Maybe you've heard the recent reports that exercise won't make us thin because it makes us hungry, particularly for junk food. Or maybe you've noticed firsthand that you eat a lot more on gym days than on days off. Either way, it raises the question: If working out only sets us up to blow our diet, what's the point?
For starters, some research suggests exercise doesn't always cause hunger but can curb it. "Exercise may lower levels of ghrelin, a hormone that stimulates appetite in the short term, while raising levels of peptide YY, a hormone that suppresses appetite," says study author David Stensel, Ph.D., reader in exercise metabolism at Loughborough University.
That's only if the workout is intense (if you can chat, forget it), but the more intense it is, the longer the benefit seems to last. "It may be that your body needs to circulate more blood to prevent overheating," Stensel explains. Because eating causes blood to flow to the stomach to aid digestion, your body dampens your appetite to prevent that.
Like all good things, this satiating effect ends about an hour later when your body starts to crave the energy it used up. And unfortunately, the desire to refuel may hit women harder than it does men. "Physical activity may raise concentrations of longer-term appetite-stimulating hormones like insulin and leptin in women," says Barry Braun, Ph.D., associate professor of kinesiology at the University of Massachusetts in Amherst.
What's up with the sexist hunger hormones? "It might be that women are wired to defend their body weight to preserve energy for pregnancy and lactation," Braun says.
Here's where frequent exercise can save the day (and our waistline). "It appears to help restore sensitivity to brain neurons that control satiety," says Neil King, Ph.D., professor of human movement studies at Queensland University of Technology. In other words, the more you do it, the more in tune you become with your hunger signals, which may aid in offsetting them.
More motivation to sweat regularly: It can lower heart disease risk, lift mood and up your odds of a longer life overall, whether you lose weight or not. Add to all that a bangin' bod, and a passing case of tummy growls is no biggie.
Beware the Exercise Halo
A great sweat session can make you feel like a health angel—for good reason, given its life-enhancing power. "But we can feel so virtuous that we reward ourselves with some not-so-healthy habits," warns Susan Bowerman, R.D., assistant director of the UCLA Center for Human Nutrition. Don't fall for these self-sabotaging thoughts:
1. My metabolism is higher after a workout, so this bite will burn right off.
Ah, the after burn effect. That's when your body uses energy to return to a resting state. "It sounds great, but even very intense exercise lasting more than 45 minutes burns less than 100 extra calories," says Philip Clifford, Ph.D., professor of anesthesiology and physiology at the Medical College of Wisconsin in Milwaukee.
The bottom line: Skip the cool-down nibble. Doing it five times a week saves you up to 500 calories—the equivalent of a spin class you don't actually take!
2. I melted mega calories this morning. I can eat what I want today.
Define mega. Research shows we grossly overestimate our sizzle. People who burned 200 calories by walking briskly thought they had burned 825 in a study at the University of Ottawa. "And they later overate by about 350 calories based on their miscalculations," says study author Eric Doucet, Ph.D.
The bottom line: Don't just guess your calories burned; tally them in a reliable way. For most women, a brisk walk zaps 5 calories per minute (225 in 45 minutes).
3. I kicked boot camp booty. I deserve a treat after my hard work.
True, but reward yourself with food and you're likely to stall your slim-down. "Run 40 minutes at a 9-minute-mile pace and you'll burn about 470 calories; grab a Starbucks Venti Caramel Frappucino afterward and you'll replace those calories plus an extra 20," Braun says.
The bottom line: "It's incredibly easy to negate the weight loss effects of exercise with a single food item, so find other ways to indulge yourself," Braun says. Try inedible rewards such as a relaxing pedicure or new songs for your workout playlist.
4. Candy bar pre-workout? Why not! Those will be the first calories to go.
Step away from the junk food: Women who ate high glycemic index foods (candy, white bread, sugary cereal) before exercising burned 55 percent less fat than those who had low-GI foods (oatmeal, yogurt), a study in the Journal of Nutrition found. "High-GI foods raise insulin concentrations, suppressing the body's ability to burn fat; low-GI ones don't," says study author Emma Stevenson, Ph.D.
The bottom line: Sweets are best in moderation—and not before the gym.