Switching from a low-protein to high-protein diet has had the most dramatic positive effect on my health and fitness I've seen in my short 35 years on this earth.
For most of my life I basically got no real protein in my diet -- cereal for breakfast, pasta for dinner, etc. I was always just a bit out of shape, and no matter how much time I spent at the gym I saw minimal improvements.
A few years ago I was inspired by a weightloss thread on reddit and "rebooted" my eating and fitness. Started running, dumped my carb-heavy diet and moved to a high-protein diet -- Greek yogurt, lots of chicken, cheese, etc.
I've lost 30 lbs and am seriously in better shape than I've ever been. My exercise level went up, sure, but the dramatic change came from my diet reboot.
So frankly, even if this were true, I'd rather live the life I've got now than go back to the low-protein lethargy of my 20s.