Trimming fat from your frame might be easier than trimming fads from the fitness industry.
Here's a current favorite of exercise physiologists and personal trainers: Working out at low intensity -- or the low end of a target heart range -- burns fat.
"Totally erroneous," says Jack Wilmore, a professor of kinesiology at the University of Texas.
"When you're lying in bed, your body gets 70 percent of its fuel from fat," Mr. Wilmore said. "When you sit up, 65 percent comes from fat. When you mosey down the hall, 50 percent is from fat. The more you exercise, the more carbohydrates you burn. Maybe 80 percent of what you're burning during exercise is carbohydrates."
Here's another remark making the rounds: Exercise increases your metabolic rate. This means that after you've worked out, your body's metabolism keeps up the momentum somehow and burns calories faster while at rest.
Not so, Mr. Wilmore says. Researchers haven't discovered that exercise has an effect on resting metabolic rate. Exercise certainly increases metabolism during exercise, he says, but when exercise stops, resting metabolism returns to its usual rate (which in part is genetically determined.)
Confusing? Of course. How are you supposed to know what effect exercise has on you?
You can listen to Mr. Wilmore, but he'll warn you there are many aspects of working out the lab boys can't explain. To wit:
"A lot of changes take place when you exercise," Mr. Wilmore says. "As you train, you become better able to use fat. Enzymes change. We can't answer why."
Peter Francis, a kinesiologist at San Diego State University, who specializes in biomechanics, says human performance research cannot be unerring because, well, humans are involved.
"The only way to know for sure how much fat a person has is to parboil him," Mr. Francis says.
Mr. Francis sees the confusion as a communication problem.
"There are fads, and there is partial misinformation," he says. "Some gimmicks are just so mindless that it only takes an ounce of education to get rid of them. Misunderstandings are much more subtle."
The low-intensity fat-burning misunderstanding is an oversimplification that came about, Mr. Francis says, because low intensity lets you work out for longer periods. You don't fatigue as quickly working at low rather than high intensity, so you can keep going.
"A calorie is a calorie is a calorie," Mr. Francis says.
He means that as long as you take in fewer calories than you use, you will break into your fat stores to supply calories needed for exertion, even if the actual fuel used at the time of exercise is not fat.