Vitamin D Called More Important for Bones Than Dietary Calcium
November 10th, 2005
By Neil Osterweil, Senior Associate Editor, MedPage Today
November 09, 2005
REYKJAVIK, Iceland, Nov. 9 - If you don’t get enough vitamin D it doesn’t seem to matter to your bones how much calcium you get.
So concluded an Icelandic study of nearly 1,000 healthy adults that found higher 25-hydroxyvitamin D levels more closely associated with normal levels of serum intact parathyroid hormone (PTH) than were high levels of calcium.
In short, otherwise healthy people with adequate 25-hydroxyvitamin D levels may need no more than 800 mg of calcium per day order to have calcium homeostasis and maintain normal levels of PTH, investigators here suggested in the Nov. 9 issue of the Journal of the American Medical Association. The recommended RDA is 400 IU to 800 IU of vitamin D.
“Our study suggests that vitamin D sufficiency may be more important than high calcium intake in maintaining desired values of serum PTH,” wrote Laufey Steingrimsdottir, Ph.D., of the Public Health Institute of Iceland and colleagues. “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.”
To determine the relative effects on calcium homeostasis of calcium intake and serum 25-hydroxyvitamin D, the authors conducted a cross-sectional study of 2,310 healthy Icelandic adults.
The participants were divided into three groups: 30 to 45 years old, 50 to 65, and 70 to 85. They were administered a food questionnaire asking about diet, including supplements, over the previous three months.
Fasting levels of 25-hydroxyvitamin D, intact serum PTH and serum calcium were recorded, and on the basis of the results, the participants were further divided into groups based on calcium and vitamin D levels.
The main study endpoint was serum intact PTH as measured by calcium intake and 25-hydroxyvitamin D levels.
Participants were excluded if they took medications that could affect calcium metabolism, or had surgical procedures that might have done so, or failed to complete the questionnaire.
Among the 944 people who completed the study, serum PTH, after adjusting for relevant factors, was found to be lowest in the group with the highest levels of 25-hydroxyvitamin D (>18 ng/ml), and highest among those subjects with the lowest 25-hydroxyvitamin D levels (<10 ng/ml). Based upon these data, the investigators define vitamin D deficiency as a 25-hydroxyvitamin D level of less than 18 ng/ml.
The inverse association between 25-hydroxyvitamin D levels and PTH was significant (P<0.01), but the relationship between calcium intake and serum PTH was not (P=0.28). Among participants with low 25-hydroxyvitamin D levels, serum PTH was significantly higher when the calcium level was also low (<800 mg/d), compared with those who consumed more than 1200 mg/d of calcium. But among those in the middle calcium intake range (800-1200 mg/d), PTH levels were not significantly different.
“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 1,200 mg/d) is not sufficient to maintain ideal serum PTH, as long as vitamin D status is insufficient,” the authors wrote.
They noted that although a cross-sectional study isn’t powered 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 investigators noted that in northern locales such as Iceland (and, presumably, in extreme southern latitudes), the population is at greater risk for vitamin D insufficiency than in more temperate climes, due to short winter days with limited sunlight.
“In our sample population, an average intake of 500 IU/d of vitamin D corresponded with a mean serum 25-hydroxyvitamin D level of more than 18 ng/mL throughout the year, but approximately 700 IU/d was required during winter,” they noted.
Primary source: Journal of the American Medical Association
Source reference:
Steingrimsdottir L. Relationship Between Serum Parathyroid Hormone Levels, Vitamin D Sufficiency, and Calcium Intake. JAMA. 2005;294:2336-2341.
Source: medpagetoday.com
Entry Filed under: Nutrition News
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