When it comes to assessing our health, most of us have tunnel vision for what the scale says. Yes, weight is an important indicator of how healthy we are, but our bodies are made up of both fat and lean tissue, which consists of muscles, bones, organs and water. Our body composition—a specific breakdown of fat, lean muscle and bone—provides a better sense of our overall health than simply weighing ourselves or looking in the mirror. Very thin people, for example, may need to build muscle and stronger bones, while muscular people may still be carrying harmful fat around their abdomens.
Body composition tests often focus on body fat percentage, which tells you how much of your weight is comprised of fat and how much of it is lean tissue. Knowing this number can give you insight into your disease risk and overall health because the more body fat you have, the higher your risk is for a variety of diseases, including diabetes and heart disease.
Plus, when you know your body composition, you can set a realistic goal weight that fits your body type, and you can better tailor your workout plan to improve your overall wellness. For instance, if you’re thin with a high percentage of body fat (anything over 38 percent for women and 25 percent for men), you can start or increase your strength training to build lean muscle and bones. If you’re overweight, you can boost your resting metabolic rate—the amount of calories you burn simply maintaining normal bodily functions, like breathing and digestion—by building more muscle tissue. And if you’re already in good shape but are still carrying around some visceral fat around your waist, you can make the necessary diet or exercise tweaks to slim down.
Doctors use a test called Body Mass Index (BMI), which is a ratio of weight to height, to determine whether a person is underweight, overweight or obese. Take two people who weigh the same but are different heights. At 150 pounds, the person who is 5 foot 8 is considered normal weight, while the person who is 5 foot 2 would be classified as overweight. Although BMI correlates strongly with body fat percentage, it’s still an indirect measure of body composition because it doesn’t distinguish between body fat and lean tissue.
There are a number of more direct ways to measure your body composition:
A device called a caliper is used to pinch, or measure, a few layers of skin, as well as the fat right underneath it (called subcutaneous fat) at several different spots on the body. These numbers are then used to estimate the percentage of body fat. Although the accuracy of this test depends on the person performing it, it’s a fairly reliable measure, and it’s inexpensive and widely available; many gyms and fitness centers offer the service.
Bioelectrical Impedance Analysis (BIA)
A handheld gadget sends a low-level electrical signal through your arms and measures how quickly it takes to pass through. (There are also BIA scales you stand on like a bathroom scale, which send electrical signals through the legs.) The faster the signal passes through your body, the lower your body fat. “Muscle contains a lot of water, fat doesn’t,” explains Dawn McCrystal, M.S., exercise physiologist at Canyon Ranch in Lenox, Mass. “So if you have more muscle, the signal goes through a lot faster.” Keep in mind that the test is dependent on hydration: If you’re dehydrated or over-hydrated, the results may be skewed. And because the test measures body fat in only one area of the body (either the arms or the legs), it’s not as accurate as some other body composition tests.
This method involves sitting on a scale that gets lowered into a pool of water. After you exhale to empty your lungs and submerge yourself completely underwater, researchers measure how much water is displaced. Using a specific formula, they can determine your body fat composition. Unfortunately, it can be difficult to determine the volume of air that remains in your lungs after you breathe out, which can skew the results. Plus, there aren’t many places that offer the test.
Dual Energy X-ray Absorptiometry (DEXA) Scan
This highly accurate test, which is often used to measure bone density, is just like getting an X-ray, except a very low dose of radiation is used—must less than if you have your chest imaged, for example. This small amount of radiation passes through different types of tissues at different rates. By using this information, weight loss physicians are able to measure your body fat, as well as muscle mass and bone mass, which tells them how much lean tissue you have. Although the DEXA is expensive and not widely available, it’s by far the most accurate test for measuring body composition. It’s considered the “gold standard” among scientists.
Once you find out your body composition, you can work with your primary care physician, a nutritionist or a trainer to create an eating and exercise plan that will help you find the right balance of body fat and lean tissue for a healthier you.