Although the importance of adequate vitamin D intake to optimal bone health has received increasing attention of late, the ideal intake has not been defined. And while it is commonly accepted that serum 25-hydroxy- vitamin D is the marker of vitamin D ?status, no collective reference value has been determined. A new study has ?now determined that calcium intake ?levels of >800 mg/day may be unnecessary for maintaining calcium meta?bolism in individuals with sufficient vitamin D status (JAMA. 2005;294:2336-2341).
An inverse relationship between serum 25-hydroxyvitamin D and serum parathyroid hormone (PTH) is recognized, but the relative importance of vitamin D and calcium intake to calcium homeostasis (as measured by serum intact PTH) had not been addressed previously.
In this cross-sectional study, Laufey Steingrimsdottir, PhD, of Landspitali-University Hospital, Reykjavik, Iceland, and colleagues randomized 2310 healthy individuals to 1 of 3 groups based on age: 30-45, 50-65, and 70-85 years. After using a food frequency questionnaire to assess vitamin D and calcium intake, the participants were further randomized into groups according to calcium intake (<800, 800-1200, and >1200 mg/d) and serum 25-hydroxyvitamin D levels (<10, 10-18, and >18 ng/mL). A total of 944 individuals (mean age, 53.7 years; 453 men, 491 women) completed all phases of the study.
After adjusting for relevant factors, serum intact PTH was found to be lowest in those with a serum 25-hydroxyvitamin D level >18 ng/mL and highest in those with a serum 25-hydroxyvitamin D level <10 ng/mL. Compared with a calcium intake >1200 mg/day, calcium intake <800 mg/day was associated with higher serum PTH in those with a serum 25-hydroxy?vitamin D level <10 ng/mL. There were significant differences between participants with the lowest and highest vitamin D intakes among those with the highest intake (>1200 mg/d) of calcium (P?= .04).
"Our study suggests that vitamin D sufficiency may be more important than high calcium intake in maintaining desired values of serum PTH," the ?investigators wrote. "Vitamin D may have a calcium-sparing effect, and as long as vitamin D status is ensured, ?calcium intake levels of >800 mg/day may be unnecessary for maintaining ?calcium metabolism."