High salt intake is a well-recognised risk factor for osteoporosis because it induces calciuria, but the effects of salt on calcium metabolism and the potential impact on bone health in postmenopausal women have not been fully characterised.
The present study investigated adaptive mechanisms in response to changes in salt and calcium intake in postmenopausal women. Eleven women completed a randomised cross-over trial consisting of four successive five week periods of controlled dietary intervention, each separated by a minimum 4 week washout.
Moderately low and high calcium (518mg versus 1284mg) and salt (3.9g versus 11.2g) diets, reflecting lower and upper intakes in postmenopausal women consuming a Western-style diet, were provided.
Stable isotope labelling techniques were used to measure calcium absorption and excretion, compartmental modelling was undertaken to estimate bone calcium balance, and biomarkers of bone formation and resorption were measured in blood and urine.
Moderately high salt intake (11.2g/d) elicited a significant increase in urinary calcium excretion (p = 0.0008) and significantly affected bone calcium balance with the high calcium diet (p = 0.024). Efficiency of calcium absorption was higher following a period of moderately low calcium intake (p < 0.05) but was unaffected by salt intake.
Salt was responsible for a significant change in bone calcium balance, from positive to negative, when consumed as part of a high calcium diet, but with a low calcium intake the bone calcium balance was negative on both high and low salt diets.