Everyone uses different things to fuel their body during a 100-mile run. You have to find what works for you through trial and error. What I do know, is what works for you as a marathon runner, probably won’t work as an ultrarunner.
Some ultrarunners use the traditional sports fuel such as Gu, shot blocks, sports beans and the like, but it’s difficult to use them throughout the race. You just get sick of it and it becomes more difficult to choke it down.
Fueling is necessary, which means you have to put something down the hatch. Perusing the aid station buffet will give you an idea of what most ultrarunners eat: various types of candy, trail mix, potato chips, boiled and salted potatoes, cookies, peanut butter and jelly sandwiches, cheese quesadillas, ham or turkey sandwiches, Romen noodles, and fruit.
Bodies run off mainly carbohydrates during exercise. You can also burn fat and more runners are turning to a low carb diet, which allows them to tap into their fat stores as fuel during their runs. This is very useful and can be very beneficial to runners who have stomach issues regularly. You have to eat a whole lot less when you’re low carb. I’ve used this strategy, but could only maintain it for eighteen months because I couldn’t get enough fats to sustain my energy output. I’ve been back on carbs for about 18 months and feel great. I’ve written two blogs on low carb running if you are interested they are here and here.
What about protein during runs? I discourage most protein while running because protein, for most people, digests slow. It sits in your stomach slowing your metabolism down. A slow metabolism means your body doesn’t get fuel quickly. You need carbs to go through quickly if you want to maintain a good pace throughout the run. You also need electrolytes and water to go through quickly. If your digestive system is working on a lump of protein, everything else is going to come through slower too.
Easily digestible proteins are fine during a race, but not too much and space it out. Nut butters are easy, cheeses are easier, and plant based proteins are easy. Meat is not easy. Many protein bars(especially over 10 grams) are not easy.
The only way you are going to figure out what works for you is by training with different things until you find a few things that work for you. I suggest you find multiple things that work because you get sick of eating the same thing every hour (or more) for up to 36 hours.
The other thing I strongly encourage is to find out what the event is using at their aid stations and make sure you can use them. Especially, electrolyte pills or drinks and other specific sports nutrition such as Gu.
My favorites: Swedish fish,Oreo cookies, fruits, peanut butter and jelly, and chick-o-sticks (all vegan by the way :0)