Differential effect of dietary salt intake on insulin sensitivity in salt-sensmve and -resistant older hypertensive humans

It has been demonstrated in younger normotensive individuals that low salt intake results in an increase in insulin sensitivity (Donovan et al. , 1993). The present study was undertaken to test whether dietary salt restriction similarly increases insulin se. sitivity in older, insulin resistant, hypertensive individuals. Insulin sensitivity was assessed by either a 2dose (low: 40 mU-m'min'1 and high: 100 mUmmin1) hyperinsuUnernic euglycemic clamp to determine glucose disposal rate (GDR: mg/kgnTnin) or a frequently sampled intravenous glucose tolerance test (FSIGTT) to determine the insulin sensitivity index (S, : xlO Vmin/nU/mL) after one week of low (920 mEq Na'/d) and high (8200 mEq Na'/d) salt diets in fifteen older (69±1 yrs), overweight (BMI: 29±1 kg/m1), hypertensive nondiabetic men (n=7) and women (n=8). GDR tow dose (n-9) GDR high dose (n=9) S, (n=6) High Salt 5. 9±0. 9 12. 2±13 25±0. 8 Low Salt 5. 9±0. 9 12. 1±1. 3 25±0. 7 These measures of insulin sensitivity are below normal suggesting that these subjects have metabolic insulin resistance. There was no overall apparent effect of dietary salt intake on insulin sensitivity whether measured by clamp or FSIVGTT. However, when grouped by salt sensitivity of blood pressure (salt-sensitive, SS; £5 mm Hg increase in mean arterial blood pressure on high salt; salt-resistant, SR : <5 mm Hg increase), measures of insulin sensitivity were lower on low salt intake in SS (n=10), but higher in SR (n=5) subjects (percent difference: -19±9 vs. 37±17, ANOVA P=0. 007). These data suggest that dietary salt restriction increases insulin sensitivity only in older, SR hypertensive individuals. We hypothesize that the decrease in blood pressure which develops in SS subjects on a tow salt diet may prevent an increase in their insulin sensitivity.