Some people eat too little.
Usually, this comes from a clear reason — as by being anorexic, dangerously poor, or digestively impaired — and such people know who they are.
Reverse advice as appropriate.
Otherwise, if you're in a developed country, you probably eat more than ideal.
- Excess body fat is common and generally harmful.
Caloric intake, to a first approximation, contributes to it.
- Eating, especially as large meals, makes one sleepy.
Being sleepy makes everything other than sleep less effective.
So, ideally, you would eat only before sleep, or in small amounts as to leave you still alert.
- The more you eat, the greater your risk of food poisoning, which is bad.
- Getting food has a cost, some mix of money and effort.
Eating less requires less food, which costs less and is less tedious.
- Gluttony is a Deadly Sin.
- Okinawan tradition finds slight undereating to be healthier.