The Definition of Lean Body Mass

Carrying too much body fat increases your risk of disease, particularly heart disease and type 2 diabetes. Body fat scales, caliper tests and medical scans tell you the percentage of fat you have on your body. Fat mass consists of essential fat -- the stuff that makes up the structure of your internal organs, bone marrow and central nervous system -- as well as the storage fat that regulates your body temperature and swells your waistband and hips. The rest of your body weight comes from lean mass, which includes your muscles, bones and connective tissues.

Lean body mass consists of your bones, ligaments, tendons, internal organs and muscles. Because of necessary fat within the bone marrow and internal organs, lean mass does include a small amount of essential fat. Body composition analysis adjust for these tiny amounts of essential fat, so you can get an accurate measure of your fat-free mass.

Having a high percentage of lean mass boosts your metabolism so it's easier to maintain an overall healthy weight. Your internal organs, for example, have a metabolic rate that is 50 to 100 times higher than their equivalent weight of fat. Muscle's metabolic rate isn't as high as that of the organs, but it's still significantly higher than that of fat tissue.

A high proportion of lean mass also reduces inflammation. In a 2006 paper published in the American Journal of Clinical Nutrition, Tufts University researchers explain that small fat cells in lean individuals promote healthy function, while the enlarged fat cells in people who have overweight or obesity promote inflammation and chronic disease.

Body composition is usually presented as a body fat percentage, not a lean mass percentage. A healthy body fat percentage is between 15 and 20 percent for men and 20 and 25 percent for women. The remaining percentage is made up of lean mass, meaning that a healthy lean mass percentage for men is 80 to 85 percent for men and 75 and 80 percent for women.

Athletes and fitness enthusiasts carry a higher percentage of lean mass due to the demands of their sport and exercise regimens. These individuals should never achieve 100 percent lean mass, however. They must carry some essential fat in the internal organs and central nervous system, which averages about 3 percent for men and 13 percent for women. Women carry more essential fat due to the needs of supporting a fetus during pregnancy and breastfeeding a new baby. You should never dip below essential fat levels or risk health complications.

You can change the amount of lean mass on your frame by building muscle and losing fat. It takes time and effort to build the tissue, but you end up looking healthier, feeling stronger and having more stamina. Combine a carefully planned program that pairs a calorie surplus of 250 to 500 calories daily with directed weight training. The most muscle you can expect to gain in a week is about 1/2 pound, at least when you're starting out. Over the course of a year of focused muscle-building work, you can gain an average 0.4 pound of muscle gain per week, since muscle growth slows down as you get more fit. Once you've developed the lean mass you want, you can shed fat by cutting your calorie intake, so you're eating 500 to 1,000 calories less than you burn a day.

Living a sedentary lifestyle or losing weight too quickly, though, will make you lose lean mass. For every pound you lose while sedentary, about 25 percent will be lean muscle mass. Eating fewer than 1,200 calories per day also encourages the loss of muscle mass. On the other hand, if you're sedentary and eating more calories than you need, you'll gain fat.

As you age, you naturally lose lean body mass. Muscle loss, known as sarcopenia, is most prevalent after age 50 and occurs at a rate of about 1 to 2 percent per year, explains a 2003 review in the Journal of Gerontology. Bone density also diminishes slightly as you age, further reducing the weight of your lean body mass. On average, you can expect to gain about 1 pound of fat per year and lose 1/2 pound of muscle from age 30 to 60. The net result is about 15 pounds of muscle lost and 30 pounds of fat gained. Your net weight may not change, but the composition shifts.

Losing muscle mass has consequences beyond changing your size and shape. You lose strength, cardiovascular capacity and calorie-burning potential. Weight training offsets the loss of muscle and fitness as you age, but it can't prevent it altogether. Nutrition, especially adequate protein intake, also helps ameliorate sarcopenia. Go for lean, complete options including poultry breast, lean steak or ground beef, fish and eggs.