Using food consumption records from more than 900 adults, the researchers determined that sufficient vitamin D levels can ensure an ideal level of parathyroid hormone (PTH) - a measure of calcium metabolism - even when calcium intake is less than 800 mg per day.
But consuming more than 1200 mg of calcium daily is not enough to maintain ideal PTH if the vitamin D status is insufficient.
The study is part of a growing body of work that points to the important role of vitamin D, and not just calcium alone, in bone health.
Bone health is a growing concern as the numbers affected by osteoporosis continue to rise, and an increasing elderly population suggests that these will grow further in the future. In Europe, osteoporosis causes around 1.1 million fractures each year.
In light of recent research, and predicting future health problems, some researchers have called for recommended intake of vitamin D to be raised but the adequate amounts needed in the diet are still not known.
The new study, published in today's issue of JAMA (vol 294, no 18, pp2336-2341), underlines the need to do further work on the RDA for this vitamin.
Nevertheless, the authors, Dr Laufey Steingrimsdottir and colleagues from Landspitali-University Hospital in Reykjavik, write that "there is already sufficient evidence from numerous studies for physicians and general practitioners to emphasise 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".
The team's findings were derived from nutrient intake data obtained from 944 healthy Icelandic adults recruited between 2001-2003.
The participants were 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). Serum 25-hydroxyvitamin D is a generally accepted indicator of vitamin D status.
After adjusting for relevant factors, the researchers found that serum intact PTH was lowest in the group with a vitamin D level of more than 18 ng/mL but highest in the group with a vitamin D level of less than 10 ng/mL.
An inverse relationship between serum 25-hydroxyvitamin D and serum PTH is well established. PTH 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.
But the researchers also observed that in people with a calcium intake of more than 1200 mg per day, there was still a significant difference in PTH between the lowest and highest vitamin D groups.
The researchers added: "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," they write.
The authors said that although this cross-sectional 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".
Vitamin D may have a calcium sparing effect, explained the researchers, and as long as vitamin D status is ensured, calcium intake levels of more than 800 mg daily may be unnecessary for maintaining calcium metabolism.
Some countries, such as the UK and Denmark, have recently begun recommending that specific groups of the population take vitamin D supplements to ensure adequate vitamin D status during periods when there is little exposure to sunshine.
But data on the market for bone health supplements shows that the vitamin is still considered to have minor importance for this application. With a mere 4 per cent share of the overall vitamins market, vitamin D comes a long way behind calcium in bone supplement sales, according to a recent Frost & Sullivan report.