If you live north of the line connecting San Francisco to Philadelphia and Athens to Beijing, odds are that you don’t get enough vitamin D. That means all of us Michiganders are probably “D-ficient.” Being deficient may increase the risk of a host of chronic diseases, such as osteoporosis, heart disease, some cancers, and multiple sclerosis, as well as infectious diseases, such as tuberculosis and even the seasonal flu.

Vitamin D is both a nutrient we eat and a hormone our bodies make. Few foods are naturally rich in vitamin D, so the biggest dietary sources of vitamin D are fortified foods and vitamin supplements. Good sources include dairy products and breakfast cereals (both of which are fortified with vitamin D), and fatty fish such as salmon and tuna.

The body manufactures vitamin D from cholesterol, through a process triggered by the action of sunlight on skin, hence its nickname, “the sunshine vitamin.”  Yet some people do not make enough vitamin D from the sun, among them, people who have a darker skin tone, who are overweight, who are older, and who cover up when they are in the sun.

As we go through the remaining month or so of a particularly harsh winter, remember to smile at the sun and take a supplement. The Institute of Medicine recommends tripling the daily vitamin D intake for children and adults in the U.S. and Canada, to minimum 600 IU per day.

Two forms of vitamin D are used in supplements: vitamin D2 (“ergocalciferol,” or pre-vitamin D) and vitamin D3 (“cholecalciferol”). Vitamin D3 is chemically indistinguishable from the form of vitamin D produced in the body according to the Harvard School of Public Health.

Talk to your doctor if you are already being prescribed medications or supplements. They should know enough about D-ficiency to make recommendations. If you would like to read more about this important topic you can find information and links at http://www.hsph.harvard.edu/nutritionsource/vitamin-d/