COMMON DIET QUESTIONS ANSWERED

Your coworker advises against eating after 8 p.m., a girl from your yoga class swears by the grapefruit diet, and your best friend warns that mixing carbs and protein can pack on the pounds. Sure, you've been hearing diet tips like these from well-meaning people for years, but is there any truth to them? To set the record straight on the most oft-repeated advice, we consulted a team of nutrition experts. They revealed which strategies you should forget and which live up to their get-slim promise.

Q. Will chewing low-cal foods like sugar-free gum and celery help me burn calories?

A. It might, but hardly enough to trigger weight loss. Gum and certain veggies are often called "negative-calorie" foods because they supposedly take more energy for your body to chew or digest than they contain.

The negative-calorie myth was put to the test when researchers at the Mayo Clinic in Rochester, Minnesota, asked people to chew sugarless gum at a rate of 100 bites per minute. After calculating the energy expended (about 11 calories per hour), they concluded that a person who chomped on a piece every waking hour of the day for a month would lose less than a pound. As for celery? All that crunching does burn energy, but it amounts to less than the 6-calorie stalk contains. The bottom line: If you really want to shed pounds, give your jaw a rest and start moving your body.

Q. Can coffee really rev up my metabolism?

A. It's true: Java can stoke your calorie-burning furnace—provided you drink it black. A study in the journal Metabolism found that the caffeine in two cups of coffee may cause a 145-pound woman to expend up to 50 extra calories over the next four hours. "Caffeine stimulates your nervous system, signaling the body to release a small amount of energy from its fat stores," says lead researcher Paul Arciero, Ph.D., an associate professor of exercise science at Skidmore College. "But stirring in milk, cream, or sugar can cause your insulin levels to rise, which diminishes that metabolic effect."

Don't try to accelerate the weight loss process by sipping black coffee all day, though. Arciero recommends not exceeding three cups in a day, as too much caffeine can cause anxiety, nausea, and headaches.

Q. Will eating after 8 p.m. make me gain weight?

A. That all depends. Contrary to popular belief, the snack you have before bedtime won't automatically be stored as fat. "The most important factor affecting your weight is how many total calories you eat each day, not what the clock reads when you eat them," says Suzanne Farrell, R.D., a Denver nutritionist and a spokeswoman for the American Dietetic Association. That said, skimping on meals during the day may set you up to overeat at night, which can pack on pounds. A recent study in the Journal of Nutrition found that the less food people ate for breakfast and lunch, the more they ate after sundown-and the higher their total calorie intake for the day.

"Our brain's satiation mechanism—its way of telling us we're ready—tends to become weaker as the day progresses," says John de Castro, Ph.D., lead study author and a professor of psychology at Sam Houston State University. "That means you may have to eat more in the evening in order to feel full." His research suggests that have having a larger breakfast, a moderate-size lunch, and a smaller dinner can help you consume fewer calories and reduce the temptation to nosh at night.

Q. Would eating carbs, fat, and protein separately help me lose weight?

A. No. While the concept of "food combining," or eating certain nutrients at specific times (and excluding others), has fallen in and out of vogue for decades, there are no proven benefits. The theory is that different food types (proteins, fats, starches, sugars, and acidic foods) require their own digestive enzymes in order to be metabolized properly. Some claim that mixing these groups or eating them at the wrong times could cause digestive issues or weight gain. For advocates of this eating style, having orange juice and scrambled eggs at a sitting, or even a turkey sandwich, is forbidden.

To determine if a food-combining diet could confer any health or weight-loss benefits, researchers at University Hospital Geneva in Switzerland put two sets of obese patients on low-calorie diets for six weeks. The first group followed a food-combining plan (eating carbohydrates at one meal and fats and protein at another), while the second ate meals that contained all three nutrients. While both groups took in the same amount of calories, those on the balanced diet actually lost about 3 pounds more than the food-combining group—and lowered their blood pressure to boot.

Q. Will blotting my pizza cut down on calories?

A. It won't soak up all of the fat and calories in your lunch, but it can make a dent. "If you're eating a medium slice of cheese pizza, swabbing it first with a napkin can remove up to 45 calories and 5 grams of fat," says Farrell.

But all the mopping in the world won't help if you're ordering the wrong kind of pie. A report from the Center for Science in the Public Interest in Washington, D.C., found that stuffed-crust and meat-lovers varieties, which can clock in at 800 calories per slice, contain more than a day's worth of fat and sodium.

If you really want to improve the nutritional profile of your pizza, skip the meat toppings and order your pie with extra veggies—like mushrooms, spinach, or broccoli—and half the cheese (which saves about 80 calories and 6 grams of fat per slice). Switching from deep-dish to thin-crust can also slash up to 200 calories and 6 grams of fat.

Q. Does exercising on an empty stomach burn more fat?

A. Yes, but you might not be able to work out as hard as you would if you'd eaten first. Researchers at the University of Ottawa in Canada asked two groups of people to hit the treadmill in the morning until they'd blasted 400 calories. The joggers who skipped breakfast burned 58 percent more fat than those who had eaten a meal before their run. But pre-workout fasting won't necessarily translate into weight loss. "People incorrectly assume that if you're using fat for fuel, it equates to losing body fat," says Nancy Clark, R.D., a sports nutritionist in West Newton, Massachusetts. "But what affects weight loss most is how many calories you've depleted during your workout and if you've sustained a deficit by the day's end."

It may sound counterintuitive, but having a 150- to 200-calorie snack at least 30 minutes before your sweat session could help you get slimmer in the long run. A study from Pennsylvania State University found that women who had a mini-meal before their workout were able to exercise up to 16 percent longer than those who drank only water beforehand. Plus, says Clark, women who exercise on empty become so ravenous after they finish that they often end up making poor food choices. Eating a banana or a granola bar before lacing up your sneakers can give you the energy you need to crank up the intensity.

Q. Can foods like cabbage soup and grapefruit help me flush fat?

A. Despite long-standing rumors to the contrary, "there's no science proving that any particular food can burn, melt, or flush away fat," says Donald Hensrud, M.D., an associate professor of preventive medicine and nutrition at the Mayo Clinic. "If a woman loses weight on a grapefruit or cabbage soup plan, it's likely because she has cut calories by restricting her intake to a handful of foods."

Hensrud's colleagues at the Mayo Clinic (which is often incorrectly credited with creating both the cabbage soup and grapefruit diets) estimated that people who follow either plan faithfully eat 1,000 to 1,200 calories a day. "You'd almost certainly lose weight eating 1,000 calories of anything, whether it's bananas or potato chips," he says. "But it will be pretty tough to keep the pounds from returning once you return to your normal diet."