Even though sunlight is free, millions of people are falling short on vitamin D, and modern habits are mostly to blame. Work, travel, and leisure now happen almost entirely indoors, leaving very little time for natural sun exposure. When we do step outside, sunscreen and clothing block the ultraviolet B (UVB) rays the skin needs to make vitamin D.

One of the foremost experts on this subject, Dr. Michael Holick, a Boston University endocrinologist and author of The Vitamin D Solution, has spent decades studying the effects of sunlight on human health. He explains that in many northern cities (for example, Boston), “virtually no vitamin D is made in the skin from November through March,” no matter how long you stay outdoors. Holick estimates that 1.5 to 2 billion people worldwide are deficient, calling it “the most common medical problem on the planet.”

Diet isn’t helping much either. Only a few foods (like salmon, egg yolks, and fortified milk) provide meaningful amounts of vitamin D, and most people simply don’t eat enough of them. Rising obesity also plays a role: because vitamin D is fat-soluble, it can become trapped in fat tissue, making less available for the rest of the body. Add it all up, and it’s clear why so many adults, especially in colder climates or office jobs, are running low on this essential nutrient.