Eat, drink and be healthy. Vitamin D: it's an essential nutrient, but getting enough of it is not necessarily easy. Jennifer Huget reports.
For most of history, humans got all the vitamin D they needed simply by being outdoors, absorbing the sun's ultraviolet light.
Ultraviolet radiation triggers the body's production (a two-step process involving both liver and kidney) of the vitamin, which is technically a hormone.
What the body didn't use right away, it stored in fat. But over the millennia, humans moved into the shade.
They migrated from equatorial regions to less sunny latitudes, switched from hunting and farming to indoor jobs and - just in the past few decades - began slathering themselves with sunscreen.
As scientists in recent years began to assess the extent and import of this diminished exposure to sunlight, they've also amassed compelling research showing just how vital vitamin D may be.
Identified in the early 20th century as the best defence against the bone-softening disease rickets, vitamin D has since been lauded for fending off osteoporosis and hip fractures in older people (by empowering calcium to strengthen bones and probably by boosting muscle strength, which helps keep people on their feet).
Some research even suggests vitamin D may work in the brain to improve balance, another boon to tumble-prone seniors.
Researchers have discovered that nearly every kind of tissue in the body is equipped with vitamin D receptors, which suggests the substance may be involved in all kinds of functions.
Though studies of vitamin D's role in cancer prevention have not proved conclusive, it appears likely the vitamin may help protect against colon cancer and perhaps against breast and prostate cancer.
It's also believed to help ward off diabetes and cardiovascular disease, and there's a growing hunch that lack of vitamin D may make people more vulnerable to such autoimmune diseases as multiple sclerosis. (That idea is supported by the fact the incidence of MS increases with distance from the equator.)
So, pretty much everyone agrees that your body needs vitamin D.
Just how much, though, is under debate.
United States guidelines established 12 years ago by the Institute of Medicine recommend 200 international units, or IUs, for children and others through to the age of 50, 400 IUs for people aged 51 to 70 and 600 IUs for those 71 and older.
But many experts now say we need more; probably twice, maybe three or even five times more.
The amount of vitamin D you need depends on several variables: the older you get, the more you need to ingest, because your body loses its facility for producing it; dark-skinned people don't produce as much as light-skinned folk; the heavier you are, the more vitamin D you need to take in.
But no matter how much you need, the question remains: what's the best way to get this nutrient into your body?
Vitamin D occurs naturally in just a few foods.
Fatty fish such as salmon, tuna, mackerel and herring top the list, but as Bonnie Liebman, director of nutrition for the Centre for Science in the Public Interest and author of many articles about vitamin D, notes, you'd have to eat an awful lot of herrings to make your quota.
Similarly, other food sources such as eggs contain such small amounts that you'd need to pull a Cool Hand Luke (the movie where a man eats 50 eggs) to consume enough.
In days of yore, people swore by cod liver oil for health benefits.
Indeed, a single teaspoonful provides several days' worth of vitamin D.
Ms Liebman points out cod liver oil also contains lots of vitamin A, and too much of that vitamin can actually increase the risk of osteoporosis and hip fracture.
Other foods that we associate with vitamin D, such as milk and breakfast cereal, don't naturally contain the vitamin but are instead fortified with it.
Even so, neither provides enough to meet present estimations of a person's needs. That leaves us with two other options: supplements and sunshine.
Ms Liebman is among the many experts who advocate meeting daily vitamin D demands through taking supplements.
Many daily multivitamins provide at least the recommended 400 IU, and popular calcium supplements also contain vitamin D. (There appears to be little risk of overdoing it; vitamin D overload is rare, and the most likely health problem it might entail is increased risk of developing kidney stones.)
But Michael Holick, a professor of medicine, physiology and biophysics at Boston University School of Medicine, likes to keep sunshine in the mix.
Prof Holick, who has been at the forefront of vitamin D research, thinks babies and kids should get 1000 IUs of vitamin D daily; teens and adults, he says, should get 2000.
Acknowledging the difficulty of getting that much from food, Prof Holick suggests combining supplements and what he calls "sensible sun exposure", which means spending a limited time outdoors with your arms and legs uncovered (and free of sunscreen), then applying clothing or sunscreen long before the danger of sunburn sets in.
