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Q. I feel the fat burning when I do ab exercises. But my midsection still has a lot of fat on it. How can I flatten out and lose a few inches from my waistline?

A. That burn you feel when you do lots of repetitions of an abdominal exercise, or when you squeeze the muscles and hold them in one position for an extended period of time, is not fat burning from your belly. That burn is simply the result of the muscles fatiguing and of the contracting muscles temporarily occluding some of the blood flow to the area. This effect is a little deceptive and may be the No. 1 reason so many people mistakenly believe that if they pummel away at their abs by doing a variety of “core” or abdominal exercises, they will shed fat from their middle.

The truth is that moving your entire body by doing cardio workouts such as walking or running—not just contracting your ab muscles—is the kind of exercise that will reduce your belly fat. Ab exercises feel like they’re doing something, and they are—just not what many people think they are doing. Ab exercises do improve the strength and/or endurance of the muscles, and make them firmer. But they don’t burn enough calories to reduce fat in the belly.  So if you have fat around these muscles, the area will look flabby, fat and/or not flat, even if you have very strong abs.

If you’ve been doing ab exercises regularly, chances are that you do have a fine set of six-pack abs—but they may be buried deep under a layer of fat. Unless you reduce belly fat, the sculpted abs won’t be visible. Try this exercise:

Slump in your chair. Put your fingertips on top of your abdominal area, around and below your belly button. It may feel mushy, especially the more fat you have in this area. Press deep so you can feel the ab muscles.

Now, sit up straight and notice how your abs flatten a bit, simply from lengthening your spine. Also notice the ab muscles tightening when you raise your ribcage away from your pelvis.

If you feel firm, you probably have fairly strong abs. But if you can’t see them, it’s not because they are not there. It’s because they are surrounded—and hidden—by fat.

Why don’t ab exercises burn enough calories to decrease fat?
If ab exercises could use enough calories to burn off excess fat, then theoretically they might help decrease belly fat. But contractions of the abdominal muscles require very little energy expenditure because they are very small muscles and they are moving only a small area of your body. In fact, lying on your back and doing a move where you, say, suck in your gut and move a leg or lift your shoulders, uses hardly any more calories than simply lying on your back and not moving at all.

At rest, the body burns around one or 1½ calories per minute. Core exercises where you lie on your back, side or face-down may burn around two calories a minute. That burn you might feel in the belly is the muscles tiring out, not the body using significant amounts of energy or burning off lots of fat.

When you use many muscles at once to move your entire body through space, you burn many more calories. For example, during a very brisk walk or bout on a cardio machine, you can burn eight to 15 calories per minute, on average. So, in 30 minutes of doing any type of cardio you are going to zap a lot more calories—and fat—from your body than you would if you spent 30 minutes doing Pilates core moves or sit-ups.

Cardio workouts burn off belly fat
Research bears this out—to date, there is no evidence that muscle endurance or muscle conditioning exercises burn off fat from the area being targeted. Cardio exercise, on the other hand, has been shown to burn off fat from all over the body—and especially from the belly.

One 2005 study in the Journal of Applied Physiology had 175 overweight, out-of-shape men and women with poor cholesterol numbers exercise for eight months. They did cardio workouts at both easy and vigorous intensities in amounts similar to walking a total of 12 miles per week, or running for either 12 or 20 miles per week. Exercising at either low or high intensities for a shorter period (the equivalent of around 12 miles per week) prevented increases in ab fat over the eight months. Those who did the greater amount of cardio exercise (around 20 miles per week) not only did not gain fat over the year, they lost an average of 7 percent of their deep and superficial ab fat. A control group who did no exercise experienced an average 9 percent increase in their ab fat.

Another study in a 2007 issue of the journal Obesity tested more than 200 unfit men and women who were between 40 and 75 years old. The participants were instructed to do cardio workouts for about an hour a day for six days a week for one year (the subjects took 12 weeks to build up to doing the full hour of cardio.) They used treadmills, bikes, rowers and/or elliptical machines. Both men and women significantly decreased their intra-abdominal fat (women by an average of 5.5 percent and men by an average of 7.5 percent). Exercisers were asked not to change their eating habits during the study, so that the effects seen could be attributed to exercise alone, rather than dieting.

The bottom line: To slim down your belly, rather than doing more ab exercises, start clocking up more cardio minutes. Unfortunately, there is no way that science has shown yet to dictate where the body burns the fat from. This may be genetically determined. So some people may lose more fat in their face and arms, whereas others may lose more in their abs or thighs. But no matter what your genetic predisposition to store fat in your stomach area, more cardio workouts can help decrease it.

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