Vitamin D Called More Important for Bones Than Dietary Calcium

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Vitamin D Called More Important for Bones Than Dietary Calcium

By Neil Osterweil , MedPage Today Staff Writer

Reviewed by Zalman S. Agus, MD; Emeritus Professor at the University

of Pennsylvania School of Medicine.

Source News Article: ABC News, Forbes

MedPage Today Action Points

Explain to patients that this study suggests that vitamin D in

sunlight, food, and dietary supplements may be more important than

dietary calcium for maintaining normal levels of PTH in healthy

adults. Point out that according to this study as long as vitamin D

status is ensured, calcium intake levels of more than 800 mg/d may be

unnecessary for maintaining calcium metabolism

Consider vitamin D supplements of approximately 500 IU/day in healthy

patients who do not get sufficient vitamin D from sun exposure and/or

diet. Patients who live in northern latitudes may require additional

supplementation to 700 IU/day in the winter months.

Review

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.