A new study has found that higher blood levels of omega-3 fatty acids, vitamin B, vitamin C, vitamin D and vitamin E are associated with better mental functioning in the elderly. You can read about this study here in the New York Times:
http://www.nytimes.com/2012/01/03/health/research/vitamins-b-c-d-and-e-and-omega-3-strengthen-older-brains.html
While many studies have found a relationship between these supplements and better mental health, no specific relationship between improving behaviours or academic performance have been found between these and ADHD. If you or your child has ADHD, it is not caused by lower levels of these vitamins, fats or minerals. Increasing the amounts may or may not be a good idea. Often individuals super dose themselves or their children based upon internet ADHD webpages or ADHD "natural care" books, thinking they are "natural" and therefore can do no harm. Not true. Before beginning any specific "natural" treatment for ADHD you need to consult with your doctor. Children, and adults have overdosed on these supplements. And they are not meant to be a substitute for scientifically supported medications for ADHD symptoms such as poor attention or focus. That scientists have found that there is a relationship between these vitamins and minerals and those that have good health doesn't mean taking these supplements will "fix" or "cure" ADHD. There is no evidence of that, and you should discuss it with your doctor. ALWAYS discuss giving supplements to your children with your MD. In some cases excessive amounts of supplements can counteract medications.
Here are a few good "skeptical" websites to look at:
http://www.skepdic.com/vitacon.html
A recent newspaper article: "More harm than good?"
Sales of vitamin supplements containing antioxidants are booming. But research suggests they don't always work - and may even increase the risk of disease. By Justine Davies
http://www.guardian.co.uk/lifeandstyle/2009/jun/23/vitamin-supplements-antioxidants-freeradicals/print
And for a really strong start on understanding all the research and what it means, try this article at Science-Based Medicine:
http://www.sciencebasedmedicine.org/index.php/another-negative-study-of-vitamins/
We have excellent, well proven interventions for ADHD. They have been used and shown to be effective for over 25 years. These include medication, behavioural interventions, classroom management programs and CBT. I suggest using these.