Background: Hypertension is associated with increased urinary calcium excretion (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-based study.
Design and methods: Out of 1577 individuals taking part in a cross-sectional survey, 743 were considered for the present analysis (407 women, 336 men) as they were all 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 690 participants who also provided 3-h fasting urine collections.
Results: After adjustment for confounders including age, and gender, 24-h UCa was significantly and independently associated with ethnic origin, BP and 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 100 mmol higher UNa predicted a 1.04 mmol higher daily UCa (P < 0.001), and a 20 mm Hg higher systolic BP predicted a 0.28 mmol higher UCa. The slopes were not significantly different by ethnic group. The ethnic differences in UCa were present when fasting UCa was used instead (1.64 [0.05] micromol/min in whites, 1.08 [0.06] in South Asians and 1.13 [0.06] 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 relationships are unlikely to be the result of differences in Ca intake or intestinal Ca absorption as they are seen also after an overnight fast, suggesting that they may reflect differences in renal tubular handling. The estimated effects of either BP or sodium intake on UCa, sustained over many years, may be associated with significant effects on bone calcium content.