Is There Such A Thing As Too Much Vitamin D?

Vitamin D is one of the many nutrients needed by the body to stay healthy. It has several functions, which include helping the body absorb calcium to build and maintain strong bones as well as blocking the release of the parathyroid hormone. If released, this hormone makes bones brittle.

The easiest way to boost vitamin D levels is to expose the skin to sunlight. Only 10-15 minutes of exposure in a summer bathing suit is required to hit the daily requirement. Unfortunately, even this may be difficult for some. Canadians experience reduced sunlight during the long winter months. Thankfully, vitamin D may also be included in the diet, either through foods like egg yolk and mushrooms, or supplements.

"Although vitamin D may be involved in regulating many of the body's systems, it is the skeleton that 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 the principal investigators of a study on vitamin D dosages and benefits. "Current Health Canada recommendations were set to prevent the bone diseases caused by vitamin D deficiency for the vast majority of healthy Canadians. But it has been more difficult to clearly establish the optimal dose of vitamin D. When we designed this study, there remained a question whether there's more benefit in taking a higher dose."

For normal, healthy adults, Health Canada recommends a total daily intake of 600 international units (IU) up to age 70, and 800 IU after age 70. Other sources, like Osteoporosis Canada, suggest adults at risk of osteoporosis, a condition characterized by bone loss, should take 400 - 2,000 IU of vitamin D. However, some people may be taking up to 20 times the recommended daily dose to prevent or treat a variety of medical conditions that might be associated with having not enough vitamin D. With so many recommended doses, which one should one follow? And is there such a thing as too much vitamin D?

These questions were answered by researchers from the Cumming School of Medicine's McCaig Institute for Bone and Joint Health. They conducted a three-year study published in the Journal of the American Medical Association (JAMA) and found that "more is better" does not apply in this case as increasing vitamin D doses will not reap any benefits.

Three hundred volunteers were divided into three groups and each group was given varying doses of vitamin D daily: 400IU, 4,000IU, and 10,000IU.

Subjects in this study had their bone density and bone strength measured using a new, high-resolution computed tomography (CT) scan of bone at the wrist and ankle called an XtremeCT used only for research. Researchers also collected fasting blood samples to assess vitamin D and calcium levels at the beginning of the study and 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 a defined segment of bone. The lower the bone density, the greater the risk for bone fracture.

Adults progressively lose BMD as they get older. This was confirmed by the XtremeCT which showed bone loss in all three groups and with significant differences in the amounts between the three dose levels.

Total BMD decreased over the three years by 1.4 percent in the 400 IU group, 2.6 in the 4,000 IU group and 3.6 in the 10,000 IU group. In other words, as the dose of vitamin D increased, so did bone loss.

Vitamin D supplementation at doses higher than those recommended by Health Canada or Osteoporosis Canada were not associated with an increase in bone density or bone strength. Instead, the XtremeCT detected a dose-related decrease in bone density, with the largest decrease occurring in the 10,000 IU per day group. More research is required to determine if high doses may actually compromise bone health.

A secondary outcome of the study indicated a potential safety concern with taking high levels of vitamin D. Although there were incidences in all three arms of the study, the investigators found that participants assigned to receive higher doses of daily vitamin D supplementation (4,000 IU and 10,000 IU) over the three years were more likely to develop hypercalciuria (elevated levels of calcium in the urine), compared to those receiving a lower daily dose. Hypercalciuria is not uncommon in the general population, but is associated with increased risk of kidney stones and may contribute to impaired kidney function.

"What we can see in this study is 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 healthy adults, 400 IU daily is a reasonable dose. Doses of 4,000 IU or higher are not recommended for the majority of individuals."