When Seeking Vitamins, Don’t Over-D It

In winter, some people load up on vitamin D supplements in search of bone health and other benefits. But these "megadoses" may backfire.


Whether in the form of a supplement or a sunny holiday, many of us will go in search of vitamin D this winter. For years we’ve known that vitamin D is essential for strong healthy bones. But according to a new study, more might not mean better.

For healthy adults under 70, Health Canada recommends 600 international units (IU) of vitamin D daily. That’s roughly the amount we produce naturally with 10 to 15 minutes of bathing suit exposure on a sunny day. At higher doses, we don’t have a good understanding of the effects.

Despite this, 3% of US adults take 4,000 IU or more vitamin D a day. They hope that megadoses will prevent or treat a variety of conditions, from heart disease to the flu. Unfortunately, this could be bad news for their bones.

In a randomized control trial conducted at Calgary’s Cumming School of Medicine, researchers followed 300 healthy volunteers aged 55 to 70. Over three years, each took a daily vitamin D supplement at one of three levels: 400, 4,000 and 10,000 IU.

Throughout, researchers measured bone density and strength using a new high-resolution computed tomography scanner called XtremeCT. The lower the bone density, the greater the risk for bone fracture.

Housed at the McCaig Institute for Bone and Joint Health, the XtremeCT shows bone microarchitecture in detail never seen before, without the need for invasive biopsies. With this new totally non-invasive ‘virtual biopsy’, the researchers expected to see dose-dependent changes. And they did, just not in the direction they imagined.

The researchers had expected to see an increase in bone density and strength at higher levels of supplementation. Instead they found the opposite: the XtremeCT detected a dose-related decrease in bone density, with the largest decrease occurring in the 10,000 IU group.

While surprising, these results should be interpreted with caution.

“It’s not like their bones are melting away, so it’s not a huge effect,” Steven Boyd, a professor at the Cumming School of Medicine and research co-lead, told CTV News.

“What the message is though, I think, is if you are taking high dose vitamin D and you are taking it for decades, that could probably add up to something important.”

While further research is needed, the researchers recommend sticking to the Health Canada guidelines and skipping the megadoses.

“What we saw in this study is that large doses of vitamin D did not improve bone density or strength,” says McCaig Institute member Dr. Emma Billington, one of the authors of the study.

“For most healthy adults, 400 IU daily is a reasonable dose for maintaining bone health, and no further bone benefit would be obtained with doses of 4,000 IU or higher.”

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Amy Noise is a science communicator who is fascinated by how and why the world works. Always learning, she is passionate about science and sharing it with the world to improve and protect our health, society and environment. Amy earned her BSc (biology and science communication) at the University of Manchester, and MSc (nutrition science and policy) at King’s College London, UK. She tweets sporadically @any_noise