Ethnic differences in intake and excretion of sodium, potassium, calcium and magnesium in South Africans

Objectives To determine any differences in the urinary excretion and dietary intake of sodium, potassium, magnesium and calcium intake in three South African ethnic groups, and to assess whether the blood pressure-cation association varies according to ethnic status. Design A cross-sectional study of 325 black, white and mixed-ancestry men and women, conveniently sampled in Cape Town. Twenty-four-hour urine samples were collected on three separate occasions for assessment of urinary electrolytes, and three 24-h dietary recalls for the corresponding urine collection times were administered by two trained fieldworkers. Para-amino benzoic acid was used as a marker of the completeness of urine collection. Results Mean urinary sodium values equate to a daily salt (sodium chloride) intake of 7.8, 8.5 and 9.5g in black, mixed-ancestry and white individuals, respectively. In normotensive individuals, black and mixed-ancestry subjects had significantly lower median urinary sodium concentrations than white subjects, but these differences were not evident between black and white hypertensive subjects. No ethnic differences were found for urinary potassium, except for mixed-ancestry normotensive individuals having a lower excretion than white normotensive individuals. Urinary magnesium excretion did not differ across ethnic groups. In both normotensive and hypertensive individuals, urinary calcium concentrations differed between all three groups, with black subjects having the lowest values, approximately less than half those of white subjects. Conclusion White normotensive subjects in Cape Town have higher habitual intakes of sodium, but also higher calcium intakes than their black and mixed-ancestry counterparts. Dietary differences may contribute to ethnic-related differences in blood pressure.