Fast Metabolism? Or Genki?

It's sort of like a trademark in some shounen anime where the main character has a superhuman appetite. They'll eat way more than the average person would eat in a week or a month. Why do you think a story writer would put that into their story and why would it catch on to other anime?

I don't know the exact reasons why the creators of those animes would put that element into their story but I can come up with an educated guess.

In Asian schools of thought such as Taoism food is regarded as a source of chi, it's not as vital as air however. Different food has different types of chi. For example meat is considered a yang food, so it would be consumed when one needs a fiery, male type of energy and  watermelon is a yang food so it would be consumed when one needs supple, female energy. Food even gives off different chi depending on how it tastes because taste is a indicator of the composition of the food. Bottom line, food gives us energy. Ok nobody would disagree with that.

Note that the food of choice is usually meat.

Why would someone consume large amounts of food. Well they say that the normal man in todays world does not need much food to survive since people are sedentary these days. Your average shounen hero is many things, but NOT sedentary. Luffy spars with whales, Goku beats up Frieza... Many similar things happen in Fairy Tail.



Bottom line is that the appetite matches the activity level.

Put the two together and now we understand that they eat tons of food because that food is converted to the extreme amounts of power they use to fight!

Keep in mind that this only works for the shounen hero who's genki and brimming with power. If you were to eat that much you'd become fat. Not to say you're not genki, you just need less food to tackle that homework and take out the trash. However, if you're on a sports team then that (small) binge after practice is just what the doctor ordered.

If you'd like to read about tao here's the books I used in my educated guesswork.
References:
Ekiken, Kaibara. "Yojokun: Life Lessons from a Samurai" (translated by William Scott Wilson), 2009.

Reid, Daniel P. "The Tao of Health, Sex, & Longevity", 1989.