Background: Toddlers in Pune, India, accustomed to low dietary calcium (Ca) intake but who were vitamin D replete have low serum ionised calcium (iCa) and inappropriately raised serum inorganic phosphorus (Pi) concentrations in the face of elevated serum parathyroid hormone (PTH) concentration. We hypothesised that dietary Ca deficiency lead to end organ resistance to PTH, thus resulting in mild hypocalcaemia and hyperphosphataemia, and that this would be reversed by oral Ca supplementation.
Methods: Fifty-one subjects (25 male; 2.4±0.8 yrs) from an urban slum in Pune were randomised to 500mg of oral Ca supplement or placebo, daily, for 8 weeks. All subjects received 20 mg of oral elemental iron, daily, as 90% had serum ferritin concentration< 12 μg/l. All subjects were examined for clinical stigmata of rickets and had a wrist radiograph performed. Serum concentration of iCa, Pi, PTH and fibroblast growth factor-23 (FGF-23) were measured at the start and end of the trial.
Results: No subject had clinical or radiological evidence of rickets. There was a significant increase in mean serum iCa concentration (p<0.001) in supplemented but not in the placebo group (p=0.32). The decrease in mean serum Pi concentration in supplemented group was greater (p<0.001) than in the placebo group (p=0.003). The mean serum PTH fell in the Ca supplemented (p=0.001) but not in the placebo (p=0.303) group. The mean serum FGF-23 concentration did not change in response to Ca supplementation.
Conclusions: From these data we conclude that low dietary Ca intake is associated with resistance to PTH.