The importance of adequate vitamin D status for optimum bone health has received increased recognition in recent years, with higher recommended intake levels being proposed by some investigators, according to background information in the article. The ideal intake is not known, and different criteria have been proposed for estimating population requirements. Serum 25-hydroxyvitamin D has been the generally accepted indicator of vitamin D status, but no universal consensus has been reached regarding which serum values constitute sufficiency. An inverse relationship between serum 25-hydroxyvitamin D and serum parathyroid hormone (PTH) is well established. Parathyroid hormone is a major hormone maintaining normal serum concentrations of calcium and phosphate and is itself regulated through levels of calcitriol and serum calcium. An insufficiency of vitamin D or calcium is generally associated with an increase in PTH.
Laufey Steingrimsdottir, Ph.D., of Landspitali-University Hospital, Reykjavik, Iceland, and colleagues conducted a study to determine the importance of high calcium intake and serum 25-hydroxyvitamin D for calcium homeostasis (metabolic equilibrium) in healthy adults, as determined by serum intact PTH.
The study included 2,310 healthy Icelandic adults who were divided equally into 3 age groups (30-45 years, 50-65 years, or 70-85 years) and recruited from February 2001 to January 2003. They were administered a semi-quantitative food frequency questionnaire, which assessed vitamin D and calcium intake. Participants were further divided into groups according to calcium intake (less than 800 mg/d, 800-1200 mg/d, and greater than1200 mg/d) and serum 25-hydroxyvitamin D level (less than 10 ng/mL, 10-18 ng/mL, and greater than 18 ng/mL). A total of 944 participants completed the dietary questionnaire.
The researchers found that after adjusting for relevant factors, serum intact PTH was lowest in the group with a serum 25-hydroxyvitamin D level of more than 18 ng/mL but highest in the group with a serum 25-hydroxyvitamin D level of less than 10 ng/mL. At the low serum 25-hydroxyvitamin D level (less than 10 ng/mL), calcium intake of less than 800 mg/d vs. more than 1200 mg/d was significantly associated with higher serum PTH; and at a calcium intake of more than 1200 mg/d, there was a significant difference between the lowest and highest vitamin D groups.
"The significance of our study was demonstrated by the strong negative association between sufficient serum levels of 25-hydroxyvitamin D and PTH, with calcium intake varying from less than 800 mg/d to more than 1200 mg/d. Our results suggest that vitamin D sufficiency can ensure ideal serum PTH values even when the calcium intake level is less than 800 mg/d, while high calcium intake (greater than 1200 mg/d) is not sufficient to maintain ideal serum PTH, as long as vitamin D status is insufficient," the authors write.
"Although a cross-sectional study such as our study is not sufficient to demonstrate causality, the association between vitamin D status, calcium intake, and the interaction between these 2 with serum PTH levels is a strong indication of the relative importance of these nutrients," the researchers write. "Although ideal intakes of these 2 nutrients need to be further defined in more elaborate studies, there is already sufficient evidence from numerous studies for physicians and general practitioners to emphasize to a much greater extent the importance of vitamin D status and recommend vitamin D supplements for the general public, when sun exposure and dietary sources are insufficient."
"In conclusion, our study suggests that vitamin D sufficiency may be more important than high calcium intake in maintaining desired values of serum PTH. Vitamin D may have a calcium sparing effect and as long as vitamin D status is ensured, calcium intake levels of more than 800 mg/d may be unnecessary for maintaining calcium metabolism. Vitamin D supplements are necessary to ensure adequate vitamin D status for most of the year in northern climates."
(JAMA.2005; 294:2336-2341. Available pre-embargo to the media at www.jamamedia.org)
Editor's Note: This study was supported by a grant from the Science Fund of St. Josephs Hospital, Landakoti, and the University Hospital, Reykjavik, Iceland.