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 risk for nearly half of all women and more than 20 percent of men whose body-fat levels are dangerously high. The good news? This research can help you live longer and become younger.

That’s because you can be a normal weight, according to your body mass index, and be toting around extra abdominal fat that the BMI doesn’t take into account.

The problem? BMI, which is computed using your weight and 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.

Wondering where you stand? Healthy body-fat percentages for women ages 20-39 are 21 percent to 32 percent; ages 40-59 are 23 percent to 33 percent; and 60-plus, 24 percent to 35 percent. For men ages 20-39, 8 percent to 19 percent; 40-59, 11 percent to 21 percent; and 60-plus, 13 percent to 24 percent. The researchers at New York University School of Medicine used a full-body scan to measure body fat in 1,400 people for their new BMI-bad-news study. But you can do just as good a job if you just grab a tape measure and throw it around your middle.

Waist size, it turns out, is a super-accurate way to measure risky belly fat. You may have some on board if your waist measures 35 inches or more for women, or more than 39 inches for men. (Put a tape measure around at your belly button — and suck in!) But the health risks that come with belly fat actually begin about 3 inches before that! So if your numbers need a trim, focus on strategies that build muscle. Lean body tissue burns calories around the clock, preventing or even reversing belly-fat expansion. Don’t simply diet; slashing calories slashes precious muscle mass, too.

Instead, four simple steps can nudge your body composition back into the safety zone, and none involves the words diet, calories or weigh-in.

Munch muscle-protecting protein. How much protein do you need? Simply 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 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.

Get chummy with healthy fats. Nuts (especially walnuts’ omega-3s and macadamia nuts’ omega-7s), 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.

Pump some iron, pull some rubber 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.

Turn in earlier tonight. Short-changing yourself on sleep leads to cravings for doughnuts and super-size colas. But that’s not all. 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 them tomorrow after “The Dr. Oz Show” while you’re doing your strength-training routine.

Mehmet Oz, M.D., is host of “The Dr. Oz Show,” and Mike Roizen, M.D., is chief medical officer at the Cleveland Clinic Wellness Institute. For information go to www.RealAge.com.