images-1Doctor’s orders: Get more sunlight on your skin.

We human beings were built to be outside. The latest evidence: widespread vitamin D deficiency. Over half of Americans have dangerously low levels of vitamin D. Why? Because we’re spending too much time indoors.

Vitamin D prevents osteoporosis, depression, prostate cancer, breast cancer, and even the effects of diabetes and obesity. Vitamin D is perhaps the single most underrated nutrient in the world of nutrition. That’s probably because it’s free: your body makes it when sunlight touches your skin. Drug companies can’t sell you sunlight, so there’s no promotion of its health benefits.

Unfortunately, office-bound Americans are spending less time than ever outdoors. As a result, the prevalence of low vitamin D levels is increasing in every segment of the general population, especially in the winter months.

The only way to ensure healthy vitamin D levels is to go outside each day. That’s because our bodies are hard-wired to be outdoors. Just as plants need sunlight for photosynthesis, our bodies need sunlight for adequate vitamin D—and consequently, for adequate calcium levels as well (calcium requires Vitamin D for absorption).

Vitamin D is produced by your skin in response to exposure to ultraviolet radiation from natural sunlight. The ultraviolet portions of natural sunlight cannot adequately penetrate glass, so you don’t generate vitamin D when sitting in your car or home. Sunscreens also impede the body’s ability to produce vitamin D.

Fish, mushrooms, fortified breakfast cereals, fortified milk, orange juice, and vitamin supplements can help slightly, but it is nearly impossible to get adequate amounts of vitamin D from your diet alone. A person would have to drink ten tall glasses of vitamin D fortified milk each day just to get the minimum levels of vitamin D into their diet. Sunlight exposure is the only reliable way to generate vitamin D in your own body.

The further you live from the equator, the longer exposure to the sun you need in order to generate vitamin D. For most of us living in the Southeast, you’ll need about 15 minutes of exposure, 3-4 times a week. And in the winter, you’ll need even more sun exposure, since the sun’s angle is lower and less intense.

In addition, people with dark skin pigmentation may need 20 to 30 times as much exposure to sunlight as fair-skinned people to generate the same amount of vitamin D. That’s one reason why prostate cancer is epidemic among black men — it’s a simple, but widespread, sunlight deficiency.

There are certainly legitimate concerns about overexposure to sunlight leading to skin cancer, but 15 minutes of natural sunlight is not likely to cause skin problems. And it is absolutely essential for the long-term health of your body.

So take a long lunch break and go for a run or bike ride. Or just step outside of your home or office for a 10-15 minutes stroll. You might as well get your daily dose of exercise along with your sunlight.