Are you getting enough Vitamin D? Sunlight through glass doesn't count

Sunlight doesn’t actually “provide” you with Vitamin D. Rather, your body produces Vitamin D when skin is exposed to the sun’s ultraviolet rays, which trigger Vitamin D synthesis. The liver and kidneys convert this biologically inert form of Vitamin D into biologically active forms the body can use to promote calcium absorption and bone health.

But sunlight consists of both ultraviolet A or UVA, which penetrates deep within the skin layers and can cause premature ageing; and ultraviolet B or UVB, which causes the redness of sunburn. It’s the UVB rays that trigger the synthesis of Vitamin D.

Many people can derive the Vitamin D that their bodies need through direct exposure to sunlight during the summer months. As little as 10 minutes a day of sun exposure is typically adequate. But for many, particularly those living in northern climes, production of Vitamin D will be inadequate during the winter months.

READ: Vitamin D and fish oils are ineffective for preventing cancer and heart disease

And you can’t get adequate UVB exposure sitting indoors or in a car. Virtually all commercial and automobile glass blocks UVB rays. As a result, you will not be able to increase your Vitamin D levels by sitting in front of a sunny window, though much of the UVA radiation will penetrate the glass and may be harmful.

“It doesn’t matter if it’s winter or summer, you will make no Vitamin D sitting in front of a window – zip,” said Dr Michael Holick, a professor of medicine, physiology and biophysics at Boston University School Of Medicine.

Those concerned about low Vitamin D levels can get more of the vitamin through foods. The best dietary source for Vitamin D is old-fashioned cod liver oil. Other dietary sources include swordfish and salmon and, to a lesser extent, fortified milk, orange juice and yogurt, as well as sardines canned in oil, egg yolks and fortified cereals. Dietary supplements are also available.

By Roni Caryn Rabin © 2019 The New York Times