i don't talk about this a lot, but i have a diet that would kill a normal person...
i have severe issues with my stomach, and live on yogurt...i take supplements and add protein and sugar to the yogurt, but thats really about 90% of my diet, about 5 pounds of it a day. i add the sugar to get close to 2000 calories a day, otherwise i start to slowly lose weight. i weighed 240 pounds at my largest, then i got a hernia which allowed a Volvulus to happen, which is your stomach or intestines twisting. my stomach got strangulated in the hernia, but then popped back in. i thought that was good, but when it popped back in, it got turned somehow. i got pretty sick, almost died, but they operated, removed a large part of my stomach due to tissue damage from the volvulus stopping blood flow, and the result is that i can't digest anything much solider than yogurt. i also developed a gluten allergy somewhere in there, no idea if that was coincidental or caused by the sickness.
so i live on protein and carbohydrates and sugar, almost no fat. i get fat free yogurt, honestly just because yogurt with a noticeable amount of fat tastes bad to me, almost rancid. i do like ice cream, and can eat it as long as it doesn't have pieces of things i can't have in it, fruit, nuts, flour like cookies and cream, or brownie chunks...i take a multi vitamin every day, and B12 and E on the advice of my doctor. i also take enough protein supplement daily along with the yogurt to bring my total to 100 grams, and enough sugar to get to at least 2k calories per day. also take fiber every day, couple of spoons full, as i get none from my diet.
i've discussed this with more than one doctor, a couple of them have told me that that my diet would be very bad for a normal person...to which i usually reply "yeah, but..."
so you can adapt to almost anything...but being able to isn't a reason to...