Urinary calcium excretion, sodium intake and blood pressure in a multi-ethnic population: results of the Wandsworth Heart and Stroke Study

Background: Hypertension is associated with increased urinary calcium excre tion (UCa). A high sodium intake increases both UCa and blood pressure (BP) . However, it is not clear whether these effects are modified by gender or ethnic origin. Objectives: To examine the relationships between BP, urinary sodium (UNa), gender and ethnic origin with both daily and fasting UCa in a population-ba sed study. Design and Methods: Out of 1577 individuals taking part in a cross-sectiona l survey, 743 were considered for the present analysis (407 women, 336 men) as they were ail untreated, had provided a complete 24-h urine collection, and had all measurements of anthropometry, BP, UNa and UCa. They were 277 whites, 227 of black African origin and 239 South Asians. Comparisons were also carried out in the 630 participants who also provided 3-h fasting urin e collections. Results: After adjustment for confounders including age, and gender, 24-h U Ca was significantly and independently associated with ethnic origin, BP an d UNa. Mean 24-h UCa was 4.62 (s.e. 0.11) mmol/d in whites, 3.33 (0.12) in South Asians and 3.16 (0.13) in blacks (P < 0.001). A 200 mmol higher UNa p redicted a 1.04 mmol higher daily UCa (P < 0.001), and a 20 mm Hg higher sy stolic BP predicted a 0.28 mmol higher UCa. The slopes were not significant ly different by ethnic group. The ethnic differences in UCa were present wh en fasting UCa was used instead (1.64 [0.05] mu mol/min In whites, 1.08 [0. 06] in South Asians and 1.13 [0.063] in blacks; P < 0.001). Conclusions: These results indicate that BP, salt intake and ethnic origin are independent predictors of UCa in an unselected population. These relati onships are unlikely to be the result of differences in Ca intake or intest inal Ca absorption as they are seen also after an overnight fast, suggestin g that they may reflect differences in renal tubular handling. The estimate d effects of either BP err sodium intake on UCa, sustained over many years, may be associated with significant effects an bone calcium content.