg(-1) min(-1) and after high salt intake 8.6 +/- 1.1 mg kg(-1) min(-1). In contrast, the mean M-value was similar after low and high salt periods in the group of individuals who had been studied after 7 days on either salt take (7.8 +/- 1.8 on low salt vs. 7.6 +/- 1.3 mg kg(-1) min(-1) on high salt). There were no differences in baseline mean serum glucose and potassium levels on either salt intake, whereas baseline mean serum insulin concentrations were significantly (P < 0.05) higher on low salt intake as compared with high salt intake in the group on diets for 3 days and for 7 days, respectively. Mean plasma renin activity, angiotensin II and noradrenaline levels were higher after low salt than after high salt intake in both groups of volunteers. The data suggest that modulation of dietary salt intake can influence insulin sensitivity in healthy volunteers. The change in insulin sensitivity is, however, an evanescent phenomenon as previously also shown for dyslipidaemia. Extrapolations from acute effects of severe salt restriction on glucose and lipid metabolism to long-term consequences of moderate low salt diet in the therapy of hypertension are therefore not warranted.