Article # 363. What Is Lean Body Tissue?

Lean body tissue influences metabolic function, disease risk and appearance. Therefore, you should understand how to manage and assess your own amount of lean tissue for positive health outcomes. On a basic level, lean tissue comprises all body weight other than fat. One lb. of lean mass is metabolically superior to 1 lb. of body fat. Understanding lean mass enables you to make informed fitness decisions.

Two-Factor Theory of Body Composition

The two-factor theory of body composition categorizes body weight as either fat mass or fat-free mass, also known as lean mass. Lean mass comprises muscles, bones, organs and connective tissues. According to the book "Advanced Fitness Assessment and Exercise Prescription" by Vivian H. Heyward, people carry essential fat and non-essential fat. For example, women should maintain a minimum of 10 percent body fat while men require a minimum of 5 percent body fat for normal physiological function. Dipping below required levels may result in impaired metabolism, chronic fatigue and reduced immune-system function.

Healthy Levels

Body composition is defined as body fat percentage, or the percentage of your total weight made up of fat. For example, a person with 10 percent body fat contains 90 percent lean mass. According to Heyward, healthy body fat percentages for 18- to 34-year-old women fall between 20 and 30 percent and increase to 25 to 33 percent for 35- to 55-year-old women. Healthy fat levels for 18- to 34-year-old men range between 5 and 15 percent and increase to 7 to 18 percent for men between the ages of 35 and 55.

Assessing Lean Mass

Although fat content cannot be measured directly, various assessment protocols can estimate your percent of lean tissue. Clinical methods include hydrostatic weighing or dual-energy x-ray absorptiometry, also known as DXA. Hydrostatic weighing measures your weight underwater while DXA performs a full-body scan. Less-clinical methods include skinfold measurement with calipers or handheld bioelectrical impedance analyzers. All methods collect data, which is then plugged into an estimation equation along with age, height and weight measurements. Usually, local gyms or universities administer fitness tests to the public for a small fee or free of cost to the members.

Manage Body Composition

You can improve body composition through reduction of fat mass or accumulation of lean mass. For example, resistance training increases both muscle mass and bone density and contributes to lean mass. On the other hand, dietary restriction and regular exercise can reduce fat mass and subsequent body fat percentages. Whatever your strategy is, maintaining a healthy body composition provides many health benefits. As always, consult a doctor before starting a diet or exercise program.