Most studies seem to support the opposite conclusion...That is, those who supplement tend to eat BETTER than those that do not. Multiple studies have born this out.
What does seem consistent is the inconsistencies the various manufacturers have regarding dosage and purity. Their record in these areas is dismal. Even without those issues, Dr. McDougall and others believe that taking concentrated nutrients, outside of the package nature provides, can cause nutrient imbalances and issues with blocking other related nutrients. He believes taking these nutrients outside of their whole package should be done with a trained health professional.
It is interesting how many of these studies say the same thing. I wonder just how "natural" it is to strip some components from their native state and compound them in a lab, then expect them to work the way they do when consumed within food. It seems we are regularly getting studies that not only show the lack of efficacy of these products but in many cases show their danger.
Here is more info from the same study. It involved 39,000 women so it wasn't a small study:http://news.yahoo.com/dietary-supplemen ... 00686.html
Also here is the actual publication abstract. I have no access to the full report.http://archinte.ama-assn.org/cgi/conten ... 71/18/1625