Can you get “too much” vitamin D in the summer?

Summer is upon us. So it’s a little easier to get the healthy sun exposure needed to activate vitamin D in your body. In fact, you may wonder–should I still take my vitamin D supplement during the summer? Will this give me “too much” vitamin D?

Those are good questions. Unfortunately, many so-called experts get the answers wrong.

To clear up the issue, I want to share with you an important study recently published in the Journal of Adolescent Health. It helps explain whether you can get too much of a good thing, when it comes to vitamin D. I’ll tell you all about that groundbreaking study in a moment. But first, let’s back up.

Vitamin D appears to benefit virtually every part of your body. And it seems to help protect you against nearly every chronic disease.

Plus, now we are learning more and more about how vitamin D works.

And as I’ve said before, this is very important to research scientists. And to the scientific world, in general. We don’t just want to know that something does work. We want to know how it works. In scientific terms, this means we understand a vitamin’s “mechanism of action” in the body.

I learned more about vitamin D’s “mechanism of action” in the body a few years ago from my colleague Dr. Michael Holick of Boston University. In 2006, I presented the keynote speech at the annual Johns Hopkins conference on natural and nutritional medicine. Dr. Holick followed my speech with another talk on vitamin D. He explained that it really acts as a hormone in the body, in addition to being an essential nutrient.

Dr. Holick says we face a global epidemic of vitamin D deficiency. And government guidelines do nothing to curb the problem. In fact, the government recommends men and women take just 600 IU of vitamin D daily. This recommendation stems, in part, over fears that you might “overdose” on vitamin D if you take “too much.”

This old, overdosing myth persists regarding the dangers of all vitamins and minerals, but especially for fat-soluble vitamins like A and D. (The one nutrient that we should be worried about is iron. Yet most doctors and government “experts” like the CDC continue to push this supplement, despite the dangers.)

But is concern about vitamin D based on actual science?

Yes, vitamin D is a fat-soluble vitamin. And that means you can store some of it in your tissues. But can you really accumulate too much vitamin D in your tissues, as some experts warn?

Clinically, this fear just doesn’t bear fruit. It is exceedingly rare to find vitamin A or D overdoses. And you certainly don’t see them when moderate doses of the vitamins are taken.

The new study published in the Journal of Adolescent Health helps underscore this practical observation.

For the study, U.S. researchers wanted to see whether healthy adolescents–with sufficient serum vitamin D levels already–would “overdose” when given a vitamin D supplement.

The researchers recruited 56 healthy adolescents, ages 11 to 19 years. And at the outset, they measured all the participants’ blood levels. None of the participants was deficient in vitamin D.

Then, the researchers randomly divided the adolescents into two groups. The two groups were matched in terms of age, race, gender, body mass, and dietary calcium and vitamin D intake.

The first group took 200 IU of vitamin D for 11 weeks. The other group took 1,000 IU of vitamin D each day for 11 weeks. (The government-recommend amount for this age group is 600 IU per day.)

At the start of the study, the 200 IU group had blood levels of vitamin D of 28.1 ng/mL. After 11 weeks, their blood levels rose to 28.9 ng/mL on average. The group that took 1,000 IU of vitamin D per day had blood levels of 29.0 ng/mL at the start of the study. After 11 weeks, that group raised blood levels to 30.1 ng/mL.

As you can see, the average change in vitamin D levels did not differ significantly between the two groups.

In addition, the groups did not differ significantly in measurements of bone turnover, calcium, glucose, insulin, or parathyroid hormone (which mobilizes calcium from bone).

So what can we conclude from this study?

When vitamin D levels are adequate (not deficient) to begin with, supplementation will not raise blood serum to higher levels. And certainly not to dangerous levels.

Obviously, this study followed vitamin D use in healthy adolescents. But I suspect further research will uncover the same results in healthy adults.

These results suggest that supplementing with 1,000 IU per day will not affect those who do not need it. But it will likely make a world of difference in those who do.

Previous studies strongly suggest that taking vitamin D can help treat or prevent many medical conditions. Those studies also suggest that 1,000 IU per day is beneficial. And it can help restore blood vitamin D levels in those who are deficient.

Given the growing importance of vitamin D, universal supplementation of 1,000 IU per day, even during the summer, appears safe for everyone. It can’t hurt those who don’t need it. And it will likely help those who do.


1. “A Randomized Clinical Trial of Vitamin D Supplementation in Healthy Adolescents,” Journal of Adolescent Health 2013; 52(5): 592-598