As long as vitamin D status is ensured, calcium intake levels of more than 800 mg/d may be unnecessary for maintaining calcium metabolism.

The importance of adequate vitamin D status for optimum bone health has received increased recognition in recent years, and some researchers have proposed higher recommended intake levels. Different criteria have been proposed for estimating population requirements, but the ideal intake level still is unknown.

Serum 25-hydroxyvitamin D has been the generally accepted indicator of vitamin D status, but no universal consensus has been reached on which serum values constitute sufficiency.

However, 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 generally is associated with an increase in PTH.

Vitamin D vs. Calcium Intake

Laufey Steingrimsdottir, PhD, 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 -- that is, metabolic equilibrium -- in healthy adults, as determined by serum PTH.

The study participants -- 2,310 healthy Icelandic adults -- were divided equally into three age groups: 30-45 years; 50-65 years; and 70-85 years. Each group completed a semi-quantitative food frequency questionnaire that assessed vitamin D and calcium intake.

Participants then were further divided into groups according to calcium intake and and serum 25-hydroxyvitamin D level. The calcium intake groups were less than 800 mg/d; 800-1200 mg/d; and greater than1200 mg/d. The serum 25-hydroxyvitamin D level groups were less than 10 ng/mL; 10-18 ng/mL; and greater than 18 ng/mL.

A total of 944 participants completed the dietary questionnaire.

After adjusting for relevant factors, the researchers found that serum intact PTH was lowest in the group with a serum 25-hydroxyvitamin D level of more than 18 ng/mL and 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 was significantly associated with higher serum PTH. At a calcium intake of more than 1200 mg/d, there was a significant difference between the lowest and highest vitamin D groups.

Vitamin D Supplements Necessary in Northern Climates

“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," say the authors.

"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," they note.

"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 two with serum PTH levels is a strong indication of the relative importance of these nutrients," the researchers explain.

"Although ideal intakes of these two nutrients need to be further defined in more elaborate studies," they say, "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," the authors recommend.