You're burning maybe 15,000 kcal per day doing this – you can't possibly eat enough to offset that. So your body progressively deteriorates, eating itself – and most people who try an FKT have precious little fat on them, so your body's eating its own muscle, too.
To slow down this process of deterioration, it's absolutely essential to get as many calories inside you as possible. It's clear from their trip reports that both Brett and Andrew were acutely aware of the need to eat as much as they possibly could, and they planned for it and carried a lot – but both of them had difficulties with their food. If nothing else, I aspire to out-eat them.
I'm planning on eating 22,000 calories in total – around 300 per hour while moving, with a 1500 calorie dump into my body (I hesitate to call it "dinner") consumed in the last hour before I stop moving at the end of day one and day two.
A major reason for my steady pacing plan in the early stages is that I want to be certain that I'm eating enough. Essentially, the purpose of day one and day two is not to be staggeringly fast. It is to deliver me to the beginning of day 3, ideally close to my planned split time, with 8 hours solid recovery sleep at low altitude, and with about 15,000 calories consumed. I think this is the only way for me to have any chance to hold the required pace through that third day.
You can't digest much protein or fat while on the move, so it's almost all carbs. Maltodextrin (short polymers of glucose) is a popular choice – the bonds are easily broken, so it's just as easily digested as glucose; in fact there's some evidence that one large malto molecule is absorbed into the gut faster than several small glucose molecules. Also, it's not sickeningly sweet.
However, I can't deal with powders such as Perpetuum. I find them too difficult to handle, especially when I know I'll be exhausted and malcoordinated. And with so much water on the trail, there's really no need to carry more than half a liter of water. So with dissolved powders, you're either spending a lot of time making frequent small batches, or you're making large batches and carrying way more water weight than you need to. I like to manage my water consumption separately – at night I will be drinking far less, but eating the same.
So, it's Power Bars. Their "C2MAX" blurb talks somewhat confusingly about a glucose/fructose mix, but in fact it's maltodextrin/fructose. They cite some thin evidence that the mixture of the two carbs yields more energy because fructose is metabolized via a different pathway. Anyway, it approximates solid maltodextrin in a form that I find edible and digestible. I've eaten over 7000 calories of the things on a 50-mile test run at race pace, with normal digestion.
It's also somewhat important that they contain virtually no fiber. The 4g insoluble fiber in a Clif Bar may be a health boon if you're eating a couple of them, but if you're planning to eat about 30 a day…. I think I've said enough.
