Twenty years ago, Jim Karas, then 24, was a husky six-foot-tall frat boy with a weight problem and a smoking habit. Today, at 171 pounds, he's the founder of Jim Karas Personal Training in Chicago, the fitness contributor on ABC's Good Morning America, and the best-selling author of The Business Plan for the Body and Flip the Switch. Jetting between Chicago and New York, Karas includes among his high-profile clients Candace Bergen, Emma Thompson, and Hugh Jackman. "Without question, the best part of my job is telling very powerful and successful people what to do," he says. "I told Diane Sawyer she had 25 pounds to lose and she hired me." In a season when most Chicagoans would rather curl up with a book than trudge to the gym, we sweated through a day with Chicago's fitness guru.

7 a.m.
The alarm wakes Karas after exactly nine hours of sleep. He reads The Wall Street Journal and the Chicago Tribune over breakfast, which is a turkey sandwich made with "60-calorie bread," sliced tomatoes, and Dijon mustard, and two bottles of water and a Diet Coke. "You'll lose weight if you stick to an eating routine," he says.

7:45 a.m.
Wakes up the kids, Olivia, 8, and Evan, 5. Karas and his wife, the actress Ellen Karas, dress them, make their breakfasts and lunches, and walk them to school at Chicago City Day, just blocks away from the family's home in Lake View. "My wife and I practice what we preach-my kids see both of us exercising and making wise food choices."

8:30 a.m.
Meets his personal trainers, Scott Wilson, Eugene Sychov, and Pete Maltese, at Fitplex (1235 N. LaSalle St.; 312-640-1235). "Most people laugh when they hear I use trainers, but on my own I'm completely pathetic," he says. Karas's workout focuses exclusively on strength training-no "mindless" cardio, he says.

9:30 a.m.
Karas treats himself to a postworkout Diet Coke.

10 a.m.
Arrives at the office of Jim Karas Personal Training (2669 N. Lincoln Ave.; 312-440-0760), home to a dozen trainers who pay house calls to high-end clients. At $95 to $135 an hour, Karas claims to be one of the most expensive trainers in the country.

1 p.m.
Lunch at Tempo (6 E. Chestnut St.; 312-943-4373). Karas orders the usual-a spinach and tomato egg-white omelet, tomato slices, and a toasted English muffin. Other favorite meals include the "Jim Karas" chicken at RL Restaurant (115 E. Chicago Ave.; 312-475-1100); the spinach, grilled fish, and steamed vegetables at Rosebud Steakhouse (192 E. Walton Pl.; 312-397-1000); and a salad sprinkled with tuna and balsamic vinegar from the salad bar at Potash Bros. Supermart (1525 N. Clark St.; 312-337-7537). "I never eat salad dressing," he declares. "It's loaded in calories."

2 p.m.
Perched on a large inflated ball, Karas spends the afternoon at his desk on the phone with clients and business partners, including Northwestern Memorial Hospital and Kraft. "The moment you trade in your desk chair for a ball, you instantly start to burn more calories because your muscles work harder to keep you balanced," he says.

4 p.m.
Karas snacks on string cheese and yogurt. "My ‘bad' snack foods are cheese and baked chips," he says. "I'd like to be buried in a wheel of cheese."

6:45 p.m.
Leaves the office for dinner with his family. A typical dinner includes steamed vegetables and chicken satay from Amarit Thai (1 E. Delaware Pl.; 312-649-0500); cheeseless pizza topped with vegetables from Pizza Capri (1733 N. Halsted St.; 312-280-5700); Boca burgers; or vegetable stew.

7:30 p.m.
Karas indulges in a couple of glasses of wine and a 100-calorie Jell-O vanilla and chocolate swirl pudding cup.

8:30 p.m.
Karas puts his kids to bed, reads (he recently finished Blink, by Malcolm Gladwell), and watches TV. "I'm hooked on The O.C. and Desperate Housewives."

10 p.m.
Bedtime. Adequate sleep is essential, says Karas, who works out four times a week. "I never, ever, ever skip my exercise."

Jim's Fast Office Workout


Illustrations: Colin Hayes

The Slow Sit-up
Works the entire lower body.
(1) Place your feet shoulder width apart, toes forward, and your hands on the armrests.
(2) While pressing into your heels, slowly lift up to standing, counting slowly to five as you do it.
(3) Once you are standing up, repeat the same motion but in the opposite direction, lowering yourself down slowly into your chair.
(4) Keep your abdominals tucked in the entire time.
(5) Perform about ten repetitions up and down.
The Chair Walk
Works the backs of the legs.
(1) Clear a path in front of you of about five to ten feet.
(2) Sit up straight in your chair with your arms on the armrests.
(3) Dig in your right heel and propel yourself forward; then do the same with your left heel. You should feel the backs of your legs working.
(4) When you get to the end of your space, turn around. Repeat the cycle five times.

Desk Pushup
Works the chest and arms.
(1) Stand up and place your hands shoulder width apart on your desk. Make sure the desk is stable before trying this exercise.
(2) Step back from your desk until you can no longer keep your heels down.
(3) With abs tucked in, slowly inhale as you bend at the elbows. Go down as far as you can to your desktop.
(4) Pause; then exhale as you press back up to the starting position.
(5) Repeat eight to ten times.