The best source of energy for our brain is fat. And the most superior fat, that our brains function at exceedingly high levels is our own! Fasting turns on certain things in our brain which actually increase it in size and intelegence. Where as carbs and obesity actually shrink our brain and diminish intelegence. This is not my own make up: world renowned neurologist and Alzheimer's specialist Dr David pearlmutter wrote it all in his book GRAIN BRAIN.
Fruit is such amazing delicious food, it's light, easy to digest and so amazing tasting... But ideally fat is where it's at for sustained performance energy. I'm the only one in my family who can eat fat in such amounts. I make everyone sick looking at me - they eat carb-based(fruits/grains/legumes)... But whenever I test out grains or too much simple carbs I get ill and have to flee to safety to my fat and veggies.
Greens and nuts are a good happy medium between the two. These are extremely sustaining.