Are calcium supplements necessary in vitamin-D-replete individuals?

BACKGROUND Vitamin D is crucial for bone health and an inverse relationship exists between the levels of 25-hydroxyvitamin D-the serum marker of vitamin D sufficiency-and parathyroid hormone (PTH), a key regulator of calcium homeostasis. The contribution of dietary calcium and vitamin D to calcium homeostasis is, however, poorly understood. OBJECTIVE To assess the relative importance of dietary calcium and vitamin D in maintaining calcium homeostasis. DESIGN AND INTERVENTION This was a cross-sectional population-based study of healthy adult volunteers from Reykjavik, Iceland. Participants aged 30-85 years were randomly recruited from a population register. Exclusion criteria included primary hyperparathyroidism and medications thought to affect calcium metabolism. To account for seasonal effects on vitamin D concentrations, equal numbers of participants were recruited monthly throughout the 2-year study period. Participants completed a general health questionnaire, and calcium and vitamin D intake were estimated using a semiquantitative survey designed to assess the entire diet over the previous 3 months. Serum PTH, 25-hydroxyvitamin D, and ionized calcium levels were measured after an overnight fast. OUTCOME MEASURE The primary outcome measure was calcium homeostasis as determined by serum PTH levels. RESULTS A total of 944 individuals completed the study; of these, 491 were women (mean age 53.7 years) and 453 were men (mean age 57.9 years). Mean dietary intake of calcium and vitamin D was above the recommended amount in all age-groups. Cod liver oil or vitamin D supplements were taken by 562 participants, the mean daily vitamin D intake being 728 IU and 208IU in the supplemented and nonsupplemented groups, respectively (P<0.001). During winter, the mean serum 25-hydroxyvitamin D level without supplements was 28.7 nmol/l compared with 46.7 nmol/l with supplements (P<0.001). In the summer, less variation was observed between the supplemented and nonsupplemented groups than in the winter, but levels were generally higher in the group taking supplements than in the group not taking them. Conversely, serum PTH levels were lower in the group that took supplements than the group that did not (36.9 ng/l versus 39.5 ng/l, P=0.02). Serum calcium levels were lowest in the group with the lowest 25-hydroxyvitamin D levels (1.23 mmol/l, P=0.01). PTH concentration was associated with vitamin D status, but not calcium intake (P<0.001), in the presence of adequate levels of 25-hydroxyvitamin D. Levels of PTH were lowest in the group with 25-hydroxyvitamin D levels >45 nmol/l and highest in the group with 25-hydroxyvitamin D levels <25 nmol/l. At low 25-hydroxyvitamin D levels, the PTH levels were dependent upon calcium intake. When the daily intake of calcium was <800 mg, the PTH levels were higher than when the daily intake was >1,200 mg (P=0.04). PTH levels in participants whose intake was 800-1,200 mg per day were not significantly different from those in individuals taking >1,200 mg daily. CONCLUSION Vitamin D sufficiency can ensure optimal serum PTH levels even when calcium intake is low.