How easy would it be for you to find (natural) laxatives? ChatGPT names Prunes and Kiwis, Broccoli, Almonds and Walnuts, Yogurt (the white non-sweet kind would be best)
Basically the above is just a who's who of healthy dieting if you ask me lol. If you need high caloric density for the "eating as much as possible" part just go for anything high in fat and or sugar. Nutella would check both boxes but this is really not recommendable from a health perspective obviously.
Competitive eaters like Beard Meats Food would just chug on double cream if they wanted to get as many calories down as possible. The only "problem" there is that it doesn't make for a very entertaining video.
Can't argue with that flawless logic.