Yes there is research that supports that high glycemic index (GI) food such as sweets and white bread deteriorate willpower. The main mechanism that is made responsible for this is the drop in blood sugar a few hours after consumption. This post has some points on it: http://lesswrong.com/lw/cmc/book_summary_willpower_by_baumeister_tierney/

More info on this can be found in the book 'The Willpower Instinct' by Kelly Mcgonigal. She says the research most supports plant-based diets and low-GI diets. Evidence for the former is better.

Furthermore I think I've heard a few things about 'hyper rewards' such as pornography, drugs and sweet food generally reducing motivation. This makes sense since these rewards were not available in ancestral times. The brain responds to strong stimuli by downregulating the neural pathways required for motivation. This should be easy to google.