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Vitamin D: How much is too much of a good thing?

When bare skin is exposed to sunlight, it makes Vitamin D, which is needed byour bodies to absorb calcium and ensure strong, healthy bones. With bathingsuit skin exposure, it only takes about 10-15 minutes of sun exposure duringthe summer to generate all the vitamin D your body needs for the day.Unfortunately, for Canadians, exposure to sunlight is diminished during thelong winter months. This results in many turning to supplements to get therequired vitamin D. For normal, healthy adults, HealthCanada recommends a total daily intake of 600 international units (IU) up toage 70, and 800 IU after age 70. Other sources, like Osteoporosis Canada,suggest adults at risk of osteoporosis, a condition characterized by boneloss, should take 400 -- 2,000 IU of Vitamin D. However, some people may betaking up to 20 times the recommended daily dose to prevent or treat a varietyof medical conditions that might be related to having not enough vitamin D.So, what is the correct dose? And, how much is too much? "Although vitamin Dmay be involved in regulating many of the body's systems, it is the skeletonthat is most clearly affected by vitamin D deficiency," says Dr. David Hanley,MD, an endocrinologist in the Cumming School of Medicine (CSM), and one of theprincipal investigators of the study. "Current Health Canada recommendationswere set to prevent the bone diseases caused by vitamin D deficiency for thevast majority of healthy Canadians. But it has been more difficult to clearlyestablish the optimal dose of vitamin D. When we designed this study, thereremained a question whether there's more benefit in taking a higher dose." Athree-year study by researchers at the Cumming School of Medicine's McCaigInstitute for Bone and Joint Health published in the _Journal of the AmericanMedical Association_ ( _JAMA_ ), showed there is no benefit in taking highdoses of vitamin D. The study followed 300 volunteers between the ages of 55and 70 in a double-blind, randomized clinical trial to test the hypothesisthat with increasing doses of vitamin D, there would be a dose-relatedincrease in bone density and bone strength. A third of the study participantsreceived 400 IU of vitamin D per day, a third received 4,000 IU per day, and athird received 10,000 IU per day. Volunteers had both their bone density andbone strength measured using a new, high-resolution computed tomography (CT)scan of bone at the wrist and ankle, called an XtremeCT, used only forresearch. The XtremeCT, located in the McCaig Institute's new Centre forMobility and Joint Health, is the first of its kind in the world, and allowsresearchers to look at bone microarchitecture in detail never seen before.Standard dual-X-ray absorptiometry (DXA) bone density was also obtained.Participants received scans at the start of the study and at 6, 12, 24 and 36months. To assess vitamin D and calcium levels, researchers also collectedfasting blood samples at the beginning of the study and at 3, 6, 12, 18, 24,30 and 36 months as well as urine collections annually. Bone mineral density(BMD) is determined by measuring the amount of calcium and other minerals in adefined segment of bone. The lower the bone density, the greater the risk forbone fracture. Adults slowly lose BMD as they age, and the DXA results showeda modest decrease in BMD over the duration of the study, with no differencesdetected between the three groups. However, the more sensitive measurement ofBMD with high resolution XtremeCT showed significant differences in bone lossamong the three dose levels. Total BMD decreased over the three-year period by1.4 per cent in the 400 IU group, 2.6 in the 4,000 IU group and 3.6 in the10,000 IU group. The conclusion was that, contrary to what was predicted,vitamin D supplementation at doses higher than those recommended by HealthCanada or Osteoporosis Canada were not associated with an increase in bonedensity or bone strength. Instead, the XtremeCT detected a dose-relateddecrease in bone density, with the largest decrease occurring in the 10,000 IUper day group. More research is required to determine if high doses mayactually compromise bone health. "We weren't surprised that using DXA we foundno difference among the treatment arms, whereas with XtremeCT, the latest inbone imaging technology, we were able to find dose-dependent changes over thethree years. However, we were surprised to find that instead of bone gain withhigher doses, the group with the highest dose lost bone the fastest," saysSteve Boyd, a professor in the CSM and one of the principal investigators ofthe study. "That amount of bone loss with 10,000 IU daily is not enough torisk a fracture over a three-year period, but our findings suggest that forhealthy adults, vitamin D doses at levels recommended by Osteoporosis Canada(400-2000 IU daily) are adequate for bone health." A secondary outcome of thestudy indicated a potential safety concern with taking high levels of vitaminD. Although there were incidents in all three arms of the study, theinvestigators found that participants assigned to receive higher doses ofdaily vitamin D supplementation (4000 IU and 10,000 IU) over the three yearswere more likely to develop hypercalciuria (elevated levels of calcium in theurine), compared to those receiving a lower daily dose. Hypercalciuria is notuncommon in the general population, but is associated with increased risk ofkidney stones and may contribute to impaired kidney function. Hyperalciuriaoccurred in 87 participants. Incidence varied significantly between the 400 IU(17%) 4000 IU (22%) and 10,000 IU (31%) study groups. If hypercalciuria wasdetected in study participants, calcium intake was reduced. After repeattesting, the hypercalciuria usually resolved. "What we can see in this studyis that large doses of vitamin D don't come with a benefit to the skeleton,"says Dr. Emma Billington, MD, one of the authors of the study. "For healthyadults, 400 IU daily is a reasonable dose. Doses of 4,000 IU or higher are notrecommended for the majority of individuals."Story Source:Materials provided byUniversity of Calgary. _Note: Content may be edited for style and length.

Reference : Science Daily