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Taking high doses of vitamin D may negatively affect bone health

A study shows that consumers have to be careful when it comes to choosing supplements

Photo (c) batuhan toker - Getty Images
Several studies have explored the pros and cons of taking a vitamin D supplement, and now researchers from the University of Calgary have explored the effects that a higher dose of the nutrient can have on the body over the long term. 

The study revealed that though many consumers may choose to up their dosage of vitamin D to support bone strength, doing so may not make much of a difference to overall bone health. 

“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,” said researcher Dr. David Hanley. “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.” 

The effect on bone density

The researchers had 300 participants involved in the study, all of whom were divided up and required to take varying levels of a vitamin D supplement -- either 400 international units (IU) per day, 4,000 IU per day, or 10,000 IU per day. 

Participants remained on the supplement regimen for three years. They had their bone density and bone strength measured at various intervals over the course of that time in order for the researchers to determine how the vitamin D was incrementally affecting their bone health. 

Older adults typically have lower bone density and strength, and these vitals are expected to slowly decrease over time. Because all participants were between the ages of 55 and 70, bone health did decrease over the course of the study at a rate that was anticipated by the researchers. 

Though each group saw a dropoff in bone health, the group taking the highest dose of vitamin D had the biggest dip in bone strength over the course of the three years. 

According to researcher Steve Boyd, consumers shouldn’t fear these results because the “amount of bone loss with 10,000 IU daily is not enough to risk a fracture over a three-year period,” though staying within the 400 IU - 2,000 IU of vitamin D per day is what the researchers recommend for the majority of consumers. 

Ultimately, taking higher doses of vitamin D didn’t harm participants’ overall health, but it also didn’t improve bone health, as was previously expected. 

“What we can see in this study is that large doses of vitamin D don’t come with a benefit to the skeleton,” said Dr. Emma Billington. “For healthy adults, 400 IU daily is a reasonable dose. Doses of 4,000 IU or higher are not recommended for the majority of adults.” 

Monitoring vitamin D intake

While many consumers try to incorporate more vitamin D into their diets, especially pregnant women planning for their newborns, researchers have found that too much vitamin D can come with adverse health effects, including kidney disease. 

The study revealed that taking vitamin D in excess, particularly over the long term, can ultimately lead to kidney damage due to too much calcium in the blood, further emphasizing the need for consumers to follow recommended doses when adding vitamin D into their routines. 

“Although vitamin D toxicity is rare owing to a large therapeutic range, its widespread availability in various over-the-counter formulations may pose a substantial risk to uninformed patients,” said Dr. Borne Auguste.

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