Cycling.  It's rare to see fats on bicycles, especially the really big ones.  If they do end up on a bike, they usually don't last.  I can't speak for downhill mountain biking though.  It's possible it has more fats in it.  I wouldn't know because I don't do it.  I stick to the pavement.  I also avoid group rides.  I have nothing against them.  It's just that I'd rather just hop on my bike when I want instead of following a schedule, and although I do have a full lycra kit, I rarely use it.  I could definitely see how having a fatty or two on a group ride would ruin it.  That said, I never see fats in cycling bunches.  So it's reasonable to assume you'd be safe from them on group rides.