As a preface, I think Jean Claude Van Damme has a very impressive body, and so does Vin Diesel.
Do you think it's easy to look good on camera? Sure, the lighting can help you if that's what they want. But you're working 12 - 18 hour days when you're on the set, and most of the craft table is always full of sugary carbs. The meals they do give you usually don't have nutrition as their top priority.
Of course, if you're a big enough star or rich enough, you may get special treatment for your food - but that's a lot of work to get to that status. I know an actor who's been consistantly on tv shows for 10+ years, and he still has to bring his own food to the set if he wants to eat healthy.
Crap! That wasn't even my point. The point was supposed to be that I remember reading a reporter asked Jean Claude why he kept on denying he did roids to reporters.
Jean said something to the effect of "Look - even if I did do 'roids, I would never tell you. I'm a role model for some kids. Kids will emulate their role models. I have to think about if some kid thinks he can look like me if he juices because I juice. There's a lot of responsibility needed here."
I had a lot more respect for him after that. To me, it sounded like he did juice, but is protecting the younger generation by setting a good example.