Body mass is one of the two variables that determines a person's body weight: the other being Earth's gravity, which pulls the body down with a constant force. Unlike gravity, however, your body mass can fluctuate in response to stimuli such as diet, exercise and disease. And while losing body mass from fat can be beneficial to health, losing lean body mass can detrimental. As the University of Dayton notes, scientists define lean body mass as any non-fat substance in the body that contributes to overall mass.
Examples
Examples of substances that comprise lean body mass include muscles, bones, internal organs, like the heart, liver, lungs, kidneys and external organs, like the skin. Smaller bodily components, such as antibodies, blood cells and enzymes, also contribute to lean body mass.
Function
Each muscle, bone, organ or other substance that contributes to lean body mass has its own specific function within the body. However, as a whole, lean body mass is responsible for providing the body with its "essential elements" for proper functioning, as the University of Texas Southwestern Medical Center states. The functions that these "elements" provide include movement, stability, respiration, digestion and immune system responses. However, while lean body mass is essential, body fat is still important. Unlike lean body mass, the primary function of body fat is to help insulate and cushion the body's joints and organs.
Importance
The body favors lean body mass over body fat. This becomes evident when a healthy person starts burning more calories than he consumes, forcing the body to convert stored body mass into energy. Instead of "attacking" lean body mass, the body primarily targets the layers of stored fat, known as adipose tissue, which accumulate below the skin. However, if there is not enough fat to burn, the body can break down muscle and could potentially turn to other sources of lean body mass.
Benefits
As the University of Dayton notes, increasing lean body mass -- such as through building muscle -- and decreasing body fat can help lower your risk for high blood pressure and cardiovascular disease. By maintaining lean body mass, you can help avoid muscle fatigue and weakness during physical activities.
Misconceptions
People often focus on body weight when they are dieting or otherwise trying to get in shape; and use the digits they see on the scale as indicators of health. However, as the University of Dayton points out, examining body composition, not weight, is a superior method for determining health. This is because body weight does not distinguish between lean body mass, which is healthy and essential for the body's functioning, and body fat, which can have negative impacts on health. In comparison, body composition refers to the breakdown of weight that comes from lean body mass and the weight that comes from fat, providing a more accurate indicator of health.
Calculation
According to the University of Dayton, you can calculate lean body mass using a simple equation: weight minus fat equals lean body mass. However, determining what percentage of your body is fat -- the second variable in the equation -- requires a separate calculation. An alternative strategy is to use an online lean body mass calculator.
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