Most weight gained during longer runs is water weight - fun fact you're more likely to drink too much water during a long-distance run than not enough. (It's Wikipedia, but it's got a lot of good sources.)
A quick search shows that Kilian Jornet competes in Ultramarathons of 100+ km where maintaining adequate nutrition is difficult mainly for logistic reasons - eating on the run is hard. So he is probably trained to eat what he can, when he can.
How much you need during training and competition is something you and your coach need to figure out. If you haven't got a coach and are looking at getting into Ultras, get one. The best advice on what too eat will come from someone well trained who can observe you in training.
As for your question, what should you eat during a race - probably nothing. Most amateurs run marathons on a big bowl of pasta the night before, some simple carbohydrates in the morning, and adequate water during the race. I'd suggest that if you are asking this question here, in the way you are, this advice will be adequate for you.