The rising and falling levels of these chemical messengers control your appetite, cravings, where your body stores fat, and more—rendering them critical players in diet success (and failure). As scientists make cutting-edge discoveries, we're learning more every day about how these gremlins operate. And once you understand their natural behaviours, you can make them your allies.
A.K.A. MS. HANGRY
NATIVE HABITAT: The adrenal glands, located on top of your kidneys.
BEHAVIOUR IN THE WILD: Can go feral in the presence of stress. Or when you feel threatened—even if only because a jerk cut you off in traffic—providing a burst of energy so you can react. Once the danger has passed, this baddie jacks up your appetiteso you can replenish the energy you just expended (even if you didn't expend much at all). Net result? Weight gain, especially around your middle.
TAME IT: First, cut off her main energy supply: caffeine. As anyone who's ever had a triple-shot anything knows, the stimulant can increase cortisol secretion. Then whip her butt with a Funny Or Die video. Laughter—or even just the anticipation of a giggle—can decrease cortisol levels by 39 percent, according to a study from Loma Linda University.
A.K.A. MR. MOJO
NATIVE HABITAT: The ovaries and adrenal glands
BEHAVIOUR IN THE WILD: This dude is best known for his tendency to help pump up your muscles, your energy, and—rawr!—your sex drive. So when he's AWOL, your muscles (along with your libido) may atrophy, leading to a sluggish metabolism and, bingo, weight gain. He starts to fade beginning in your twenties or when you take certain forms of birth control.
TAME IT: Coax him back to his previous glory with resistance training, such as a barre class or other weight-bearing exercise (aim for a minimum of 30 minutes, three times a week), which revs up your testosterone production. Cutting back on sugar may also help him thrive—research has found eating too much of the sweet stuff can turn off the gene that regulates the amount of active testosterone in your body.
A.K.A. MS. COULDN'T EAT ANOTHER BITE
NATIVE HABITAT: Adipose tissue (fat cells)
BEHAVIOUR IN THE WILD: She normally has your back, regulating appetite by signalling when you're full and should stop eating. But if the fat cells where she hangs out start disappearing—that is, if you successfully lose some weight—she makes herself scarce as well. And when leptin dwindles, you don't get the "Okay you're done now" message, so you keep eating, making this gal one of the main drivers of yo-yo dieting. Lugging around extra pounds for too long can also wear her out and cause her to quit working, a condition known as leptin resistance.
TAME IT: Avoid fad diets that encourage quick weight loss—and the inevitable regain that leads to leptin resistance—and instead take a steady approach to cutting calories. Also, make sure you're getting plenty of zinc; low levels have been linked with decreased leptin output in healthy people, say researchers. Load up on zinc-rich foods such as beans, beef, and cashews to keep this heroine hormone happy, and don't rush through meals: Slowing down gives leptin more of a chance to do her thing.
A.K.A. MR. MUNCHIES
NATIVE HABITAT: The gut
BEHAVIOUR IN THE WILD: Working in tandem with leptin, this hungry fellow regulates appetite. When your stomach nears empty, he bellows to your brain that it's time to chow, stat. After you've munched your lunch, he chills for about three hours. But when you lose weight, your body thinks it's starving and tries to reinstate its "set point" (hey, it doesn't know you're doing this on purpose). So it ramps up ghrelin production, increasing hunger to make you plump back up. One study found overweight adults who lost an average of 30 pounds had 20 percent higher levels of ghrelin than they did prior to slimming.
TAME IT: A full stomach will send ghrelin packing, so load up on bulky but low-calorie foods like produce, popcorn, and broth-based soups. And while you're at it, try tucking yourself in a little earlier. Even low levels of sleep deprivation can increase your ghrelin levels and lead to more body-fat storage.
A.K.A. MS. TEMPTATION
NATIVE HABITAT: The ovaries
BEHAVIOUR IN THE WILD: Like her sister sex hormone progesterone, this seductress is responsible for the roller coaster of cravings that accompanies your menstrual cycle. She's a complicated creature: When she's the least active, just before your period starts, you're most susceptible to binge eating. And when she rises to the height of her power, right before ovulation, appetite is suppressed. But paradoxically, if this little lady gets too strong she can overpower her sister progesterone, and the resulting condition, called estrogen dominance, can make weight loss extremely challenging, if not impossible.
TAME IT: To keep her in proper balance, make sure you're getting enough fibre, which binds to estrogen and moves it out through your bowels so it doesn't go back into circulation. Aim for a pound of vegetables per day—it sounds like a lot, but a green smoothie, salad, and a broccoli-and-pepper stir-fry will ensure you hit your target.