The main role of vitamin D is to keep your bones healthy, primarily by helping you absorb intestinal calcium. In recent years, however, vitamin D deficiency has gained a lot of attention for being associated with a long list of diseases ranging from heart disease and cancer to diabetes mellitus, depression, dementia, colds, multiple sclerosis, and premature death (Click for Benefits of Vitamin D video).

As a result, many physicians, including myself, have been screening for vitamin D deficiency in people who are at risk—such as those who get limited sun exposure without sunscreen, have dark skin, are older, or obese—and have been recommending vitamin D supplementation.

However, researchers are beginning to caution that the zeal for vitamin D supplementation has outpaced scientific evidence of its effectiveness. Among them are JoAnn E. Manson, MD, DrPH, and Shari S Bassik, ScD, both from the Division of Preventive Medicine at the Brigham and Women’s Hospital in Boston. In a recent commentary in the Journal of the American Medical Association, they add:

“Although moderate doses of vitamin D may be beneficial, more is not necessarily better—and may be worse.”

So, should you take a vitamin D supplement? And, how much do you really need?

The Vitamin D Dosing Dilemma: The four most important facts you need to know

Vitamin D has only been proven to benefit bone health.

Although there have been a flurry of studies suggesting vitamin D may do much more, the design of those studies limit proving other benefits.

As background, interest in vitamin D was first spurred by ecologic studies, which observed that people who lived in regions of the world with greater sunshine had lower rates of cancer and heart disease.

Subsequently, lab tests found a possible link: nearly every tissue in the body contains vitamin D receptors, or proteins that bind vitamin D, including the prostate, the heart, blood vessels and muscle.

However, the majority of studies associating vitamin D with health benefits are observational studies—studies that observe an association between vitamin D deficiency and a particular disease. By design, these studies do not prove vitamin D deficiency causes the disease, or that replacing vitamin D would prevent developing the disease.

Large interventional randomized controlled trials are underway to determine if supplementing with vitamin D can improve cancer and heart disease outcomes. Thus far, such “gold standard” studies have been too small to make conclusions.

Too much vitamin D may be as harmful as too little.

The relationship between your vitamin D level and your health is not linear. Rather researchers describe the relationship as a U shaped curve—when your level is too high or too low, your risk of developing disease is highest. The sweet spot is being on the horizontal part of the curve.

Many labs define the horizontal part as ranging between 30ng/mL to 80 ng/ml. However, the “normal,” or optimal level for vitamin D, has not been established. According to the Institute of Medicine, a level of 20ng/mL or higher may be sufficient. In other words, many people flagged as deficient may be just fine. Similarly, levels in the upper range of “normal,” such as those greater than 50ng/mL, have been associated with higher rates of pancreatic cancer, cardiovascular events, and all-cause mortality. For now, I think a reasonable recommendation would be to target a level between 30ng/mL to 50ng/mL.

You do not need high dose vitamin D supplements.

The recommended daily intake (RDA) of vitamin D is 600 IU daily for people age 1 to 70 and 800 IU daily for 71 and older. The Institute of Medicine considers this amount adequate for more than 97.5% of the United States and Canadian population. While doses up to 4000 IU daily will not cause harm, these higher doses are not necessary for the general population.

The current recommended daily amount of vitamin D is based on requirements for bone health. As results of ongoing large studies that are looking at the benefit of vitamin D for chronic disease prevention become available, we may find that this recommendation may prove too conservative and higher amounts may be necessary for preventing diseases such as heart disease and cancer.

The most practical way to get vitamin D is through a supplement.

Vitamin D is both a nutrient and a hormone. Yet, unlike other nutrients, there are very few foods that naturally contain vitamin D. The best sources are fatty fish, such as salmon, tuna and mackerel, and fish liver oils. In the American diet, most of our vitamin D comes from foods fortified with vitamin D— such as milk, breakfast cereals, and some types of orange juice. However, the amount of vitamin D through fortified food is often not enough to add up to your daily recommended amount. For example one cup of milk contains approximately 100 IU.

Vitamin D is considered a hormone because we can also make it in our body. By some estimates, we can make adequate vitamin D from 5-30 minutes of full body sun exposure twice a week without the use of sunscreen (SPF 8 or greater). However, the most practical way to get adequate vitamin D—without increasing your risk of skin cancer and ensuring that your dietary intake doesn’t fall short—is through a supplement.

More to come

By late 2017, the results of four large ongoing trials looking at the risks and benefits of supplementing with higher doses of vitamin D than the current recommendation will start to become available. These studies, the largest led by Dr. Manson, will definitively determine the amount of vitamin D needed for prevention of cancer, cardiovascular disease, diabetes, depression, infection, and other conditions.

Until then, it’s best to avoid getting too little, which is commonly the case without a vitamin D supplement, or too much from high dose supplements.

Question: Do you know your vitamin D level? Are you taking a vitamin D supplement? You can reply using Twitter, Facebook, or LinkedIn.