Vitamin D review: Latest studies explore benefits and risks

By John Murphy, MDLinx
Published June 21, 2019

Key Takeaways

Taking extra vitamin D doesn’t help the heart, according to the authors of a new study in JAMA Cardiology. It’s somewhat surprising news because, in previous observational studies, researchers have indicated that lower levels of vitamin D are linked to greater risks for heart attack, stroke, heart failure, and mortality due to cardiovascular disease (CVD). So, it seemed completely reasonable that upping vitamin D levels with supplements would lower these CVD risks.

But in this new meta-analysis, which included 21 randomized clinical trials and nearly 83,300 patients, investigators determined that vitamin D supplementation isn’t linked to lower risks for major adverse cardiac events such as heart attack, stroke, CVD mortality, and all-cause mortality.

“The findings suggest that vitamin D supplementation does not confer cardiovascular protection and is not indicated for this purpose,” the authors concluded.

However, “it also should be emphasized that vitamin D therapy in patients with chronic kidney disease and hyperparathyroidism is definitely indicated, and such therapy has established cardiovascular benefits, including blood pressure reduction, reduced electrolyte derangements, and overall reduced cardiovascular mortality rates in patients on hemodialysis,” wrote authors in a related commentary in JAMA Cardiology.

Perhaps we don’t yet know everything there is to know about vitamin D. To that end, here are some of the top vitamin D-related reports from 2019.

Vitamin D doesn’t prevent type 2 diabetes

In previous studies, researchers have found that people with low levels of vitamin D have a higher risk for type 2 diabetes. So, in a recent trial—the largest of its kind—investigators tested whether vitamin D supplementation would help patients already at high risk for developing diabetes. Participants took 4,000 IU of vitamin D per day—the upper limit of the recommended intake. But, in the end, they didn’t have any less risk for developing diabetes than those in the placebo group.

“The message is there’s no magic pill,” said the study’s lead author, who added that only two things can help prevent diabetes. Read more about what can prevent diabetes.

Vitamin D could help cancer patients live longer

Vitamin D, if taken for at least 3 years, could help people with cancer live longer, according to researchers who presented their findings at the American Society of Clinical Oncology (ASCO) annual meeting on June 3, 2019.

The researchers performed a meta-analysis and found several trials, involving more than 79,000 participants, that compared the use of vitamin D to a placebo, and also had data on cancer incidence and mortality. From this analysis, the researchers concluded that vitamin D was associated with a significant reduction in cancer-related mortality compared with placebo.

But there are still many unknowns, the authors cautioned, such as how much of the vitamin to take, how much longer vitamin D extends survival, and why it even has this effect. Learn what else these authors discovered.

Vitamin D doesn’t reduce cancer risk

Although taking vitamin D supplements may extend survival (as described above), it doesn’t lower cancer risk, according to results of the largest-ever clinical trial to assess vitamin D for cancer prevention.

Even after 1 year of taking vitamin D supplements above the recommended level, participants in the vitamin D group had the same incidence of invasive cancer (6.1%) as participants who received placebo (6.4%). Read what the researchers also found out about vitamin D and cardiovascular risk.

Does sunscreen shortchange your vitamin D?

If sunscreen can reduce the sun’s harmful effects on the skin, does it also inhibit the body's production of vitamin D? To find out, investigators went to sunny Tenerife, the largest of Spain’s Canary Islands, to put different sunscreens to the test. Their findings indicated that sunscreen protected the skin from sunburn without compromising vitamin D levels.

“Sunlight is the main source of vitamin D. Sunscreens can prevent sunburn and skin cancer, but there has been a lot of uncertainty about the effects of sunscreens on vitamin D,” said the study’s lead author. “Our study, during a week of perfect weather in Tenerife, showed that sunscreens, even when used optimally to prevent sunburn, allowed excellent vitamin D synthesis.” See how ultraviolet light also factored into the calculation.

More vitamin D improves memory, but slows reaction times

We all need vitamin D. It helps build and maintain healthy bones and teeth. Without enough vitamin D, you’d develop osteomalacia—more commonly known as rickets. The recommended daily amount of vitamin D for the average adult is 600 IU.

While it’s essential to get enough vitamin D, too much can also cause problems. In a study led by Rutgers University researchers, overweight or obese older women who took more than three times the recommended daily dose of vitamin D showed an improvement in memory and learning. However, these women also had slower reaction times, which may have increased their fall risk. Find out how the researchers made this connection.

You can get too much vitamin D

A recent case report illustrated the danger of taking too much vitamin D. A 54-year-old man presented with a high level of creatinine, suggestive of acute kidney injury or malfunction. He reported that he had just returned from a trip to Southeast Asia where he spent much of his holiday sunbathing. He also revealed that he had seen a naturopath, who prescribed high doses of vitamin D, even though he did not have a history of bone loss or vitamin D deficiency. Over 2.5 years, the patient took 8,000–12,000 IU of vitamin D daily. As a result, he had very high levels of calcium in the blood, which left him with significant kidney damage.

“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,” wrote the authors of the case report. Read what else they had to say about the “unfettered use of vitamin D.”

Vitamin D for ED

Vitamin D is primarily known to help bone health and for its role in calcium-phosphorous homeostasis—but results of a recent study showed it may also improve erectile dysfunction (ED). The researchers reported that men who received vitamin D supplements showed increased testosterone levels and improved erectile function. The trial included only 41 men, so a larger study is needed to confirm the results. But what mechanism might be behind vitamin D’s erectile effect? Click here to find out.

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