IMO the obesity trend we're seeing is simply the result of a typically very unhealthy diet combined with a very sedentary lifestyle.
For example,
- Most common cause of death in U.S.: Heart attack
- How frequently a heart attack kills in U.S.: Every 45 seconds
- Risk of death from heart attack for the average American man: 50%
- Risk of death from heart attack for the average American man who consumes no meat: 15%
- Risk of death from heart attack for the average American man who consumes no meat, dairy products or eggs: 4%
Presumably vegetarians eat less junk food too, but numbers like these speak volumes.