There isn’t a vitamin more important to your health than vitamin D. You need it to build and maintain strong bones. Your immune system is crippled without it. Vitamin D deficiency appears to raise the risk of almost every chronic disease, from cancer to diabetes to cardiovascular disease to age-related cognitive decline to auto-immune disease.

Yet professor and nutrition researcher Peter Horvath at the University of Buffalo School of Public Health and Health Professions in Buffalo, New York, reports that during Buffalo’s winter months, nearly 50% of the population in Buffalo has insufficient levels of vitamin D, while 25% are outright deficient.

“Every cell in your body,” he says, “is responsive to vitamin D. If you’re deficient, you won’t see the health effects for years and it could take months to get your levels back up.”

Dr. Horvath is probably using the standard definition of vitamin D deficiency—a blood level of less than30 ng/mL (or 75 nmol/L). However other vitamin D experts define deficiency as anything below 50 ng/mL (or 125 nmol/L). Using this definition, upwards of 75% of our population is estimated to be deficient.

Our skin makes vitamin D from UVB sun rays. For a long time it was assumed that most people could store up enough vitamin D during the sunny summer months to see them through sunless winters. (Vitamin D is a fat soluble vitamin and is stored in our fat cells.) Also, the only known deficiency disease was rickets, a bone deformity, which was rare.

However more recent research has revealed that vitamin D has many more functions in the body than just building healthy bones, and that we need much more than previously thought. How much is still a matter of debate. US government health agencies have generally accepted 30 ng/mL as sufficient. Although they recommend supplementation, they recommend relatively low doses, and once again this year, the United States Preventive Services Task Force has rejected routine vitamin D screening for healthy people.

At Beyond Health we believe this is a big mistake. Supplementing with vitamin D is inexpensive and safe, and taking adequate amounts can pay big dividends. We agree with experts in vitamin D research who recommend maintaining a blood level of 50-70 ng/dL (or 125-175 nmol/L).

You can learn your vitamin D status by asking your doctor to do a blood test called the 25(OH)D. Your levels should be at their lowest now that winter is coming to an end.