We studied the effect of dietary NaCl restriction performed in middle-aged/older adults (8 men and 3 women;60+/-2 yrs) with moderately elevated systolic BP (SBP: 139+/-2;DBP: 83+/-2 mmHg). We measured urinary and plasma MBG levels, SBP, aPWV (randomized, placebo-controlled, cross-over design). We also explored the associations of MBG excretion with SBP and aPWV across conditions of 5 weeks of a low NaCl (LS;77+/-9 mmol/day) and 5 weeks of a normal NaCl (NS;144+/-7 mmol/day) diet. Markers of oxidative stress were also assessed. SBP (127+/-3 mmHg vs. 138+/- 5 mmHg;P<0.05) and aPWV (700+/-40 cm/sec vs. 843 +/- 36 cm/sec, 17% of reduction, P<0.05) were reduced following a 5-week low sodium vs. 5-week normal sodium diet (randomized, cross-over design). Urinary MBG excretion (weekly measurements) was lower during the LS vs. NS condition (25.4+/-1.8 vs. 30.7+/-2.1 pmol/kg/day, p<0.05) and was positively correlated with SBP (r=0.39, p<0.001) and sodium excretion (r = 0.46, p<0.001) across the 10 weeks. These relations persisted during the NS but not the LS condition (p<0.005). MBG excretion also was positively related to aPWV (r=0.51, p<0.05) and endothelial cell expression of the oxidant enzyme NAD(P)H oxidase-p47phox (r=0.53, p<0.05). In summary, the present results demonstrate that dietary sodium restriction reduces renal MBG excretion in middle-aged and older adults with moderately elevated systolic blood pressure. Our findings suggest that this reduction in MBG may contribute, possibly via reduced oxidative stress, to the reductions in large elastic artery stiffness and SBP that accompanied dietary sodium reduction. Given age-associated increases in arterial stiffness, SBP and salt sensitivity, these results echo recent calls for reduced sodium intake in the American diet, particularly in the rapidly growing, at-risk population of middle-aged and older adults. These results show, for the first time in humans, that dietary salt restriction reduces MBG excretion, and this is associated with reductions in SBP and aPWV, and with improved elasticity of blood vessels. In addition, MBG is positively related to SBP during sodium intake typical of an American diet, complimenting existing evidence in rodents/humans fed experimentally high sodium diets.
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