Just can't get rid of your flabby belly, even though you do sit-ups and crunches until you're ready to drop? Maybe you're so mortified by the condition of your abs that you've relegated your bikini and low-rise jeans to the dark recesses of your closet.

Before you dump on (or just plain dump) your workout for not giving you killer abs, you should know the real culprit may be your diet — or even your high-stress lifestyle.

If you want to go from fat to fab abs, new research shows that what you eat is just as important as how — or even how much — you work out. And lifestyle counts, too, because stress doesn't just mess with your head, it also can induce a pooch.

Following are six strategies from the country's leading weight-loss, nutrition and stress experts, all designed to get you flat abs in just four weeks (see our "4-Week Fiber-Up Guide," left), plus three delicious, lowfat recipes that will fill you up without making you look or feel bloated.

Tip 1. Eat more fiber.
Not eating enough fiber may be a major reason women are getting fatter and flabbier. To ditch the fat and show off firm, beautiful abs, you need to eat at least 25 grams of fiber daily, says leading fiber researcher David J.A. Jenkins, M.D., Ph.D., D.Sc., professor of nutrition at the University of Toronto, and a member of the National Academy of Sciences (NAS) Food and Nutrition Board. Fiber, which is the indigestible part of fruits, vegetables and whole-grain foods, helps you achieve flat abs for three reasons:

The "bulk" factor Fiber is like a dry sponge. When it combines with the water in your digestive tract, it makes everything move through more quickly.

The "fill" factor Because high-fiber foods like fruits and vegetables supply plenty of bulk to your meals without adding a lot of calories, they keep you feeling full longer and help you lose weight, according to a study at the Human Nutrition Research Center at Tufts University. Researchers concluded that lowfat diets work only if they're also high in fiber-rich foods like fruits, vegetables and whole grains, all of which fill you up on fewer calories and less fat. In contrast, lowfat diets that are low in fiber and high in sugar, salt and preservatives can lead to bloating and weight gain.

In a study conducted by Barbara Rolls, Ph.D., a professor at Penn State University and co-author of The Volumetrics Weight-Control Plan (HarperTorch, 2003), subjects who ate vegetables as part of their meals consumed about 100 fewer calories and didn't make up for the caloric deficit later. While saving 100 calories a day may not sound like much, it translates into losing 10 pounds in one year. Use just this one trick — and there goes your tummy!

The "chew" factor "High-fiber foods require more chewing and take longer to eat," explains Leslie Bonci, M.P.H., R.D., author of the American Dietetic Association Guide to Better Digestion (John Wiley & Sons, 2003). "Because your mouth is more involved in the eating of high-fiber foods, you feel more satisfied with a high-fiber meal."

How to add fiber to your diet comfortably One cautionary note: It's important to add fiber slowly but consistently to prevent gas. "Make higher-fiber choices throughout the day; don't have all your fiber in one bunch," Jenkins says. "This is particularly important with viscous fiber — a type of soluble fiber found in beans, oats and barley that also has the benefit of lowering blood cholesterol," he says.

For best results, increase your fiber intake slowly over the course of one month and drink plenty of water to keep food moving through your system as quickly as possible.