How taking too much vitamin D puts your health at risk

Vitamin D is important for our health, but too much of it can have negative consequences.
Vitamin D is important for our health, but too much of it can have negative consequences.

For a while now, there's been a constant promotion of vitamin D supplements, largely by the nutrition industry and other non-medical institutions. However, according to a recent study, taking high doses of this vitamin can be harmful.

What’s sometimes referred to as the "miracle supplement" because of its lengthy list of possible benefits, can have a downside too. A three-year study by researchers at the Cumming School of Medicine’s McCaig Institute for Bone and Joint Health showed there is no benefit in taking high doses of vitamin D. And even more bad news – too much can actually decrease bone density.

Skeleton most affected

Published in the Journal of the American Medical Association (JAMA), the study followed 300 volunteers between the ages of 55 and 70 in a double-blind, randomised clinical trial to test whether an increase in vitamin D dosage would also increase bone density and strength.

“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 also one of the principal investigators of the study. 

Many people commonly turn to vitamin D supplements during the winter months when sunlight is diminished so that they can get the required amount. This vitamin is a nutrient that the body makes from sunlight and is also found in fish and fortified milk. It is needed by our bodies to absorb calcium and ensure strong, healthy bones. 

Read: For one man, too much vitamin D was disastrous

Participants in the study

A third of the study’s participants received 400 International Units (IU) of vitamin D per day, a third received 4 000 IU per day, and the final third received 10 000 ID per day. All volunteers had their bone density and strength measured using a new, high-resolution computed tomography (CT) scan of bone at the wrist and ankle, called an XtremeCT – the first of its kind in the world.

Fasting blood samples were also collected at the beginning of and during the study so that vitamin D and calcium levels could be assessed. Urine collections were completed annually. 

The results indicated that total bone mineral density (BMD) decreased over the three-year period by:

  • 1.4% in the 400 IU group
  • 2.6% in the 4000 IU group
  • 3.6% in the 10 000 IU group

Bone mineral density test can provide a snapshot of your bone health and is determined by measuring the amount of calcium, as well as other minerals in a defined segment of bone. The lower the bone density, the greater the risk for bone fracture.

Adults slowly lose BMD as they age, and the use of the high resolution XtremeCT showed significant differences in bone loss among the three dosage levels.

“We were surprised to find that instead of bone gain with higher doses, the group with the highest dose lost bone the fastest," says Steve Boyd, a professor in the CSM and one of the principal investigators of the study. 

Recommended daily intake

Growing up, our parents advised us to drink plenty of milk to build strong bones, but the amount of vitamin D in (fortified) milk doesn’t come close to what we really need. Our main sources of vitamin D are sunlight and ultraviolet light exposure which causes the skin to synthesize vitamin D. It only takes around 10–20 minutes of sun exposure during summer to generate the vitamin D your body needs for the day.

The Nutrition Information Centre of the the University of Stellenbosch (NICUS) provides a detailed adequate intake amount of vitamin D for people of all ages, and notes that people with dark skin require around 20 minutes per day of sun exposure, and young people with light coloured skin need about 10 minutes per day of casual sun exposure of their face and hands.

However, the study notes that some people may be taking up to 20 times the recommended daily dose, in the form of supplements, to either prevent or treat a variety of medical conditions related to a vitamin D deficiency. But how much is enough?

"For healthy adults, 400 (IU) daily is a reasonable dose," says Dr Emma Billington, MD and one of the authors of the study. “Doses of 4 000 IU or higher are not recommended for the majority of individuals,” she added.

Image: iStock
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