I must be healthy and become little bit fat?

Yes: Yes indeed for your age.

Healthy and fat.: It's one thing to want to be healthy - that's great. And you can be healthy and overweight, as studies have shown. But aspiring to be fat is misdirected; being overweight usually at least suggests health risk, if not in the short term then in the long term. You can be healthy without being overweight, of course.