As Mr. Strautmanis walked away from a White House news conference one morning, the president caught up with him and called out, “Strautmanis, Cornell says you’re not showing up for your workouts.”

“And that’s when I knew this was going to be a different experience for me,” said Mr. Strautmanis, who has forsaken cheeseburgers, taken up egg whites and lost 20 pounds since he started twice-weekly sessions with Mr. McClellan in a gym at the Eisenhower Executive Office Building.

An unassuming 54-year-old with a compact body and an easy laugh, Mr. McClellan has become the unlikely man at the center of a group of top staff members who work out with him, gossip with him, talk about him and even rat one another out to him, especially when someone  that means you, David Axelrod  slips up and gets an extra scoop of ice cream in the White House mess. (But the real culprit, aides joke, are the M&Ms found throughout the West Wing).

“He has this gift where he works you pretty hard and you don’t even realize it,” said Mr. Axelrod, a former senior adviser to the president who shed 25 pounds on Mr. McClellan’s watch. “And then you go home and you can’t lift up your arms.”

Mr. McClellan grew up practicing martial arts, eventually earning a black belt, and as a college student realized that he had a knack for working with people. He owns Naturally Fit, a personal training and wellness center in Chicago, and now spends part of his week in Washington at Mr. Obama’s request.