Vitamin D sufficiency appears to be more important for bone health than is high calcium intake, according to Laufey Steingrimsdottir, Ph.D., of the Public Health Institute of Iceland, Reykjavik, and associates.
Both nutrients are known to influence calcium homeostasis, but the relative contributions of each haven't been studied before, they said (JAMA 2005:294:2336–41).
Although ideal intakes of these two nutrients “need to be further defined in more elaborate studies, there is already sufficient evidence from numerous studies for physicians” to further emphasize the vital importance of vitamin D to bone health, Dr. Steingrimsdottir and associates said.
Physicians should recommend vitamin D supplements for the general public “when sun exposure and dietary sources are insufficient,” they added.
The researchers assessed the relative importance of calcium intake and serum levels of 25-hydroxy vitamin D for maintaining calcium homeostasis in a study of 944 healthy white residents of Iceland. The 491 women and 453 men, aged 30–85 years.
Most Icelanders take vitamin supplements or cod liver oil to supply vitamin D because there isn't sufficient sunshine there throughout the year for adequate biosynthesis of vitamin D. Most also have a relatively high calcium intake, chiefly through the consumption of dairy products. In this study, the mean intake of both vitamin D and calcium were well above recommended levels in all age groups, although there was great variation in both.
Vitamin D status was found to ensure ideal values for serum parathyroid hormone, even when calcium intake was not sufficient to maintain those PTH levels.
In addition, mean serum ionized calcium levels, a more precise marker of calcium homeostasis and thus of bone health, were dependent on serum 25-hydroxyvitamin D levels, but not on calcium intake.
“Although sufficient intake of both nutrients is certainly important, our study indicates that as long as vitamin D status is secured by vitamin D supplements or sufficient sunshine, calcium intake levels of more than 800 mg [per day] may be unnecessary for maintaining calcium homeostasis,” the investigators noted, adding that high calcium intake levels “may have other beneficial effects not addressed in this study,” such as possibly protecting the gut lumen against polyp formation.