At the risk of ruffling a few feathers…. FALSE!
The first point that I want to make is that the 80% diet and 20% exercise rule is a total myth.
Because of 2 things:
For these two reasons dieting only seems to be more important because it takes more effort and typically the results seen from dieting incorrectly vs correctly are astronomical. Dieting is the “missing link” but you will only get halfway there doing one or the other.
Now why do gym rats look better than perpetual dieters?
Because of reason #2 stated above, you will see people who focus on exercise in much better shape than the reverse. Many people diet incorrectly and to be honest, no one knows what the “right” amount of anything is these days. The fact that it changes with each generation is a key indicator. Dieters won’t see the results they want because quite frankly, they don’t know what they’re doing. It would take you many years to find out exactly what your body needs to produce what you want it to produce. Some people get lucky, but dieting right varies so much person to person. We have more sound knowledge of the human movement system.
Another reason is because exercise builds muscle and “tones your body” and dieting takes the layers off to make it more visible and controls how much you get out of your workout.
Just to look at the extremes on each end: