There is no clear scientific evidence of cause/effect by diet or nutritional problems. However, research suggests that certain food products may play at least some role in affecting ADHD symptoms in some people.
One thing I know about ADHD and a nutritional link is the Feingold Diet or an elimination diet. This diet is based on a belief that there are benefits of excluding certain foods.
Here's a great explanation of this diet along with other information about ADHD:
Medical Drs. believe whatever is good for the brain is likely to be good for ADHD. Higher clean protein, complex carbs vs. sweet or simple carbs, and omega-3 fatty acids. And eating a variety of nutrient-dense whole foods for a complete, balanced array of vitamins, minerals, and phytonutrients.
Similar info on diet has been found in studies for bipolar disorder in addition to stress and weight management. Here's a good article by Dr. Peeke about BD.
I hope you find some of this helpful!