Have you had your vitamin D level checked lately? Chances are good that it has dropped since last summer.

Unless you are taking a supplement, most of your vitamin D is made by your skin when it is exposed to sunlight. Winter sun is too weak to do the job, even if you could stand to expose enough skin in the frigid air.

A surprisingly high proportion of people become deficient in vitamin D at this time of year. A recent study of pregnant women demonstrated that vitamin D blood levels (25-hydroxyvitamin D) peaked in the summer and dropped in the winter (Paediatric and Perinatal Epidemiology online, Dec. 20, 2013).

Why might this matter? In the past several decades, researchers have discovered that vitamin D is important for more than building strong bones. Virtually every cell depends upon vitamin D for normal functioning.

According to studies, this nutrient enhances the immune system and is critical for normal cell division. That’s why people deficient in vitamin D may be more susceptible to cancer, infections (including influenza) and autoimmune conditions such as arthritis, diabetes and multiple sclerosis.

Common conditions such as heart disease and high blood pressure also may be related to poor vitamin D status (Archives of Toxicology, December 2013). It is less clear, however, that taking vitamin D supplements will reverse the damage.

Grandmothers in cold climates have had an empirical way to deal with low vitamin D during winter. They have been administering cod-liver oil to their family members for centuries.

Traditional cod-liver oil tasted and smelled awful. Children who were dosed with it remember this as torture even when they reach a ripe old age. Nevertheless, this winter tonic was an excellent source of vitamin D and vitamin A at a time when these nutrients were in short supply.

Today, doctors are debating how much vitamin D people need and what is the optimal blood level. There may be some individual variability in how well people utilize supplemental vitamin D, so it might make sense to add enough so that 25-hydroxyvitamin D reaches an appropriate level. There are details about the benefits of vitamin D and how much to take, along with the differences between D-2 and D-3, in our Guide to Vitamin D Deficiency. Anyone who would like a copy, please send $3 in check or money order with a long (No. 10), stamped (70 cents), self-addressed envelope to: Graedons’ People’s Pharmacy, No. D-23, P.O. Box 52027, Durham, NC 27717-2027. It also can be downloaded for $2 from our website:

We recently heard from a physician who was having trouble with shoulder arthritis. Out of curiosity, he had his vitamin D level measured and discovered that it was quite low. After he started taking supplements to bring the level back to normal, the pain in his shoulder subsided. He is convinced that the vitamin D made a difference in the quality of his life.

Clinical trials now under way may answer the question about the benefits of extra vitamin D. Until then, people need to monitor their own levels so they can stay healthy.