Are you defficient? Too little vitamin D puts more than bones at risk

Nutrition Action Healthletter, Nov, 2006 by Bonnie Liebman

Osteoporosis, muscle weakness, gum disease, diabetes, insulin resistance, arthritis, multiple sclerosis, and cancers of the breast, colon, pancreas and prostate.

That's a partial--and still growing--list of the illnesses that vitamin D may help prevent. How might one vitamin do so much?

Vitamin D isn't like any other vitamin. It's a hormone that has receptors in most--if not all--cells in the body. It affects how cells grow, proliferate, and "specialize," how the body makes bone, muscle, and insulin, and how the immune system fights disease ... or itself.

And many experts are now convinced that we get far too little.

How much vitamin D do we need?

It's been nearly 10 years since the Institute of Medicine issued the latest recommendations--200 to 600 International Units (IU) a day, depending on age. And those levels were based on how much it would take to prevent rickets, the disease characterized by bowed or deformed bones.

"There's been a huge amount of data since 1997," says researcher Bess Dawson-Hughes of the Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University in Boston.

"The evidence has just exploded--not just on bones, but diabetes, infection, insulin resistance, various cancers," she adds. And it looks like we're getting too little vitamin D for all of them.

Most vitamins occur naturally in a variety of foods. Not vitamin D.

"Initially, humans didn't require vitamin D in their food supply," says researcher Bruce Hollis of the Medical University of South Carolina in Charleston, "because over millions of years we evolved a mechanism to produce it in our skin."

Relying on the sun's ultraviolet rays to make vitamin D worked fine when all humans lived near the equator. But "approximately 50,000 years ago, small bands of people, almost certainly darkly pigmented, migrated gradually from sub-Saharan Africa to more northern latitudes," Hollis explains.

Further from the equator, UV rays were scarce, leading to rickets, the vitamin D deficiency disease that deforms bone, including the pelvis. "Populations couldn't survive if they became ricketic because women couldn't deliver a child," says Hollis. "Both would die at birth."

Scientists suspect that the light-skinned races lost their skin pigment so they could absorb more of the sun's UV rays.

"Northern populations became depigmented fast," explains Hollis, because the mutation that led to lighter skin had a huge survival advantage.

"People of color couldn't survive in limited sun," he adds. The exception: Eskimos have endured, because they live on fatty fish, the only food that is rich in vitamin D.

BONES

Vitamin D helps the body absorb calcium, so it's no surprise that giving people vitamin D--with or without calcium--can boost the density of hip and some other bones (though it appears to have little impact on the spine). (1)

Does vitamin D actually keep bones from breaking? Yes, says Dawson-Hughes, "but only if you look at studies in which people took enough to get their blood levels high enough."

In a meta-analysis that pooled the results of those studies, people who took 700 to 800 IU a day had a 26 percent lower risk of hip fractures than similar people who took a placebo. (2)

In other studies, "the doses of vitamin D were similar, but there wasn't a step up in blood levels of D," notes Dawson-Hughes, probably because participants stopped taking the supplements. (3)

For example, in a British study of more than 5,000 older people who had already broken a bone, "the researchers gave 800 IU a day, but only half of the participants were taking any pills after two years of the five-year study," she explains. "Compliance probably trailed off further after that."

The bottom line: 1,000 IU of vitamin D a day could substantially lower the risk of broken bones in older people, says Dawson-Hughes. But, she adds, it's not just vitamin D's impact on bone that would ward off those fractures.

MUSCLE

"Vitamin D affects bone mass and strength, but it also lowers the risk of falling by improving balance and muscle performance," says Dawson-Hughes. "It's a two-pronged benefit."

Muscle tissue has receptors that are specifically designed to accept vitamin D, which suggests that the vitamin must have a key role in muscle function.

"When researchers gave vitamin D to older women, they saw an increase in protein synthesis, which means an increase in muscle growth and size," Dawson-Hughes explains.

Older people with higher blood levels of vitamin D also do better on tests that require muscle strength and balance. (4) "The higher their vitamin D, the faster they can walk eight steps and get out of a chair," she says. "It's a very striking association."

GUMS

Periodontal disease is the leading cause of tooth loss, especially in older people. It's caused by chronic inflammation, which leads to receding gums. Eventually, the tooth starts to wear away.

"Several studies have seen an association between low vitamin D levels and periodontal disease," says Dawson-Hughes. "A link is also turning up with gingivitis, which is the precursor to periodontal disease." (5)