Body Composition: Fat-Free Mass and Fat Mass

In the fitness world, we use body composition to describe the proportions of fat, bone, water, connective tissue, and lean muscle in the human body. When referring to body composition, we are talking about the percentage of body mass that is both fat mass and fat-free mass (Lean Mass: L%, Fat Mass: F%). When we talk about measuring body fat percentage, we must understand, first, that all fat is not accumulated equally, stored equally, and nor is it measured equally because where it is located and how it’s distributed have huge impacts on our overall health.

Secondly, we must differentiate between the body fats. Subcutaneous fat is the most visible fat, it’s the (measurable) fat located just under the skin; this fat is found around the midsection, the hips, the arms, and the buttocks. Visceral fat is the deeper fat that wraps around internal organs … woven between muscles, and insulates organs. Visceral fat is also considered to be the more dangerous of the two body fats because as the amount increases, it replaces muscle tissue and suffocates the vital organs it’s designed to insulate.

Body Mass Index vs. Body Composition
BMI is NOT a measure of body fat or a measure of body composition; it is an assessment of body mass relative to height. Body composition does NOT rely on BMI but instead, body composition attempts to quantify body fat percentages though measuring subcutaneous and visceral fat of an individual on particular areas on the body.  BMI will not tell you how much of your body weight is fat or how much is muscle or other tissue. BMI is best used as a general assessment of a larger group. The greatest limitation of BMI is that it tends to classify an individual with greater muscle mass (lean or fat free mass) as overweight or obese, when they are in exceptional shape; whereas, an individual who is in poor shape can be classified as healthy when their fat mass is actually higher than their muscle mass.

Body composition, on the other hand, measures how much of your weight is fat-free and how much is lean/muscle mass, and how much is bone, water, and connective tissues. Essentially, body composition separates the subcutaneous fat tissue from the muscle mass (lean or fat-free mass) allowing for a much more accurate assessment of an individual’s status of health. The downside of the body composition is that it is not as simple as the BMI and is not as useful or practical for larger groups.

Measuring Fat Mass & Fat Free Mass
Body Mass Index (BMI) is a score that results from measuring a person’s mass (weight) and height. Body mass is divided by the square of height, commonly conveyed in units of kg/m2, resulting from mass in kilograms and height in meters. Typically, one’s BMI is determined by using a chart or a table, ultimately identifying levels of fatness and then categorizing people into ranges labeled:  “underweight”, “healthy”, “overweight”, and “obese”.  Your BMI number will tell you a great deal about your risk of heart disease and diabetes. As your BMI increases, your risk for developing weight related diseases also increase.

Body fat can dramatically fluctuate with age in children and teens and during the adolescent years, so the amount of body fat varies between boys and girls. When measuring mass in children and teens, age and sex specific BMI tests are utilized. In adults, BMI is moderately correlated with more direct measures of body fat; while, in children and teens, BMI “can be considered an alternative to direct measures of body fat.”

When in Doubt …
There are some relatively cheap and handy tools for assessing body fat. Clients can get an idea of their baseline measurements for better understanding of their body mass index with regards to body composition. The quality and accuracy of the methods of measuring body fat percentage is something to consider. The simplest and easiest are by scale, handheld device, or skinfold testing and include some margin of error. Underwater weighing or air displacement, are the most accurate but can be complicated, expensive, and time consuming.

It’s always best to stay consistent. When used together, BMI and body composition can be very effective tools for controlling weight and body fat. Have your body fat assessed in consistent increments of time, i.e. bi-monthly. Have your body composition assessed with the same tool, by the same person, and at the same time of day.