The summer sun can boost your bone health, immune function, and glucose control as it tamps down inflammation. How does it do that?

Ultraviolet-B rays hit precursor vitamin D in your skin cells, providing the energy needed for your body to synthesize vitamin D2 or D3.

Unfortunately, the prevalence of an indoor, sedentary, nutritionally poor lifestyle means that 35% of U.S. adults are deficient of vitamin D. And it's a greater problem among those with darker skin, who are 15 to 20 times more likely to have severe deficiency. 

In addition, people who are obese have a 35% higher chance of vitamin D deficiency.

What's the best summertime sun strategy?

North of a line that runs from New York City to Sacramento, California, 5 to 15 minutes of midday sunlight on arms, legs, and face three times a week can maintain healthy vitamin D levels in Caucasian adults.

Darker-skinned people should add more time but make sure not to burn. Use sunblock with micronized zinc at all other times, and reapply every 10 to 15 minutes.

But what can you do during the winter (or if a blood test shows you're chronically deficient)?

A new study reveals that consistently getting aerobic exercise indoors prevents your vitamin D level from decreasing during the darker days of winter. Study participants did 30-plus minutes four times a week, including one high-intensity interval training session.

Still need a boost?

Take vitamin D2 or D3 supplements year-round — 1,000 IU daily is a basic routine. Add vitamin K3 and make sure to eat foods such as salmon and mushrooms. Find recipes in my "What to Eat When Cookbook."