I was wondering this.
Everyone needs a basic level of protein, vitamins and minerals. Presumably the need for these increases somewhat when the body is being put under the sort of stress that Steve & Kurt are currently doing. I am assuming that the increase would not be proportional to the work done. The big one is energy, and I would have thought that the requirement is directly proportional to the amount of work done. I recall discussing with my nephew, a GP, my calorie intake when I was doing over 7000 miles a year. He expressed an interest in this (he's a cyclist as well when he gets the chance).
From memory, his back-of-a-fag-packet calculation was that I was burning about 175000 extra calories a year compared to the recommended daily allowance. That works out to about 480 calories a day extra. Given that serious HAMR entrants are expecting to do 11 times as many miles as I was, that implies an additional 5280 calories a day over and above the 2500 recommended for adult males, therefore 7780 calories a day. Given that I was trundling round at about 11 mph and they are doing lots more than that, that will increase their calorific requirement compared to mine. That will be somewhat offset by the fact that I weighed about 115kg, lots more than them.
An extra 5000 to 6000 calories a day implies a lot of bulk intake. That's going to need a lot of output as well as input, I would have thought.
I am not a nutritionist, just an interested speculator on the internet.