|Image by David Richfield from Wikipedia|
To be clear, vitamins and supplements are great, but only when taking the recommended daily amount in a normal, healthy individual. Supplements may be necessary only if there's a deficiency, but not when you are a healthy individual.
The New York Times wrote a nice expose regarding this issue along with why the FDA doesn't say anything about it (vitamin lobbyists passed a law in 1976 prohibiting the FDA from saying anything).
So, let's take a look at the studies mentioned in the NYT documenting the adverse affects of too much vitamins:
The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. N Engl J Med. 1994 Apr 14;330(15):1029-35.
"Unexpectedly, we observed a higher incidence of lung cancer among the men who received beta carotene than among those who did not (change in incidence, 18 percent; 95 percent confidence interval, 3 to 36 percent)... more deaths from hemorrhagic stroke were observed among the men who received this supplement than among those who did not. Total mortality was 8 percent higher (95 percent confidence interval, 1 to 16 percent) among the participants who received beta carotene than among those who did not, primarily because there were more deaths from lung cancer and ischemic heart disease."
Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease. N Engl J Med. 1996 May 2;334(18):1150-5.
"After an average of four years of supplementation, the combination of beta carotene and vitamin A had no benefit and may have had an adverse effect on the incidence of lung cancer and on the risk of death from lung cancer, cardiovascular disease."Meta-analysis: high-dosage vitamin E supplementation may increase all-cause mortality. Ann Intern Med. 2005 Jan 4;142(1):37-46. Epub 2004 Nov 10.
A dose-response analysis showed a statistically significant relationship between vitamin E dosage and all-cause mortality, with increased risk of dosages greater than 150 IU/d.Antioxidant supplements for preventing gastrointestinal cancers. Cochrane Database Syst Rev. 2004 Oct 18;(4):CD004183.
We could not find evidence that antioxidant supplements prevent gastrointestinal cancers. On the contrary, they seem to increase overall mortality.Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev. 2012 Mar 14;3:CD007176. doi: 10.1002/14651858.CD007176.pub2.
We found no evidence to support antioxidant supplements for primary or secondary prevention. Beta-carotene and vitamin E seem to increase mortality, and so may higher doses of vitamin A. Antioxidant supplements need to be considered as medicinal products and should undergo sufficient evaluation before marketing.
There are many more studies that show the adverse affects of taking mega-vitamins as well as additional studies that show that taking mega-vitamins makes absolutely no difference. Of course, with any medical issue, there are also studies that support the consumption of mega-vitamins, but the studies are not as rigorous nor as conclusive.
What to do?
I would suggest taking a regular multi-vitamin daily which contains enough vitamins to supplement any deficiencies that may occur from insufficient dietary intake... Otherwise, additional vitamin supplementation is not necessary unless a known deficiency is present.