This could indeed be the underlying physiological mechanism. However, a recent paper of Chiou and colleagues in Psychological science an interesting psychological mechanism of consumers taking multivitamins. In two studies, they show that taking a simple pill (which participants believed was a multivitamin) generated a psychological license to exercise less and indulge in unhealthy food choices. Taking vitamins gives you 'permission' to do something bad, while still feeling good about yourself.
|Vitamins; a license to indulge in Dutch Spice Nuts (pepernoten)|
Maybe this explains why the huge popularity of dietary supplements has not translated in healthier people.