Is there any reason for a healthy adult to take a daily multi-vitamin?
My intuition tells me that an adult eating a healthy and balanced diet will get all of the nutrients she needs and that taking a daily mulit-vitamin would be pointless.
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There answer is not necessarily a simple "yes" or "no."
Way too little of a vitamin is a terrible thing, and way too much can be just as bad, although you must reach rather high levels to contract vitamin intoxication. A "what's the harm" attitude towards multivitamins is arguably defensible, as you're not running a risk of vitamin intoxication from following the recommended dosage. It rather boils down to whether or not you're willing to spend the money and go through the daily routine, when it's not at all obvious that you should need to.
Vitamin supplements are all about avoiding a vitamin deficiency. There really is no added benefit to getting more vitamins once you're at sufficient levels. The trick is that it's not that easy to keep track of whether or not you are.
The rule cannot be generalized into saying "Healthy adults don't need vitamin supplements, because in order to be healthy, you must not have a vitamin deficiency." You're not likely to catch scurvy unless you're doing something terribly wrong. That's not what this is all about. There can be more subtle effects of small vitamin deficiencies. Vitamins can help you fend off other diseases, for instance:
Vitamin D from a multivitamin or single supplement can lower the risk of colon and possibly many other cancers. * harvard.edu
The process of self-diagnosis should not simply be "Do I suffer from scurvy? No; ergo I'm getting all the vitamin C I need." Rather, you should be asking my self "Am I avoiding colon cancer as best I can?" That question is trickier to answer.
No, you do not need vitamin supplements if you get sufficient levels of vitamins anyway. There is no added benefit to getting more than enough vitamins. But I can't generalize that into "eating a healthy and balanced diet", as per your question. First of all, that phrase probably entails different specific details depending on the individual. But, more importantly, I'd like to stress that it's not all about your diet. The richest source of vitamin D, for instance, is exposure to the sun.
If you live north of the line connecting San Francisco to Philadelphia and Athens to Beijing, odds are that you don't get enough vitamin D. The same holds true if you don't get outside for at least a 15-minute daily walk in the sun. African-Americans and others with dark skin, as well as older individuals, tend to have much lower levels of vitamin D, as do people who are overweight or obese. * harvard.edu
It is worth looking into whether or not you belong to any of the risk groups, to assess whether or not you would be in need of supplements, or perhaps, and preferably, a change of habits. For a healthy adult to take multi-vitamins sounds to me like hedging your bets, but a pill a day won't get you intoxicated either.
Vitamin supplements are not demonstrably useful for healthy people taking a healthy diet
It is worthwhile referring directly to some of the primary literature for this question (especially so since I was heavily criticised for selectively quoting the @david-hedlund answer above in another question Are Vitamins ingested in natural food more effective than those ingested in supplements? ).
It is quite hard to give a proper comprehensive review because of the volume of studies that have been done. But the tone of other reviews is well summed up in this 1990 BMJ article:
Healthy people eating a healthy diet do not need them... So is any benefit to be expected for ordinary people from vitamin supplements?
Expert committees do not think so-at least for the populations of affluent countries. The American Institute of Nutrition and Society for Clinical Nutrition recommends that "healthy children and adults should obtain adequate nutrient intakes from dietary sources. Meeting nutrient needs by choosing a variety of foods in moderation, rather than by supplementation, reduces the potential risk for both nutrient deficiencies and nutrient excesses. Individual recommendations regarding supplements and diets should come from physicians and registered dietitians."'" The United States National Research Council could find "no documented reports that daily multiple vitamin-mineral supplements, equaling no more than the recommended dietary allowances . . . are either beneficial or harmful for the general population." Dismissive recommendations like this are incomplete unless they give guidance on the outlines of an adequate diet-for example, "people eating a good diet that includes bread and cereals, vegetables and fruit, meat or meat substitutes and dairy products do not require vitamin and mineral supplements." The consensus is clear: "healthy adult men and healthy non-pregnant, non-lactating women consuming a normal varied diet do not need vitamin supplements."
But there have been plenty of proposed advantages to certain supplements since then. Here are a few summaries of more recent work.
A study about the effects on cancer and heart disease was summed up by a news story in the BMJ like this:
The evidence that vitamin supplements are useful in preventing cancer or heart disease is not conclusive, the US Preventive Services Task Force, an influential US government advisory panel, has said.
Moreover, β carotene supplementation may do more harm than good in patients with lung cancer, the panel found (Annals of Internal Medicine 2003;139:51-5)...
...The literature review failed to find any good quality studies that showed any beneficial effect of taking vitamins A or C on cardiovascular health or in reducing the risk of cancer. Some observational studies did show a reduction in the risk of breast and colon cancer in women taking vitamin A, but the researchers could not control for confounding factors.
(original source Annals of Internal Medicine)
Here is the conclusion of a more recent study from the BMJ on 2010 on cardiovascular protection from some supplements:
Conclusion This study does not support the routine use of dietary supplements containing B vitamins or omega 3 fatty acids for prevention of cardiovascular disease in people with a history of ischaemic heart disease or ischaemic stroke...
A recent Cochrane review on the effect of vitamins with antioxidant properties on mortality concluded (my emphasis):
We found no evidence to support antioxidant supplements for primary or secondary prevention. Vitamin A, beta-carotene, and vitamin E may increase mortality. Future randomised trials could evaluate the potential effects of vitamin C and selenium for primary and secondary prevention. Such trials should be closely monitored for potential harmful effects. Antioxidant supplements need to be considered medicinal products and should undergo sufficient evaluation before marketing.
While not a comprehensive survey, I hope this gives a general flavour of the key results.
It is also worth noting that there are trials which show big and significant results from supplements, but the relevant to the general population is questionable. The most significant is the use of Folic Acid in pregnancy which significantly reduced the incidence of Neural Tube Defects (Cochrane review here). But this effect is largest when there are clear dietary deficiencies. There are also some recent studies showing some correlation with vitamin D levels and mortality especially from cancer (but not the converse that taking extra vitamin D will reduce the incidence see the research in the BMJ in 2010 here). And, in any case, the best way to get Vitamin D is to go outdoors in bright light not to take pills.
In summary, I'm not trying to claim vitamins are never useful. But their use in the general population is not demonstrably beneficial. Some people (e.g. computer programmers who never see natural light) might benefit from supplements like vitamin D and pregnant women (especially those with dodgy diets) should take folic acid. There is a big industry out there trying to persuade us all to take their supplements; we should ignore them until they provide us with better evidence the supplements are worth taking.
The only way to know whether you have enough vitamins through your diet is to get an detailed blood test.
Otherwise there's Vitamin C and Vitamin D for which they are separate arguments.
Other mammals produce for themselves a lot of vitamin C. A few million years ago our gene for vitamin C production mutated and doesn't work anymore. As a result people started to advocate that you should take supplements to get to a Vitamin C level of other mammals.
As far as Vitamin D goes we get it from the sun. As we evolved outside of the office and now spends a lot of time inside the office there a argument that we probably don't produce enough Vitamin D on our own. There was a study that showed that taking 4000 IU per day significantly reduced preterm pregnancies. Pregnant women who don't see that they get additional Vitamin D are acting irresponsible.