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 to stop eating—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 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 weightloss 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 Should I eat a doughnut at morning work meetings, or have nothing at all?

A Doughnuts are the better choice, but just marginally. “Not only are they excessively high in fat, but doughnuts are also made with sugar and white flour, carbohydrates that are quickly absorbed into your bloodstream,” says Farrell. “You may feel satisfied during your meeting, but you’ll probably start feeling hungry shortly thereafter.” Over time, these breakfasts may lead to weight gain: A Saint Louis University study found that women who chose carb-rich meals over higher-protein ones ate about 400 more calories and had stronger cravings over the next 36 hours.