Vitamin D lowers daily dose of Calcium needed.
December 5, 2005
Subject: Adequate doses of vitamin D lower the amount of calcium needed to maintain healthy bone.
Most women have low levels of vitamin D. With adequate vitamin D they absorb calcium better and don't need the large daily doses of calcium their doctors tell them to take. Two recent studies shed light on this situation.
A study done in Iceland and published in JAMA in November says that women who take supplemental vitamin D can get by with about one-third of the RDA for calcium without compromising their bone health.
The November 9, Journal of the American Medical Association reports that in women getting no more than 200 international units (IU) of vitamin D, the need for calcium was high—at least 1,200 milligrams per day. That's the RDA for Calcium in the United States . In these Icelandic women it took about 18 nanograms per milliliter (ng/ml) of Vitamin D to lower the parathyroid hormone (PTH), a common test for adequate vitamin D.
For Icelandic women who got about 500 IU of vitamin D per day from cod liver oil, the need for calcium, was only about 800 mg per day a third less. [i]
Not everyone gets exactly the same results.
Robert Heaney published an article in the October Journal of Steroid Biochemistry & Molecular Biology in which he concludes that bones don't get all the calcium they need until vitamin D levels reached 32 ng/ml, a level almost twice what the Icelandic research suggests. [ii]
In an earlier study, Heaney tested women in Nebraska in the springtime of two successive years. Each time, the women were fed a breakfast containing 500 mg of calcium. The first year, the women hadn't been taking vitamin D supplements and their blood levels were around 20 ng/ml, roughly comparable to high values in the recent Icelandic study. On average, the women absorbed only about 22 percent of the calcium from their food.
The next year, the same women took vitamin D supplements for more than a month prior to the testing, about 1,200 to 1,500 IU per day. As a result their vitamin D levels averaged 35 ng/ml. Their calcium absorption increased from 22 percent to 37 percent. In other words adequate vitamin D boosts calcium absorption from food or supplements by 70%. [iii]
Let's do the math and let's use Heaney's Nebraska ladies as they live closer to home. A woman without vitamin D supplements absorbs 22 percent of the 1200 mg of calcium she takes each day. That's about 275 mg of calcium. With adequate vitamin D she may only need to take around 700 mg of calcium to get the same benefit.
Most researchers agree that people in the US need at least 1,000 IU of vitamin D per day. Heaney suggests that they may need more. If one accepts Heaney's 32 ng/ml value as the target level to prevent bone loss, then people should get a 2,600 IU of vitamin D per day.
Most of the experts seem to think the Icelandic research, which suggests that 500 mg of vitamin D per day drops the daily dose of calcium by a third, may be too good to be true. Most accept the results in principle though. If our vitamin D levels are sufficient, lower doses of calcium are required. Suggested doses of Vitamin D vary from 1,000 iu to 2,600 iu/day. Suggested target blood level of 25(OH)D for bone is 32ng/ml or higher). At this level women may only need 700 mg of calcium a day to maintain their bone density.
[i] Steingrimsdottir, L. and G. Sigurdsson. 2005. Relationship between serum parathyroid hormone levels, vitamin D sufficiency, and calcium intake. Journal of the American Medical Association 294(Nov. 9):2336-2341. Abstract available at http://jama.ama-assn.org/cgi/content/abstract/294/18/2336
[ii] Robert P. Heaney . The Vitamin D requirement in health and disease. The Journal of Steroid Biochemistry and Molecular Biology. Volume 97, Issues 1-2 , October 2005, Pages 13-19