Tammi Leader Fuller, 55, is outdoors most of the time, running her Malibu, Calif.-based sleepover camps for grown-ups. Yet, she knows she does not get enough of D, the "sunshine vitamin," because it requires ultraviolet rays to be absorbed.
"I'm always covered with sunscreen because I've had skin cancer," said Leader Fuller (www.campowerment.com). "So I'm in the sun, but my D level was way low. Now I take a 10,000 IU supplement a day and eat vitamin D foods, and my level is almost normal."
Vitamin D deficiency is not unique to people in cloudy Northern states, said Kim Larson, a registered dietitian/nutritionist in Seattle and a spokeswoman for the Academy of Nutrition and Dietetics. It's a byproduct of our lifestyle."
"We stay inside in the North to keep warm, or in the air-conditioning in the South to keep cool," Larson said. "Instead of going outside to play, we're inside, on our computers. When we're out, we wear clothing or sunscreen to protect us from cancer."
Although the advent of D-fortified milk in 1932 eliminated widespread rickets (soft bones) among children, milk is no longer a diet staple. Gone are the "Ozzie and Harriet" days when almost every meal included milk.
About 42 percent of adults are D-deficient, according to the most recent National Health & Nutrition Examination Survey. The percentage varies a lot by race: Blacks have the highest rate, 82 percent.
Most people she tests in Seattle lack enough vitamin D, Larson said, and it is especially prevalent among teens. "They complain of being sore and tired, and you think it's because of their busy sports schedules," she said. "But they aren't getting enough D."
Half of the 60 adults in his 2014 study did not have enough D, said Peter Horvath, associate professor of exercise and nutrition sciences at the University of Buffalo in New York state. "You might not see the effects for years, though, so you don't know it," he warned.
D is the closest thing to a magic bullet in the vitamin world, affecting our health from every angle. It maintains our calcium and phosphorous levels, which in turn strengthen our immune system, keep us sharp and help prevent heart disease, high blood pressure, Type 2 diabetes and many types of cancer. D helps prevent osteoporosis (weak bones) and osteomalacia (bone pain).
Doctors prescribe high D levels for the treatment of a range of ailments from autoimmune disorders to multiple sclerosis.
Getting the right combination of food and sunlight, though, is complicated.
Dark-skinned people are at greater risk for D deficiency because their skin shields them from sunlight. Older people's skin is less able to process sunlight. Being overweight means D is jailed by your fat tissues instead of being used efficiently.
Many common medications, including diuretics and anti-seizure drugs, counteract vitamin D.
Ask your doctor to give you a vitamin D test, which may not be part of an annual exam, Larson said. Then a dietitian can prescribe a custom combination of foods and supplements.
For Gail Rubin, 56, of Albuquerque, N.M., for example, a 2,000 IU (international units) supplement of D keeps her on track. She's a breast cancer survivor, has osteoporosis in her genes, eats D-rich foods but avoids the sun -- all of which affect her D intake.
Rubin and Leader Fuller said they read food labels because scouting vitamin D in the grocery store is tricky. Outside of cod liver oil and some fish, few foods meet the daily requisite of 600 IUs for people ages 1 to 70 and 800 for ages 71-plus, as recommended by the Institute of Medicine of the National Academies.
"Fortified" cereals vary from 10 percent of the recommended level of D in Raisin Bran to 25 percent in Total. The cup of 2 percent milk you add to your cereal gives you only 25 percent.
Horvath favors D-rich sun-dried mushrooms. "Either eat mushrooms, which you can dry yourself in your backyard, or add the powdered form when you cook something like an omelet," he said.
Scan grocery purchases for the "USP" stamp or the www.consumerlab.com triangular logo, Larson said. They tell you those goods have been checked by independent labs.
Ideally, also get some midday sunlight, sans sunscreen, although experts concede this is not a reality for many of us. During the winter, the Earth's tilt away from the sun reduces your exposure. "Then, you could lie outside naked in the snow and not get enough sun because of the sun's angle," Horvath said.
In the next decade, we'll see a "burst of research about vitamin D," Larson said. "We're learning more about the effects of D on autoimmune diseases, cancer and heart disease, especially."
This will underscore the importance of D, Horvath said, the vitamin we must chase "despite our immobile, indoor lifestyles."