I read this - http://www.nytimes.com/2011/08/21/magazine/do-you-suffer-from-decision-fatigue.html?_r=3&pagewanted=all – article in the NY Times about the willpower muscle in your brain. After making constant decisions, the muscle wears out, and you either make hasty, impulsive decisions, or you are unable to make decisions.
Towards the last half of the article, the author refers to studies indicating that glucose gives your willpower muscle energy. When you eat sugar, it is thereafter easier to make rational decisions. This is why dieters have such a hard time dieting – they need glucose to maintain willpower, but glucose is what dieters typically avoid!
I’ve noticed this in my own life. Just yesterday, I was feeling really tired and unable to do my work properly. I had a candy bar, and finished out the rest of my workday with zeal. This inspires me to start eating fruit and nuts in the afternoon, when my willpower is fading. In order to avoid junk foods and be able to work properly, I should eat a couple pieces of fruit to help me keep going.
What an interesting topic! I would be pleased to see more studies on this topic, and perhaps policy changes based on the findings.