Does Catholicism consider body building to be a sin? I am an amateur body builder. I have heard that the Roman Catholic church categorizes this as a 'sin'. Is this in line with church beliefs?
The short answer is “no.”

First of all, the Church is not in the habit of listing all possible sinful behavior. The vast majority of possible behaviors are left to the prudent judgment of the faithful: only when certain things are particularly grave violations of the natural law does the Church speak out about them. (See the Catechism of the Catholic Church [CCC], 1954-1960, for an overview of the natural law.)

Hence, when we consider the morality of body building, it is sufficient to apply the principles of natural law, because in fact the Church has no official teaching about it.

It is fairly evident that, in and of itself, body building does not offend God, neighbor, or self in any way, and hence it is not sinful.

Of course, there are any number of excesses that can go along with body building: using steroids or other performance-enhancing drugs (unless they are being used to treat some kind of disorder), for example, would be immoral (see CCC 2291), as would damaging one’s health for the sake of body building (see CCC 2289). Another possible pitfall is that body building can become an obsession: spending excessive amounts of time in what is essentially a recreational activity can be immoral. Moreover, body building can be the occasion for vanity: of seeking the approval of others in a disordered way.

However, as long as body building is viewed as an essentially recreational activity, or as legitimate means to train for sports, there is no problem with it.