Vitamin D is an interesting nutrient. It's actually a hormone, mostly produced by exposure of skin to sunlight. Only about 10% of your body's active vitamin D comes from foods you eat.

When your body produces vitamin D from the sun and absorbs it from food, the liver and kidneys convert it into the hormone calcitriol.

In your body, activated vitamin D helps increase the absorption of calcium, so it can enter your bloodstream, helps prevent calcium loss from the kidneys, and promotes the formation of new bone cells.

It also plays an important role in bolstering your immune system, pushing calories into muscle rather than fat, countering depression, and heart disease, as well as helping prevent the progression of prediabetes to Type 2 diabetes.

A 14-year Chinese study shows that people with prediabetes (almost 100 million Americans) and lower blood levels of vitamin D were far more likely to progress to Type 2 diabetes than those with healthy levels of D. And if their triglyceride levels were also elevated, the risk was even greater.

Because of how much time most people spend indoors, we recommend that you get a blood test to check your D levels and take a daily supplement of 1,000 IU of D3 (or more if the test indicates that).

You can also boost your D with wild-caught salmon and other foods discussed. Get the best recipes in Dr. Mike's "What to Eat When Cookbook."