Because vitamins cannot be created from scratch we are obligated to eat them. For eons of time they came exclusively from foods, but beginning in the 1930's, vitamins have also been available as little round pills. It is increasingly popular to incorporate these into the daily routine. Currently more than half of Americans take a vitamin or supplement daily, generating many billions of dollars in annual sales for the industry.
As a college student, being health conscious and believing the usual propaganda from vitamin companies and fitness magazines, I thought it would be a good idea to start taking a multivitamin. It seemed like it wouldn't hurt, and there were some vague benefits either hinted at or promised outright.
Eventually I noticed a change in my habits. After taking the morning vitamin, a little voice in my head started helping me justify eating junk and sitting around too much. In other words, because I got my recommended daily allowance early, the pressure to do all the right things throughout the day was off. This wasn't anything conscious, and it wormed its way in gradually, so it took me a few years to figure it out. Once I realized what had happened I was disgusted and threw the vitamins in the trash.
Last summer a Taiwanese group reported a fascinating study that describes what happened to me 20 years ago. Chiou and colleagues wondered why people in the developed world take more vitamins and supplements than ever before even while public health has not improved (it's actually worse by a number of measures).
Their study involved giving placebo pills and calling them vitamins. When people believed they got a vitamin their behavior changed. They were more likely to "engage in hedonic behavior", they walked and exercised less, and they chose an all-you-can-eat buffet over a healthy meal. Chiou called this an "illusory invulnerability caused by taking dietary supplements". Basically people take a vitamin, pat themselves on the back, and think they can get away with more afterwards. I found I had become one of those people.
The food industry loves fads involving vitamins and supplements because it's relatively easy for them to manipulate the nutrients and thus manipulate the public. Whether the trend is low fat, no fat, low carb, high fiber, fortified with calcium, high in beta carotene, high in antioxidants, gluten free, vitamin fortified, or high in omega 3's, it doesn't matter. The processed food companies will either put it in or take it out, whatever they think we want, and they will then market it to us.