Image courtesy of Marie Claire

The most potent weapon in your antiaging arsenal may be scaling back at mealtime.

IT CAN HAPPEN AT ANY AGE: One day you're eating what you want with no consequences. The next day, one stray cookie and your jeans don't zip. Elisabetta Politi, nutritional director at the Duke Diet & Fitness Center in Durham, North Carolina, notes that each year your resting metabolism burns approximately 10 fewer calories -- 100 fewer per decade -- meaning you can slide up into the next dress size all too easily if you don't gradually adjust your diet over time. Plus, as you age, "your old techniques don't work anymore," says Heather Bauer, a New York City-based registered dietician and coauthor of Bread Is the Devil. "You can't binge and starve your way to a healthy weight." Research suggests that even exercise can't counteract the evils of age-related weight gain. Recent studies indicate that metabolism is less responsive to exercise than we had thought. New York-based fitness instructor and holistic health coach Craig Smith believes that the food you eat accounts for a full 85 percent of your body's appearance, while a gym routine dictates only the remaining 15. "No workout will give you the results you want unless you change your diet," he says. We polled the country's top nutritionists and doctors and a swath of svelte professional women to find the best body strategies for every age.

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20s

THE SCIENTIFIC SCOOP: Most women's basal metabolic rate -- a key determinant of metabolism speed -- can drop by 5 to 10 percent between their mid-teens and early 20s, due to a rise in reproductive hormones, says Dr. Jeffrey Morrison, a family doctor and nutritionist based in New York City. "As a woman enters prime childbearing years, estrogen -- which increases body fat -- rises," he explains. At a cellular level, mitochondria, which convert glucose into energy, become less efficient, impairing the body's ability to burn fat and sugar, says Oz Garcia, a New York City-based wellness and aging expert whose clients include Hilary Swank and Heidi Klum. "As kids, our mitochondria work at full blast," he says, but they slow with age.

THE EATING STRATEGY: Aim to get a full third of your diet from protein, suggests Morrison, who says he's seen patients become vegetarians only to find it harder to shed weight when they consume inadequate amounts of protein, which is necessary to maintain muscle mass and optimize metabolic function. Case in point? Jewelry designer Suzanne Somersall, 29, lost 5 pounds from cutting back on calorie- and carb-dense Nature Valley granola bars and eating more lean protein like chicken, salmon, and hummus. In addition, New York nutritionist Joy Bauer suggests swapping sugar- and calorie-heavy coffee drinks, the domain of college all-nighters, for skim-milk lattes. Milk provides calcium (women need 1,200 milligrams daily to help build bones before menopause) and protein. And add vegetables like spinach to get folic acid, which is important for women who want kids.

THE EXERCISE PLAN: Experts agree that establishing a consistent fitness routine, say 30 minutes of cardio three to four times a week to establish muscle tone and drive metabolism, is a must. Good habits now will pay dividends later.

6 Surprising Ways to Lose Weight

30s

THE SCIENTIFIC SCOOP: Research done at the Cleveland Clinic shows that while portions of the human skeleton continue growing through the mid-20s, by her 30s, a woman's vertical growth has stopped and the hormones responsible for boosting muscle and bone strength fall off dramatically. Experts say those growth hormones also help prevent glucose absorption in fat cells, and when there is a deficiency, it's hard to lose weight. On top of that, pregnancy and breast-feeding mean many women temporarily increase their nutritional intake, and stress -- brought on by full-fledged careers and family life -- can cause overeating and trigger the release of cortisol, a hormone that signals to the body to store fat around the midsection.

THE EATING STRATEGY: "In your 30s, every day can be a frantic whirlwind," says Bauer. To stabilize blood sugar and maximize energy, she suggests starting the morning with a breakfast of Greek yogurt, which has lots of calcium and twice the protein of regular yogurt, and making a peanut butter sandwich on whole-wheat bread for a mid-morning snack. (The slow-burning carbs help you feel fuller longer, as does the fat in the peanut butter.) Elisa Dahan, a 33-year-old mother of two and the Montreal-based co-creative director of outerwear line Mackage, eats Nutella on whole-wheat toast every morning, saying it satisfies her sweet tooth and gives her something to look forward to the night before. For dinner, in lieu of bread or pasta, Dahan has a salad and barbecued salmon. One habit busy women should avoid, experts agree, is unintentionally sabotaging a diet by casually polishing off high-calorie foods (like uneaten French fries) from your child's or partner's plate. "You have to create controlled moments when you can eat," advises Bauer. So steam a big plate of spinach to snack on while your kids have dinner, or order a side of grilled asparagus at restaurants.

THE EXERCISE PLAN: Full-body conditioning, like in a cardio class with weights, will torch calories and build muscle simultaneously, making it a time-efficient way to get in shape during your 30s, says Smith.