If you think vitamin D is just for bones, think again. Recent research suggests that high doses of vitamin D may reduce the risk of everything from cancer and multiple sclerosis to diabetes and high blood pressure. Supplements may or may not be the answer.

In a report published in the January issue of the journal Bioassays, Trevor Marshall, Ph.D. suggests that vitamin D deficiency, long interpreted as a cause of disease, may actually be the result of the disease process.

Marshall goes on to suggest that the use of supplements are harmful, because they suppress the immune system, and interfere with the body’s ability to fight disease and infection.

Marshall’s assertions are in sharp contrast to those of others in the research community, most notably Drs. Holick and Hollis, both of whom have spent decades researching vitamin D.

What we do/don’t know about vitamin D
First of all, vitamin D isn’t a vitamin at all, it’s a sterol. Vitamin D is manufactured in the skin by the sun. The amount made depends on the time of day, season of the year, geographic location, skin pigmentation, sun exposure, and age.

Vitamin D is fat soluble, so the body can store it for days or months. But if it is sequestered in fat, it may not be available to the body, which may explain why obese individuals are more likely to be vitamin D deficient. Unlike other vitamins, vitamin D is found naturally in only a few foods—oily fish, eggs, butter, and human milk. To ensure an adequate intake of vitamin D (or what we think is adequate intake), foods such as milk, orange juice, and breakfast cereals are enriched or fortified with vitamin D. Because the sun is the main source of vitamin D, it is hard to get enough vitamin D from food alone, so many doctors recommend supplements as well.

Making a case for testing
A recent meta-analysis, published in the Archives of Internal Medicine, reported a 7 percent reduction in total mortality rates among people taking vitamin D supplements (400-800 IU a day). Dr. Bruce Hollis of the Medical University of South Carolina is among those advocating routine vitamin D testing. Hollis has been researching vitamin D for 30 years, and is highly respected in the medical community.

“An intake of even 800 to 1,000 IU a day won’t be enough for some people,” says Hollis.

“The body’s ability to make and utilize vitamin D varies from person to person, which is why testing can be important,” he adds.

“Exposing your arms and legs to the midday sun without sunscreen can produce high levels of vitamin D, but can also cause skin cancer, so supplements are the best option,” states Hollis. “A blood test can help you decide how much supplement you need.”

Many doctors feel that it is too early to recommend routine testing for everyone. Large long-term clinical studies that would justify routine screening have not yet been done. But individuals with low bone density, especially those over the age of 60, might want to talk with their doctor about testing.

Recommended daily intake
Currently, the Institute of Medicine (IOM) recommends a daily intake of 200 IU for people 50 and younger, 400 IU for people 51 to 70, and 600 IU for those 71 and older. However, many researchers believe these guidelines are too low and recommend 800 to 1,000 IU of a day.

In 2007, fifteen leading experts urged the IOM to increase its recommended intakes. The Canadian Cancer Society now advises all Canadian adults to take 1,000 IU a day during fall and winter months, and year round for older and darker-skinned people.

Are all supplements the same?
Scientists thought they knew the answer to that question—citing previous studies showing that vitamin D3 (cholecalciferol) was more potent than vitamin D2 (ergocalciferol). However, a more recent study published in the December issue of the Journal of Clinical Endocrinology and Metabolism found that one form of vitamin D was just as effective as the other in maintaining adequate levels of 25-hydroxyvitamin D. Holick, lead author of the study, concluded, “More studies are needed to determine whether the media the vitamin D is dissolved in ( e.g. alcohol, oil, or sugar) effects bioavailability or catabolism.”

Currently, the official upper limit for vitamin D supplementation is 2,000 IU. But a review article in the American Journal of Clinical Nutrition concluded that as much as 10,000 IU a day is non-toxic.

What about cod liver oil? It is considered the richest source of vitamins A and D. The standard dose is 1 to 3 teaspoons a day, with just 1 teaspoon containing 4,500 IU of vitamin A. Capsules of cod liver oil contain less oil, but the amount of vitamin can still add up, especially when combined with the vitamin A in foods and from a multivitamin, if you take one.

Recent studies have found that as little as 6,000 IU of vitamin A each day can interfere with bone growth and promote fractures. And women of childbearing age should not exceed 10,000 IU of vitamin A because of the risk of birth defects. Also, cod liver oil is made from livers, which filter toxins. Contaminants, such as polychlorinated biphenyls (PCBs) can concentrate in the liver, making cod liver oil a likely source of contaminates compared to other fish-oil supplements.

So what’s a person to do?
Obviously more data is needed. But it is always best to talk with your health care provider before taking any supplements. If Marshall’s hypothesis (albeit highly controversial) is correct, and vitamin D supplementation only makes diseases worse, perhaps researchers will conclude that there is no safe substitute for sunlight after all. But if Marshall’s hypothesis proves to be false (as abundant data suggest), and supplements are recommended, researchers will need to develop guidelines for supplementation—recognizing that individual needs vary greatly.

In the meantime, if you’re headed to the beach, exercise caution—too much of a good thing may apply to both sun and supplements.