There was tremendous hope for the approach of preventing cancer by popping a vitamin pill.  And, the first large study testing a combination of vitamins in a nutritionally deficient Chinese population at high risk of gastrointestinal cancers showed promise.  

However, the results of almost every study since found that the people who were randomized to receive the supplement (rather than the placebo) did not get less cancer. In fact, in some studies, the people who got supplements actually got more cancer! Here's a quick summary of major findings:  

RCTs that didn't pan out:  

Beta-carotene:  In two separate studies across the globe (here and here) smokers taking high doses of beta-carotene unexpectedly developed MORE lung cancer than those taking placebo. What went wrong? The beta-carotene doses found in normal diet (like in sweet potatoes, carrots, etc.) is much lower than the doses given in the studies. Other nutrients in plants that might work together with beta carotene to prevent cancer were not included in the supplements.

After the studies came out, researchers determined that in the lungs of smokers, the high doses of beta-carotene might have acted like a pro-oxidant (causing DNA damage) instead of an anti-oxidant. In two other studies of mostly non-smoking men and women, beta carotene didn't have an effect on cancer risk either way (here and here).    

Selenium:  A trial of selenium and skin cancer found no protection for skin cancer, but it did suggest a lower risk of prostate cancer, which led to another large RCT. This second study, called SELECT, gave either selenium, vitamin E, both or placebo, to some 35,000 healthy men. It found no effect either way of selenium or vitamin E on prostate cancer.  

Vitamin C:  Several studies have found no association between vitamin C in preventing or causing cancers or heart disease.  

Folate:  Although folate is associated with a lower risk of developing colorectal (colon) cancer, a recent RCT showed that in people who have had polyps removed, high doses of folic acid slightly increased the number and size of new polyps.  What this suggests is that if the cancer process has already started, high dose supplements might feed the process.  

Multivitamins:  At least 50% of US adults take a vitamin supplement. The United States Preventive Task Force reviewed the evidence and concluded that taking multivitamins (in adults) will not reduce the risk of cancer or other chronic diseases. Multivitamins contain several of the nutrients discussed above, though usually in lower doses than the trials.     

To be determined  

Vitamin D:  Although some observational studies (here and here) provide some support for a role of vitamin D in colon cancer prevention, doses of 400 IU/day did not lower colon or breast cancer risk in a RCT. A new large trial testing vitamin D at higher doses (and fish oil) is enrolling participants now, so results won't be out for several years. Vitamin D is hotly debated, and experts are understandably guarded about recommending any supplements to lower cancer risk until the evidence is solid. We say "remember beta-carotene" the way historians say "remember the Alamo."  

More Promising  

Calcium:  I've saved the best news for last. A large RCT in the United States showed that, in people who have had colorectal (colon) polyps removed, taking calcium lowers the risk that polyps will grow back by 15%. There is one hitch, though: men who consume or take high doses of calcium (say, >1,500 mg) may be at slightly higher risk of developing prostate cancer, according to some observational studies. For women, this is obviously not a problem. Until we know more, men may want to keep calcium intake between 800 mg and less than 1,500 mg. Supplements usually contain 500 mg, and a cup of milk contains 300 mg.  

It's worth noting that the Institute of Medicine, the organization that comes up with the recommended dietary intake levels that you see on food packages, also figures out what doses of vitamins might lead to toxicity. Out of approximately 30 food constituents reviewed, they've assigned an "upper tolerable limit" on about half of them. That means there's documented evidence that too much can be harmful for half of the best-studied nutrients. For less well-studied supplements, well, we know even less.  

We all know that too much of a good thing is rarely good, yet somehow have it in our heads that 'more is better' when it comes to vitamin supplements.  I think that's because these nutrients are found in the foods your mom told you to eat more of. The catch is that supplements usually include isolated nutrients, often in much higher doses-and sometimes different chemical forms-- than that found in nature.  

To wrap up, here are the important things to know:   1) Supplements are not the same as food. No supplement can fully replace a healthy diet.

2) More is not always better. Make sure to tell your health care provider about any supplements you are taking.

3) Supplements are not regulated by the FDA. That's right - no oversight. Given the fact that we know toxicities and overdoses can occur, and that some trials have shown harm from supplements, it's wise not to go overboard on supplements.  

The take-home message?  There may be other good reasons to take supplements, but at least for now, prevention of cancer is not one of them.