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.
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:
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.
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:
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:
(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:
A recent Cochrane review on the effect of vitamins with antioxidant properties on mortality concluded (my emphasis):
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.