Image courtesy of Prevention

If you're like many women over 40, you've probably noticed that it's become a lot easier to gain a few pounds than to lose them. The foods that you ate without care in your 20s and 30s now stick to your body like glue, adding bulk to your midsection. The good news: The solution to a slim, firm body at 40-plus is no farther than your fridge. Research shows that, when combined with a little regular exercise, what you eat and when you eat it are your metabolic secret weapons for building muscle mass, the body's prime calorie-burning tissue and a key driver of your metabolism.

"The main culprit that slows metabolism and often leads to yo-yo dieting is what I call shrinking muscle syndrome," says Caroline Apovian, MD, director of the Nutrition and Weight Management Center at Boston Medical Center and the author of The Overnight Diet: The Proven Plan for Fast and Permanent Weight Loss. Starting at age 30, most people begin to lose about half a pound of the metabolism-revving tissue each year. Poof! Gone, just like that. And at age 50, the rate doubles. "The average sedentary woman may have lost nearly 15 pounds of muscle by the time she reaches her late 50s, a change that could cause her to gain nearly the same amount in body fat," says Wayne Westcott, PhD, a Prevention advisory board member and the director of fitness research at Quincy College in Massachusetts.

But too-tight jeans, a flabby midsection, and an increased risk of diabetes don't have to be your future. (Your chances of winding up with all of the above increase with each pound of muscle you lose.) Here's how to safeguard muscle mass and turn up the heat on your body's natural calorie-frying furnace.

What's Your Metabolism Type?

METABOLISM BOOSTER #1 Keep Tabs on Protein

You already know to keep calories and fat in check, but you'll fan the flames of your metabolism by putting another nutrient on your radar: protein, the building block of lean muscle mass. Each time you eat a protein-rich food -- say, a piece of fish or cheese -- your body goes to work, breaking it down into smaller particles called amino acids. "The amino acids enter your bloodstream and are then absorbed by your muscle tissues and other cells," says Douglas Paddon-Jones, PhD, director of exercise studies at the University of Texas Medical Branch. "Once the amino acids end up in your muscles, your body starts putting them back together -- sort of like Legos -- into your muscle tissue." This is called muscle-protein synthesis, and it's the process your body uses to build and maintain muscle mass.

However, just like those rehab-loving design gurus on HGTV, your body breaks down muscle as regularly as it builds it. "All of the cells in your body need protein to function. When there aren't enough amino acids from food available in the bloodstream, the body will start to break down and harvest amino acids from your muscle in order to keep more vital cells -- like the ones in your brain and other organs -- functioning," says Dr. Paddon-Jones. "This is a natural, continuing cycle. Muscle-protein synthesis goes up after you eat a meal with protein, and your body switches back to muscle-breakdown mode a few hours after you've eaten. Normally, the ups and downs equal out and your muscle mass stays the same." However, eat too little protein for too long and your muscles start to shrink, eventually causing your metabolism to take a nosedive.

And new research suggests that many of us may need more protein than we realize. The current RDA is 0.8 g of protein per kilogram of body weight, but several studies have found that 1 to 1.2 g may be more protective against age-related muscle loss. Dr. Apovian uses a slightly higher amount -- 1.5 g/kg of ideal body weight -- to successfully help herself, as well as her patients, shed body fat and maintain lean muscle mass. According to her, if you're 5-foot-5 and 130 pounds, you should aim for about 90 g of protein. While that may sound like a lot of protein, it's doable when you break it down. Four ounces of chicken or beef provides nearly 30 g in one shot, and a single serving of Greek-style yogurt packs nearly 20.

While most Americans consume plenty of protein, research shows that some women begin skimping on the muscle-sustaining nutrient as they age, consuming less than the RDA. Calorie-conscious dieters also tend to cut back on protein, when they should be doing the opposite. "Cutting back causes your body to rob your muscles for energy, leaving you thinner but also flabbier and weaker," says Dr. Apovian. "Not only does losing muscle make your clothing fit poorly, but you begin to burn fewer calories, so even if you're eating the same amount, you can easily regain the weight you shed." Having less muscle mass also makes you weaker, making it harder to do simple activities, so you become more inclined to crash on the couch. Eventually, the scale climbs back up and you start all over again, chipping away at muscle mass and putting the chill on your metabolism with each diet you try.

"However, keep in mind that calories still count, especially if you're looking to drop a few pounds," says Christine Gerbstadt, MD, RD, the author of Doctor's Detox Diet: The Ultimate Weight Loss Prescription. "If you're increasing protein intake, you need to cut back elsewhere."

Case in point: In a 10-week preliminary study led by Dr. Apovian and Dr. Westcott, baby boomers who exercised regularly and followed a moderate-calorie diet (1,200 to 1,500 calories for women; 1,500 to 1,800 for men), while simultaneously increasing their protein intake to 1.5 g/kg of ideal body weight, lost nearly 5 times more weight than participants who exercised without changing their diets. They also lost 4 more pounds than exercisers who increased protein intake but didn't keep calories in check. Even better: The calorie- and protein-conscious group gained more muscle, reduced their blood pressure, and dropped 2 inches from their waists.

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