Can vitamin D protect from cancer?

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We're living in the glory days of Vitamin D, which is now touted as a miracle cure-all treatment for, well, practically everything. Doctors are testing their healthy patient's vitamin D levels and deficient or not, many choose to pop a D supplement, just in case.

Every day a new study catches the attention of reporters - if only for a brief moment - who are quick to announce that vitamin D deficiency can be blamed on yet another ill. Judging by some headlines the sun vitamin can cure diabetes, heart disease, cancer, hypertension, depression, obesity and even the common cold.

Scientists are known killjoys, and have been trying to curb the enthusiasm for vitamin D supplementation, noting that most of the vitamin D studies are observational, and that much like other vitamin supplementation disappointments (remember the vitamin E, A and C crazes?) it very well might be that healthy people tend to have higher levels of vitamin D (an explanation of why that's so later), which does not mean that taking vitamin D will make you healthier. Correlation doesn't prove cause, experts remind us, and they suggest we await further research.

Last November the Institute of Medicine Committee released its report stating that vitamin D is important for bone health, has not been a proven panacea to any other illness, and that most people have adequate amounts of vitamin D (the committee's lower limit cutoff is 20 nanograms of vitamin D per milliliter of blood, while many labs shoot for 30).

Vitamin D and cancer

Three of the vitamin D experts who were part of the Institute of Medicine's committee published a new commentary in the New England Journal of Medicine, elaborating why the panel thinks vitamin D isn't ready for prime-time in cancer prevention.

The beginnings were promising -- several observational studies showed that lower levels of vitamin D were correlated with cancer. But association studies have important limitations, the authors warn, especially where vitamin D and cancer are related.

Obesity, lack of exercise and poor nutrition are known risk factors for cancer. Those precise risk factors tend to be associated with lower levels of vitamin D - vitamin D is sequestered in body fat, therefore overweight people have lower measured levels of this vitamin, non-exercisers tend to spend less time outdoors absorbing the sun and make less of the vitamin, and people who pay less attention to healthy nutrition may get less vitamin D from food or supplements.

And let's not forget the reverse causation bias: People who are not in great health will stay indoors, eat poorly, and therefore have lower vitamin D levels.

Randomized trials, in which people are randomly assigned to be treated either with an active ingredient, or with a placebo, are much more reliable than observational or case-control studies. There are very few randomizes clinical trials looking at the effect of supplemental vitamin D on cancer incidence. The results of these have been disappointing, inconsistent and inconclusive and research hasn't established a cause-effect between vitamin D and cancer as of yet. The receptor for vitamin D is expressed in most of our body; therefore it is possible that this nutrient plays a role in cancer prevention or progression.

Several new trials assessing vitamin D supplementation's effect on cancer are underway, and we can remain hopeful that their results will support vitamin D's benefit for conditions other than bone health, but at this point the authors conclude that "the existing evidence falls short".

Practical approach to vitamin D

Vitamin D maintains normal blood levels of calcium and phosphorus, and is critical to forming and maintaining our bones.

Our body makes most of its vitamin D: Our skin produces vitamin D after exposure to ultraviolet light from the sun. Just 10 minutes of sun exposure daily will usually get us enough production to prevent deficiency, and the extra vitamin produced is stored for rainy days. Foods with naturally occurring vitamin D aren't common, and include fatty fish, eggs and mushrooms. Many other foods are fortified with vitamin D: Milk, flour, cereals and margarine are a few examples.

The new reference vitamin D intakes issued by the Institute of Medicine are 600 IU/day for most people aged 1-70 years (see link for other age groups). People who rarely get sun exposure and dark-skinned individuals should probably take vitamin D supplements. While our body never makes too much vitamin D, even if we're out in the sun all day, day after day, it is possible to suffer vitamin D toxicity (which can cause kidney stones and tissue damage) when we ingest too many supplements, therefore the upper limit recommended by the Institute of Medicine report is around 2000 IU/day.

Wouldn't it be nice if we could prevent a myriad of illnesses by a treatment as simple as a vitamin pill? Unfortunately, research has not proven that vitamins cure any condition besides vitamin deficiencies (which are very uncommon in the US), and more is not better when it comes to these essential vitamins. For now, I'd stick with Michael Pollan's rule #40: "Be the kind of person who takes supplements - then skip the supplements." Becoming that person isn't easy - it requires adopting good eating and exercise habits - but popping a vitamin isn't a shortcut to getting the same health benefits.

Dr. Ayala

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