Raise your hand if you've sworn off sugar, cut out alcohol, and committed to a punishing gym ritual—yet those last five pounds are sticking around like an unwanted guest.

A new study in Science suggests that even ardent self-deniers can't shed extra padding if the bacteria in their system is unbalanced. Researchers at Washington University in St. Louis studied sets of twins—one fat, one lithe—and found that the thinner siblings had gut bacteria that spike the metabolism and suppress appetite, while the heavier ones were shackled with species that do the opposite.

"If you have the wrong makeup of your microbiome, it's virtually impossible to drop pounds," says Raphael Kellman, M.D., author of The Microbiome Diet.

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Unless, that is, you hack your body with probiotics. Yes, the stuff in yogurt that Jamie Lee Curtis hawks for digestive issues could burn calories and curb cravings when Draconian diet and exercise plans aren't enough.

Which is why probiotics are transcending the dairy aisle. Swank health-food stores and juice bars have caught on to the microbes' fat-torching potential and are incorporating them into what you already consume or making bacteria-laden fermented foods more available.

Moon Juice in Venice, California, sells custom probiotic shots to add to your cold-press, New York City's Juice Press blends them into smoothies (like the drinkable faux yogurt Vital Force), and ready-to-eat chain Organic Avenue stocks shelves with probiotic chia-berry cups and kimchi bowls. Walk into Whole Foods or the like and you'll find tempeh, miso, kefir, and fermented beverages like Body Ecology's CocoBiotic coconut juice, kombucha, even ginger beer—all of which boost waist-trimming bacteria in your stomach.

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Skeptics warn that more research is needed to find out which strains do what. Plus, probiotics aren't regulated by the FDA, meaning manufacturers can make vague claims without backup. Still, the anecdotal evidence is persuasive. Stella Metsovas, a certified nutritionist and author of The 21-Day Digestive Health Detox, prescribed a half-cup of probiotic-rich foods daily to a group of clients, who lost an average of 21 pounds over six months without cutting calories.

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"The right types of probiotics can absolutely improve metabolism and help with weight loss," Kellman says. The trick is getting an ideal mix of good and bad bacteria: The wrong microbes cause chronic inflammation, which leads to fat-inducing insulin resistance (exercise can't counteract that). An excess of certain strains can also trigger cravings for sugar and fat. To help strike that balance, down probiotics at least twice a week and let the bugs do the work for you.

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Term to Know: Psychobiotics

Scientists at UCLA's David Geffen School of Medicine found that people who regularly take probiotics exhibit brain activity associated with greater emotional stability. They believe that particular strains of gut bacteria can reduce stress, boost mood, and lessen pain by influencing certain hormone levels, including cortisol, serotonin, and oxytocin. Different microbe families affect different neurotransmitters, so to reap as many benefits as possible, try a supplement with a variety, like Healthy Origins 30 Billion Probiotic ($38 for 150 capsules; amazon.com).

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