So well, in fact, that Mr. Bush has started buying a new wardrobe to replace oversize shirts and having wide pants that no longer fit his diminished figure taken in. Besides following the Paleo diet, Mr. Bush is subjecting himself to almost daily sessions on a treadmill or laps in a pool, aided by a successful knee operation recently. (He declined to discuss his newfound fitness.)

This is not, of course, Mr. Bush’s inaugural stab at slimming down.

“I went through several different diet phases” with Mr. Bush and his wife, said Josh Butler, the executive chef at the governor’s mansion during Mr. Bush’s tenure (among them: a vegetable phase).

One spring, he tried giving up breakfast and lunch for Lent, telling a constituent that the lengthy sacrifice left him famished.

Those who know Mr. Bush say he is refreshingly candid and, for a man in public life, self-deprecating about his difficult relationship with weight.

“He’s been very open about his own struggles,” said Art Smith, a Chicago chef and author who has cooked for Bush family events in the past.

Not even schoolchildren were spared Mr. Bush’s frankness. As governor, he received a stream-of-consciousness email from Matthew Ross, a middle school student who wondered whether Mr. Bush liked pizza. “Mom and me had pizza for dinner. We like Little Caesars,” Matthew explained.