Vitamin D: More isn't always better

Sept. 17, 2019—Vitamin D helps the body absorb calcium, which is needed for strong, healthy bones. But it can be hard for some people to get enough vitamin D naturally. And bone density also tends to decline with age. That's why many older adults take vitamin D supplements.

In the U.S., the recommended daily allowance (RDA) for vitamin D is 400 to 800 international units (IU), depending on your age. But 3% of U.S. adults say they take much more—at least 4,000 IU per day.

According to a new study published in JAMA, not only do higher doses not provide extra protection, they may actually speed up bone loss.

Surprise results

The study tracked 311 Canadian adults 55 to 70 years old for three years. All of the participants were healthy and without osteoporosis at the start of the study. Researchers split them into three groups that took daily supplements of vitamin D at different levels:

  • 400 IU.
  • 4,000 IU.
  • 10,000 IU.

To measure bone density, researchers used both standard bone-density scans and special high-resolution CT scans. While the standard scans showed no difference between the three groups, the CT scans revealed a surprising trend.

Researchers had expected that higher doses of vitamin D would help preserve bone mass. But just the opposite happened: The higher the dose, the faster the adults lost bone mass.

Even at 10,000 IU, the amount of bone loss still wasn't enough to cause a fracture over the three-year study period, the researchers said. But it did raise red flags about the risks of high doses of vitamin D.

Participants who took daily doses of 4,000 and 10,000 IU of vitamin D also had a raised risk of hypercalciuria—high levels of calcium in the urine. This condition is linked to an increased risk of kidney stones. And it may harm kidney function as well.

The bottom line

Overall, more research is needed to establish just how harmful high doses of vitamin D might be. But for now, overloading likely isn't worth the risk.

Learn more about how vitamin D affects your health—and how to get the right amount.

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