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.
From the horses mouth. So to speak.
Catholic Forum Question: Is it a sin to body build?
"I really don't see it as being anything more than self-glorification and vanity."
"The probability is quite high that this activity would lead one away personally or others away from God."
"wow, great responses! I tend to agree that it really does depend on the individual, and his/her own disposition and intentions."
I doubt everyone has these views. Here is a relevant bible verse: 1 Tim 4:8 "For physical training is of some value, but godliness has value for all things, holding promise for both the present life and the life to come."
God wants your heart. Keep him first above body building. Just because that forum says these things doesn't mean it is the view of the church as a whole. @steve makes good point. I would talk with a spiritual leader you can trust.