If you're a believer in
Dr. John Berardi's principle of G-Flux, then wouldn't that pendulum swing both ways? If metabolism can be improved by eating more/exercising more, doesn't it then make sense that metabolism could be slowed by eating less/expending less energy?
And if metabolism can be improved in a matter of weeks/months, what makes us think that metabolism can't also be slowed in a matter of weeks/months of undereating?
Sorry to jump in, but it's an interesting conversation...