Nutrition is a pretty important part of ultra-running. Whilst you can get used to running without eating too much by becoming a more efficient fat burner, for races and for optimum performance you really need to stoke the fire as you go. In my training I tend to rely on fat-burning, largely because gels are frigging expensive, and I'm too cheap to buy them if I can help it. So I very rarely eat anything on my runs, even on my long (25-30ish miles) ones.
But even the most efficient fat-burner still needs carbs to function - "fat burns in a carbohydrate fire" I believe is the cliché that is bandied around. For this reason, I tend to use gels when I race, and will often make my way through a 100 miler predominantly on these, with the odd bit of fruit thrown in at the aid stations. I don't go too nuts; for instance I probably had 5 gels in 8 hours at the recent SDW50 - so about 1 every hour and a half. This works out fine for me, and gives me the energy I need to keep going, without causing major stomach-related issues or borderline diabetes.
Below is a little review of the two main brand of gels that I use - TORQ and GU. I don't really know the ins and outs of physiology and metabolism (I believe that the head-bone is connected to the arm-bone?), so I won't go too much into the "sciencey" side of things. The main thing that I care about is how they taste and how easy they are to eat while running. It's simply a way of getting energy into my face at the end of the day. Beyond that, I don't notice a huge difference between them; I know what I like and what works for me after a few years of trying various things out and failing miserably every now and then. I'll pretty much eat anything I'm given, but these are the ones that I will go for if given the choice.
