In recent years, as disappointing research cast a dark cloud over the hyped benefits of some vitamins and nutrients, vitamin D has been a ray of sunshine.
It’s partly because vitamin D is naturally produced in our skin when exposed to the sun’s ultraviolet rays.
But, it’s also because its reputation as a bone builder and disease fighter has been borne out by increasingly solid evidence.
“More studies are showing that vitamin D in the body is more diverse and more important than what we had thought,” says Cheryl Rock, a registered dietitian and professor of nutrition at the University of California San Diego Medical School. “We’ve known that it’s crucial for the absorption of calcium, but now we think it may relate to cancer because its biochemical activity is so diverse.”
In 2007, researchers from UCSD’s Moores Cancer Center found that people with high blood levels of vitamin D had half the risk of breast and colon cancer of those with low levels.
A review of dozens of studies and research in the American Journal of Public Health says that women with the lowest vitamin D levels are five times more likely to get breast cancer than those with the highest levels, and the vitamin slows the progression of the disease. Low levels of vitamin D also correlate with up to a 70 percent increase in the risk for prostate cancer and double the risk of colon cancer.
And in a Harvard School of Public Health study in 2006, researchers found that people who took in the highest amounts of vitamin D cut their risk of pancreatic cancer almost in half compared with those with the lowest intakes.
What’s the cancer link? Research has shown that vitamin D helps regulate cell growth, a fundamental biological process that goes haywire in cancer.
But it may be more than just cancer risk and bone health that benefit from increased vitamin D.
In a study published earlier this year, researchers at Harvard Medical School found a strong link between vitamin D deficiency and cardiovascular disease. Researchers followed more than 1,700 members of the Framingham Offspring Study (children or grandchildren of the original participants in the 1948 Framingham Heart Study) for more than five years. They found the rate of cardiovascular disease events such as heart attacks, strokes and heart failure were from 53 percent to 80 percent higher in people with low levels of vitamin D in their blood.
A study in the June 2007 edition of Archives of Internal Medicine found a “significantly higher” prevalence of hypertension, diabetes and high triglyceride levels in individuals with lower levels of vitamin D.
Scientists are not sure what mechanisms connect vitamin D with reduced risk of heart disease and diabetes, but some studies have shown that the vitamin can lower inflammation by increasing levels of anti-inflammatory messengers.
Researchers are careful to point out that this study does not prove that taking vitamin D supplements reduces heart attacks and strokes. That can only be done with a large clinical trial in which vitamin D is compared with a placebo.
A deficiency of the vitamin is also believed to weaken the immune system. Some research shows that increased vitamin D may also protect against multiple sclerosis and rheumatoid arthritis, in which the immune system attacks the body’s own healthy tissue.
Vitamin D functions a bit differently from other vitamins. The inactive form of vitamin D, made by our skin as a result of sun exposure or obtained from food, is like a reservoir of raw material stored in our livers, fat tissues, muscles and blood. When needed, it’s converted to an active form, actually a hormone, that tells various parts of the body what to do.
While much of the research presents a positive and hopeful picture of how the sunshine vitamin can benefit us, it only works if we get enough of it. And, according to some medical and nutrition experts, many of us don’t.
One-third to one-half of otherwise healthy, middle-aged to older adults have low levels of vitamin D in the United States, Harvard researchers say. A University of California San Diego study found that the average U.S. adult intake of vitamin D is only 230 international units (IU) daily. The government’s current recommendation for vitamin D is 200 IUs a day for people up to age 50, 400 IUs to age 70, and 600 IUs for people over 70.
Many experts say the government guidelines are too low, because we’re not getting enough of the vitamin from the sun or our food.
Although the conscientious use of sunscreen has successfully filtered out the harmful cancer-causing UV rays, it’s also reduced the amount of vitamin D we’d normally get from the sun. During winter when the sun is low and people are not outdoors as often, people’s vitamin D levels drop significantly.
The problem is particularly serious in colder parts of the world, in people with dark skin, the elderly and those who are diligent about avoiding sun exposure.
Some researchers say we need at least 800 to 1,000 units of vitamin D daily, probably taken in the form of a supplement, to reap the health benefits. The studies that showed a link between the vitamin and reduced cancer and cardiovascular disease risk used at least 1,000 units of vitamin D every day.
Not everyone agrees that the recommended dosage be increased. The American Cancer Society favors keeping the current recommendation of 200 to 600 IUs for now, cautioning that more than 2,000 units is viewed in the government nutritional guidelines as potentially dangerous.
“In excess, vitamin D can be very toxic,” says Rock, who admits that the recommendation may need to be increased to 800 units daily. “Taking 2,000 IUs is the highest dosage in which you don’t see adverse effects. Any more than that and all bets are off.”
Some medical/nutrition experts advocate going to the source for more vitamin D.
“Just 10 to 15 minutes of sun on your face, arms or back twice a week is enough to keep your blood levels of vitamin D up. As we get older, we make the conversion less efficiently, so as we age we may need to get a little more sun exposure, maybe 20 minutes instead of 15,” says Rock, who notes that people with dark skin don’t convert the sun to vitamin D as readily, so they may also need more sun time.
Researchers from UCSD point out that people living closer to the equator have lower incidences of some cancers, including colon, lung, breast, ovary and prostate.
“Just don’t get carried away,” Rock says, noting that prolonged sun exposure increases the risk of skin cancer significantly. “You don’t need to spend hours in the sun and look like Malibu Barbie to get enough vitamin D.”
Relying on food for an adequate supply of vitamin D isn’t as easy, because not that many foods contain the vitamin.
Oily fish like sardines, salmon, mackerel and tuna all have vitamin D. So does cod liver oil. And eggs have a little bit.
“The good news is that many more foods are fortified with vitamin D. Milk has been fortified with vitamin D by law (since the 1930s to prevent the bone-deforming disease rickets). Many cereals are fortified and so are some cheeses and margarines and orange juice,” Rock says.
It’s important to try to get the right kind of vitamin D.
The vitamin has two main forms, D-2 and D-3. Vitamin D-2 is from plant sources and is the type often contained in many multivitamins and fortified foods. Vitamin D-3 is from meat sources and is considered to be the more potent and accessible. It’s what we make when our skin is exposed to sunlight.
“D-2 is not as bio-available as D-3, and our body doesn’t absorb it as well. So, if you take 400 IUs of D-2, you might not be getting all 400 units,” says Linda Copp, a registered dietitian and instructor of nutrition at San Diego State University.
Word is getting out to the supplement and food industry that vitamin D-3, or cholecalciferol, is the preferred form, and more of them are starting to offer it in their products, noting it boldly on the label.
How do you know if you’re lacking in vitamin D? You probably don’t know for sure. But if you rarely get any time in the sun and don’t eat or drink foods containing or enriched with the vitamin, or if your skin color is dark, chances are you may not be getting enough.
However, instead of guessing and haphazardly increasing your vitamin D dosage, Rock suggests having a blood test, which usually runs $100 to $200.
“It’s possible to measure the amount of vitamin D you have in your blood. Your health insurance probably won’t pay for it, but it’s not that expensive. And if you’re really concerned, it probably is a good idea to ask your doctor about being tested,” Rock says.
Beth Wood contributed to this article.
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