Before Jane Fonda showed us how great 52 can look and 43-year-old Nolan Ryan pitched a no-hitter to the best team in baseball, many people assumed that getting slower and wider were facts of midlife.

The average American male picks up six pounds between his 25th and 35th birthdays, notes the federal Centers for Disease Control in Atlanta, and the average woman gains nearly eight pounds during that decade -- most of which is fat.

But as more and more fortysomething athletes -- like football star Lyle Alzado, boxer George Foreman and baseball catcher Carlton Fisk -- demonstrate that it's possible to have gray hair and a great body, aging baby boomers are realizing that they, too, can shed their middle-age spread.

And it's not just a matter of vanity, it's a matter of health. Excess fat can increase your risk of a wide variety of ailments, from heart disease to high blood pressure, diabetes, gallstones and some types of cancer.

"There is no biological reason for men and women to get fatter as they get older," says physician Kenneth Cooper, executive director of the Aerobics Center in Dallas. "If you take care of yourself, exercise and watch what you eat, you can expect minimal -- if any -- increase in body fat with age."

Most Americans, however, get less active with age but eat the same or more. The result is stored fat, from taking on more fuel than they're burning. That's why the latest studies show that the best way to lose fat and keep it off is by both diet and exercise, together.

Dieting without exercising is "the most inefficient method of losing weight," says Peter Wood, an associate director of the Stanford Center for Research in Disease Prevention. "When you diet, your basal metabolic rate goes down and you burn fewer calories."

Over-reliance on diet for weight loss -- particularly extremely low-calorie, nutritionally poor diets -- is associated with health risks, including electrolyte imbalance, which can lead to heart arrhythmias. Extreme diets can prompt your body to rob its nonfat mass (such as muscles, bone and water) for fuel. So though you may lose weight, you won't necessarily lose fat.

Weight lost through such diets is typically regained, "since most people don't like to diet," Stanford's Wood says. "As soon as you begin consuming enough calories again, your body will assume its old shape. So there's only one thing to do: become more active."

If you must pick one -- either diet or exercise -- to lose fat, you're better off exercising. "Our studies show that overweight men who exercise can lose a substantial amount of weight -- comparable to amounts lost by dieting -- but without dieting at all," he says. "And they keep the weight off better than do the people who lose weight by dieting. We've found that behavioral changes relating to exercise are retained better than behavioral changes relating to eating."

Exercise gives added punch to fat loss because activity boosts your metabolic rate so you burn more calories. And your metabolic rate stays elevated for up to 75 minutes after activity ends. This is why some fitness professionals advise exercising before meals, since -- if your metabolism is going at top speed while you're eating -- it will burn more of the food calories passing through your system.

Exercise that builds muscles -- such as strength training -- can also give you a fat-burning edge, since muscular people tend to have higher metabolic rates and burn more calories while they are at rest. Muscles are like the body's engine and are more metabolically active than fat tissue. Powering up this "engine" means using more fuel, which helps reduce fat stores.

"Every pound of muscle we lose lowers our metabolic rate by about 50 calories per day, and every pound of muscle we gain raises our metabolic rate by about 50 calories per day," says exercise physiologist Wayne Westcott, strength-training consultant to the YMCA of the USA. "Research shows that basal metabolism slows down at the rate of 3 to 5 percent per decade. This may be more closely related to our muscular conditioning than to our chronological age."

While strength training can indirectly boost fat loss, aerobic exercise is still the best calorie burner. The American College of Sports Medicine's new guidelines for adult fitness recommend performing aerobic activity -- such as walking, running, bicycling, swimming or cross-country skiing -- three to five days per week at an intensity that raises the heart to 50 to 80 percent of its maximum for a duration of 20 to 60 minutes. In addition, they recommend strength-training sessions of moderate intensity at least two times per week.

Losing weight at any age is largely a matter of arithmetic -- burning more calories than you consume -- to deplete your body's fat stores.

"Visualize one pound of body fat as four sticks of butter," suggest Kenneth and Mildred Cooper in "The New Aerobics for Women". "It takes about 3,500 calories to make one pound of body fat. To lose one pound, you have to burn 3,500 excess calories.

"Simple arithmetic says that lopping off 500 calories a day from your ideal-weight caloric intake will cause you to lose one pound a week (500 calories x 7 days = 3,500 calories)." But since the idea is to lose fat, not just weight, they note, "it's better to cut only 250 calories a day and burn off the other 250 calories through some form of exercise that tones your muscles while you're shedding fat."

Bodyworks appears on alternate Tuesdays.