It's fun to do sth.是将动词不定式做了主语,那么该动词不定式能改成动名词吗,为什么?

It's fun to do sth.是将动词不定式做了主语,那么该动词不定式能改成动名词吗,为什么?
因为It's fun to do sth=To do sth is fun.
那么有没有It's fun doing sth呢?
另外，Eating food is good for your health能不能改为It's good for your health eating food.