Personal experience: just because you're riding a bike does not mean that you're getting good exercise. Because I travel all over LA, I have been part of various riding groups. There is a pattern - those who are overweight, never keep up with the group on the hills. Sure, one might argue that they don't keep up because they are fat, but that's a fallacy. An overwhelming theme I have seen is that people who do not lose weight when they ride almost always choose the easier road. For example, they will skip rides that are hilly or long. They will join slower groups rather than push themselves to be part of the fastest groups possible. Or they will stroll leisurely up a hill and only be reunited with the group at a lunch stop. Everyone can pretty much ride a century, but if your average is 13MPH and includes a whole lot of stops, is that really the type of grueling century most of us have in mind? Then there are people who first join a club and are overweight, but you can see how hard they try to keep up with the fastest group. Without exception, these hard working few always end up losing the weight becoming regulars in the club.

When I first started riding, I thought that everyone who was overweight was a beginner. It took me months to figure out what was going on. I have myself lost over 20lbs and it was not easy. I used to come home after my Saturday rides and be in so much pain that I lied in bed the entire day. Only if you're making that sort of effort and not losing weight can you justify genetics. The problem is that physics still wins because if you're burning so many calories, you are guaranteed to lose weight even if we assume that you maintain your unhealthy diet. Changing your diet is also part of your commitment to exercise and health, but if you don't lose at least some sizable chunk of fat once you start riding, you're doing it wrong.