Dr. Gregor's research has also shown multivitamins are a distraction... https://www.youtube.com/watch?v=5fgVDT0qw88

But these studies are on general population, different from the prison population which is probably undernourished. And the article cites a study on prisoners which shows vitamins (and supplementary fatty acids) reduce violent behavior: http://bjp.rcpsych.org/content/bjprcpsych/181/1/22.full.pdf