Weight loss is hardly a laughing matter here in the Obese Nation, where two-thirds of adults are overweight or worse. But the newest aha! breakthrough in the battle of the bulge just might prompt a snicker — and maybe make you healthier.

One can lose weight by laughing. No joke.

Instead of yo-yo dieting, try ho-ho dieting.

Before guffawing at the notion that mirth reduces girth, be aware that purposeful laughing is gaining a following. Thousands of laugh clubs worldwide now invite people to, well, laugh out loud together. A hybrid branch of psychology called "laughter therapy" is finding its way into hospitals and nursing homes with mood-lifter activities. A new exercise movement called Laughtercising has created guidebooks and laugh-track CDs of nonstop hooting and howling to get the yuks started. Even scientists are examining the good laugh in clinical studies.

"When it comes to the weight-loss arena, I ask myself: Is laughter a gimmick or a gift?" says Katie Namrevo of Bellevue, Wash. "Some people don't take this seriously."

But she does. On the back cover of her 2004 book, "Laugh It Off! Weight Loss for the Fun of It," is a "before" photo showing her as a frumpy 50-year-old and an "after" photo as a 54-year-old who says she laughed off 35 pounds. Included in the $29.95 book is a laugh-track CD to jump-start your weight-reduction hilarity.

Namrevo was a "stress eater" who had tried all the diets and pills, she says. They only added stress and she didn't lose an ounce.

One day, after watching a TV program on laughter therapy, she headed to the fridge to "medicate" and decided to try laughing instead. Loud, long and hard, like a lunatic.

Giving new meaning to the phrase "belly laugh," Namrevo says she found that laughing 30 seconds to five minutes as often as 10 times a day, she no longer craved food. She began losing weight and she had more energy and a desire to exercise. "Laughing is a happy and healthy thing to do," she says.

Which may mean it's only a matter of time before Robin Williams and Chris Rock join the ranks of fitness trainers.

Laughing exercises "will definitely become a part of all the fitness clubs and yoga centers," predicts Thomas Varkey, a business consultant who two years ago founded the Laughter for Life club in Boston. Members meet for 25 minutes twice a month for yoga-inspired, roll-on-the-floor laugh-o-ramas.

Varkey's laughter club is one of about 1,000 in the United States and 3,000 worldwide. Most clubs are founded by "laughter leaders" trained and certified by either of two laughter-advocacy organizations, Laughter Club International, based in Mumbia, India, or the World Laughter Tour, based in Gahanna, Ohio.

"It gives a lot of exercise to our body and a kind of well-being," says Varkey. "The well-being helps us not to eat too much. When we are depressed, we tend to eat more. Laughing is antidepressant medicine."

When Chicago public relations professional Betty Hoeffner decided last year to make a CD of uproarious laughter, friends thought she was crazy. But she had been using concerted laughter to reduce stress for years and was convinced that laughing 10 minutes a day would reduce stress for others. So she founded the Laughtercising program that gradually builds up people's ability to laugh hard for 10 minutes at a time. And she produced the 60-minute "Laugh It Off" CD to trigger laugh contagion.

"It's just laughing, but you have to work up to the 10 minutes just as you would in any exercise program," says Hoeffner, who sells the $10 recording through online retailers and at her Laughtercising and HeyUgly Web sites, the latter dedicated to increasing self-esteem in teens. Sales are "picking up," she says. "It has just been word of mouth — ha! ha!"

Hoeffner breaks into a big raucous laugh as a spontaneous demonstration of proper technique. "You just keep going and going and you work up such a sweat and your abs are aching," she says. "You get so much energy you'll be vacuuming your house at 10 o'clock at night. Just try it!"

"The laughter industry is really funny to me," says physician Patch Adams, an alternative-medicine advocate and the icon of the health benefits of laughter, who returned last week from taking 32 clowns, a third of them high school students, on a tsunami relief trip to Sri Lanka.

"The clearest connection (of laughter to weight-loss) is that depression, boredom and loneliness are the gigantic reasons why people eat gigantic quantities of trash and fatness," says Adams, who founded the Gesundheit! Institute in suburban Washington and West Virginia, which works to bring fun and creativity to healthcare. "It's not really laughter that is a great power, but the life that leads to laughter and the readiness to laugh at things."

Science is finding that laughter alone produces biological benefits. A study from the University of Maryland School of Medicine in Baltimore, presented last month to the American College of Cardiology's annual conference, showed that daily hearty laughing increases the flow of blood by expanding vessels constricted by stress.

"A belly laugh is internal jogging," says William Fry, associate professor emeritus of clinical psychiatry at Stanford University. Laughing involves "a great deal of physical exercise and muscular behavior" — 15 facial muscles plus dozens of others all over your body that flex and relax. Your pulse and respiration increase, oxygenating the blood.

"Laughing 100 to 200 times per day is the cardiovascular equivalent of rowing for 10 minutes," Fry calculates.

How often do people laugh per day? "Far more than they realize," he says, adding that any kind of laughter sets the respiratory apparatus and its muscles into motion.

Which means even brown-nosers who laugh too hard and too long at the boss' jokes are getting healthier, if not ahead.