A study shows that, for many people, less is more when it comes to vitamin D.

There's no question that vitamin D can help build strong bones. But there may be a sweet spot when it comes to how much.

A study published in the Aug. 27 issue of JAMA found that, compared with people who took moderate amounts of vitamin D, adults who took large amounts daily not only didn't see additional gains in bone density, but in some cases ended up worse off.

"People often assume that if some is good, more is better," says Dr. JoAnn E. Manson, the Michael and Lee Bell Professor of Women's Health at Harvard Medical School. "This is generally not the case, and certainly is not true of vitamin D. While there is no question that vitamin D and calcium are essential to bone health, it appears that very high doses of vitamin D don't provide further benefits for bone health and may actually have a harmful effect."

Testing high-dose vitamin D

Researchers gave more than 300 healthy adults daily doses of 400 international units (IU), 4,000 IU, or 10,000 IU of supplemental vitamin D. All participants underwent bone mineral density tests, which are designed to assess bone health, when the study began and after they'd been taking the supplements for three years.

To put those doses in context, 600 IU is the recommended daily amount of vitamin D for adults through age 70; 800 IU is recommended for people above age 70; 4,000 IU is considered to be the upper end of the tolerable intake level; and 10,000 IU is considered a megadose, well beyond what is typically advised, says Dr. Manson.

"Compared to just a modest dose of 400 IU a day, the study found no improvement in bone density at the higher doses and even a suggestion that there might be some harm by reducing bone density," says Dr. Manson. "This has been found in other randomized trials of very high-dose vitamin D."

Too much of a good thing

Some professional societies encourage people to take as much as 5,000 IU to 10,000 IU of vitamin D each day. This study provides new information that might prompt some to rethink those recommendations.

"This is further evidence that high doses are not advisable," says Dr. Manson. "In terms of bone health, once you get to a certain level of intake, increasing that amount isn't going to be beneficial."

There is also some evidence that people are overdoing vitamin D supplementation on their own. Researchers looking at national survey data gathered from 1999 to 2014 found a 2.8% uptick in the number of people taking potentially unsafe amounts of vitamin D — that is, more than 4,000 IU per day, according to a 2017 research letter published in JAMA. And during the same time period, the number of people taking 1,000 IU or more of vitamin D daily increased nearly 18%.

However, while most people should stick to a moderate daily dose of vitamin D, some people do need higher amounts, says Dr. Manson. These include people with conditions that prevent the body from properly absorbing nutrients, such as Crohn's disease or ulcerative colitis and people who have had gastric bypass surgery. But in most people, high doses are unnecessary, and potentially harmful.

More information coming on the potential effects of vitamin D supplementation Beginning in 2000, research into vitamin D's role in conditions beyond bone health began to expand rapidly, and people started taking vitamin D supplements in hopes of warding off everything from heart disease to cancer. The science is finally catching up. Harvard Medical School professor Dr. JoAnn E. Manson is the principal investigator of the ongoing Vitamin D and Omega-3 Trial (VITAL), a large study (more than 25,000 participants nationwide) investigating whether vitamin D and omega-3 supplements can lower risks for cancer, cardiovascular disease, and other conditions. She says while there is strong support for vitamin D's role in bone health and growing evidence that it may reduce cancer-related deaths, the evidence that it prevents conditions unrelated to bone health is not yet conclusive. "Research on vitamin D supplementation has been mixed, and especially when it comes to randomized clinical trials, it has not shown clear benefits," she says. Longer-term follow-up of VITAL and results from other ongoing trials may provide additional information about any links between vitamin D and disease prevention. So far, Dr. Manson and her team have published two meta-analyses (which combine data from multiple studies) of randomized trials of vitamin D. One, published in Annals of Oncology in February, found that vitamin D does appear to help reduce the risk of death from cancer, but doesn't seem to prevent cancer itself. This might mean vitamin D could make tumors less aggressive, less invasive, or less likely to metastasize, she says. The benefits emerged after people had been in the trial for at least one year. However, when it came to heart disease, vitamin D supplementation seems to fall short. A meta-analysis that Dr. Manson and her team published in June in JAMA Cardiology showed no benefit from vitamin D supplements in preventing heart attack or stroke. In the coming months, more results from VITAL are expected to show whether vitamin D plays a role in protecting cognitive function; warding off such conditions as depression, heart failure, and diabetes; and reducing frequency of infections.

The take-home message

When it comes to vitamin D there are some simple strategies you should follow to protect your bones.

Stick to moderate amounts. "It's reasonable to take a small or moderate amount of vitamin D, such as 1,000 IU a day, if you're concerned about whether you're getting enough," says Dr. Manson. Supplements are best for people who tend to have little vitamin D in their diet.

Good food sources include fatty fish, such as salmon, tuna, and sardines, fortified dairy products, including milk, cheese, and yogurt, and cereals.

People who spend very little time outdoors in the sun may also want to take a supplement. Vitamin D is nicknamed "the sunshine vitamin" because your body produces it after sun exposure. Your skin is less able to produce vitamin D as you age. People over age 65, generate only one-fourth as much vitamin D as they did in their 20s.

Address unique risks with your doctor. Large doses of vitamin D (such as more than 4,000 IU a day) should be taken only under the advice of your health care provider.

Taking too much supplemental vitamin D can be toxic in rare cases. It can lead to hypercalcemia, a condition in which too much calcium builds up in the blood. This could cause deposits to form in the arteries or soft tissues. It may also predispose you to painful kidney stones, which are small, pebble-like masses, formed when high levels of minerals in your urine start to crystallize in your kidneys.

Choose food over pills. If possible, it's better to get your vitamin D from food sources rather than supplements. To find a good source, check the nutrition labels on packaged foods to see how much vitamin D they contain.

Image: CharlieAJA/Getty Images

Disclaimer:

As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.