Is your bathroom scale lying about whether your weight is healthy—or a health threat? The obesity epidemic is bad enough, but along comes a study that indicates that the widely used, 200-year-old healthy-weight formula is underestimating the health risk for nearly half of all women and more than 20 percent of men whose body-fat levels are dangerously high. The good news is this research can help you live longer and younger.

The problem? You can be a normal weight according to your body mass index (BMI), but you might be toting around extra belly fat that the BMI doesn't take into account. BMI, which compares your weight to your height, can't distinguish between lean, sexy, healthy muscle and excess body fat—especially belly fat, which raises your risk for heart attack, stroke, diabetes, cancer, worsening arthritis, dementia and more.

Here are healthy body-fat percentages for women:

• 20 to 39 years old: 21 to 32 percent
• 40 to 59 years old: 23 to 33 percent
• 60 years or older: 24 to 35 percent

For men, healthy body-fat percentages are as follows:

• 20 to 39 years old: 8 to 19 percent
• 40 to 59 years old: 11 to 21 percent
• 60 years or older: 13 to 24 percent

For their BMI-bad-news study, the researchers at New York University School of Medicine used a full-body scan to measure body fat in 1,400 people, but you can do just as good a job if you just grab a tape measure and measure your waist. Waist size, it turns out, is a super-accurate way to measure risky belly fat. You may have some body fat on board if your waist measures 35 inches or more for women, or more than 39 inches for men.

How do you calculate BMI? Put a tape measure around you at the belly button level—and suck in! The health risks that come with belly fat actually begin about 3 inches deeper than your un-sucked-in middle. If your waist size needs a trim, focus on strategies that build muscle. Lean body tissue burns calories 'round the clock, preventing or even reversing belly-fat expansion. Don't simply diet. Reducing calories reduces precious muscle mass, too. Instead, take these four simple steps to nudge your body composition back to a healthy BMI (none involve the words diet, calories, or weigh-in):

1. Eat muscle-protecting protein. To learn how much protein you need, multiply your weight in pounds by 0.4. If you're 165 pounds, that's about 66 grams of protein. Get yours from fish (32 grams of protein are in 4 ounces of salmon or trout), skinless poultry and plants (kidney beans, nuts and edamame have 16 to 18 grams of protein per cup). With vegetarian protein, you also get fiber, protective plant phytochemicals and no saturated fat.
2. Snack on healthy fats. Nuts (especially walnuts and macadamia nuts), fish, avocados, seeds and a splash of canola oil are bursting with unsaturated fats that help your body listen up when leptin (the "I’m full" hormone) says, "Put down the fork and back away from the table." Eat fish a couple of times a week and enjoy a small handful of nuts every day to restore your body's natural leptin sensitivity. Choosing these hunger fighters instead of foods brimming with saturated fat helps because that greasy stuff actually turns down your body's production of leptin.
3. Pump some iron, try resistance bands, or leverage your own body weight. Aim for three 20-minute strength-training sessions per week. Using a weight that exhausts you with 12 repetitions builds muscle; resistance bands are great no-impact exercises that are good for posture; and chin-ups, push-ups or sit-ups (knees bent) that use your own weight, build muscle safely and effectively.
4. Head to bed early. Short-changing yourself on sleep leads to cravings for doughnuts and super-sized colas. Sleep deficits also raise levels of stress hormones that order your body to store the extra calories in your torso, so turn out the lights at 9:30 or 10 tonight. Set your DVR to record your favorite late-night shows, then watch 'em tomorrow after The Dr. Oz show, while you're doing your strength-training routine!