It all started with the important discovery of vitamin deficiency diseases like scurvy, pellagra and rickets and now it's become a huge industry. You can hardly venture onto public transport without seeing adverts for supplements on railway stations, in tube trains or on the backs of buses. Anyone would think vitamins and minerals were to the body what oil is to the engine of a car - a simple top up and smooth running is assured.
It's very hard for the average consumer to make informed decisions in this area - the products sound tempting and "everyone's" taking them, so they must be good, mustn't they?
Doctors tend to be conservative on the subject - "you only need vitamins if you have a vitamin deficiency disease" would be their traditional position statement.
"Nutritionists" are far more liberal. They believe that vitamins not only prevent diseases but can also "heal" all manner of ills. They recommend a vitamin (or six) for every ailment. As if that extra oil in the car could mend a broken engine part. They tend to be overly eager to support their beliefs by citing evidence from experiments involving cells and mice.
In the middle ground we have the lucrative supplement industry with a vested interest in convincing us that we need supplements and so getting us to part with our money.
So what's the scientific take on all this?
Evidence cannot convince the scientific community unless it involves a large group of humans (not genetically modified mice), taking vitamins over a long period of time, with a "blind" comparison group taking placebos. Only then can you check whether the supplements are safe and whether any positive effects result. Such trials are, of course, very expensive and are consequently rather rare. Such studies that do exist have sometimes shown no benefit, or even demonstrated harm to those who take vitamin supplements for long periods.
In summary - the better quality evidence has generally been "disappointing" inasmuch as it has failed to show that supplements reduce your chances of avoiding colds or cancers, or that they increase your chances of living longer. Vitamins cure vitamin deficiency diseases, but evidence that they are curative in other areas is very flimsy indeed.
The other problem for consumers is that vitamins are very lightly regulated. In Europe, now, you cannot even hint on packaging that a food has an active health benefit. There are only a few examples that are approved by the European Food Standards Agency (notably oats and the cholesterol-lowering products such as Benecol margarine). Food packaging is still allowed to bear words like "fortified with Vitamin A" or "a source of vitamin C" as long as they contain minimum amounts of the relevant nutrient.
Where food supplements are concerned, as long as they are not actually toxic or contaminated and as long as they make their claims in qualified terms such as "may help support a healthy immune system" they are very lightly regulated. In other words there is not a lot of consumer protection.
Cochrane systematic reviews are a good place to look if you want to check the best evidence about supplements. A new book has been published which goes into more myth-busting detail and it looks like interesting reading.