The finding appears in the latest Journal of the American College of Nutrition.

It points to the increased need for people to take daily vitamin D supplements, says lead researcher Robert P. Heaney, MD, of Creighton University in Nebraska.

Americans up to age 50 are advised to take 200 IU (international units) of vitamin D daily. From age 51 to 70, the advised dose is 400 IU. For people over age 70, it's 600 IU.

Vitamin D is also made in the skin when it is exposed to sunlight. However,
concern about skin cancer has caused many people to limit their time in the
sun. In addition, during the winter across the northern half of the U.S., there is an insufficient amount of the sun's rays reaching the skin to stimulate production of vitamin D, a news release states.

To look at this issue, Heaney and colleagues conducted two studies in the spring, one year apart, each involving 34 postmenopausal women with apparently normal blood levels of vitamin D.

In the first study, the women were first pretreated with a dose of .02 mg of vitamin D -- on alternative days, for three weeks -- to quickly increase their blood levels of vitamin D.

On the "study day," their blood was tested before breakfast. Then, they were
given 500 mg calcium supplements -- but no vitamin D supplement -- to mimic a
standard low-calcium breakfast. They were also given a low-calcium lunch a bit later. Their blood was tested before breakfast and at frequent intervals for 10 to 12 hours afterward to determine calcium absorption.

In the second study, women did not receive the pretreatment with vitamin D.
They were simply given calcium supplements plus 200 IU of vitamin D
supplements on the study day, plus a low-calcium lunch, and their blood was
tested at similar intervals through the day.

The first study's results showed the positive effects of extra vitamin D over
the long haul -- the women's blood showed from 45% to 65% greater levels of calcium.

Along with calcium supplements, people should take vitamin D to promote
calcium absorption, Heaney states