Starting a New Diet? Get to Know the Fat-Free Mass Index

Why Fat and Muscle Mass Paint a Fuller Picture of Your Progress

By Michael Ormsbee, Ph.D., Florida State University

Fitness programs often include a weigh-in at regular intervals, but the number on the scale doesn’t tell the whole story. Discover why the fat-free mass index is a much more accurate way to track progress, as it looks at both fat and muscle mass. You’ll also learn the role that essential and storage fat play in your health.

Close up of hand dieting planning on table with healthy food
Knowing your body composition while starting a new fitness program, gives you a more accurate picture of your progress and overall health. Photo by Arturs Budkevics / Shutterstock

Understanding the Fat-Free Mass Index

Contrary to what television shows like The Biggest Loser would have you believe, weight loss is not always the best indicator of health. For instance, during a new fitness program, you might actually gain weight.

This weight gain might cause you to become discouraged, even though you’re, in fact, gaining muscle and losing fat. To assess your progress or overall health, you should also consider your body composition—not just your weight or Body Mass Index (BMI).

Body composition is the relationship between how much muscle and how much fat make up your weight.

To account for fat-free mass, or muscle mass, the fat-free mass index was developed. It is similar to BMI, but it takes your fat-free mass in kilograms, divided by your height in meters squared.

This index can quantify your body composition more accurately when you know that you have extra weight that is mostly from muscle. It’s a different index than BMI and may be more useful in certain cases.

“While the fat-free mass index is not used nearly as much as BMI, a few research papers have attempted to come up with normal fat-free mass index values for men and women,” said Dr. Michael Ormsbee, Associate Professor in the Department of Nutrition, Food, and Exercise Sciences at Florida State University.

A 2002 paper in International Journal of Obesity and Related Metabolic Disorders found the average fat-free mass index for young women is about 15 kg/m2, and for young men is 19 kg/m2.

But these values change slightly after age 35, showing that fat mass is gained as we age, most likely due to a sedentary lifestyle. The number of options that scientists have developed to better quantify amounts of fat and muscle indicates the importance of knowing this information.

Gender and Essential Fat

Why is fat mass index helpful?

  • It helps you know your health status and risk of disease, such as cardiovascular disease, pulmonary disease, and type 2 diabetes. If you have a lot of fat deposition, particularly around your waist and belly near your major organs, your chance of developing these types of diseases increases dramatically.
  • Knowing your current body composition gives you greater clarity about your starting point if you decide you need a change. You will be able to track your body’s changes over time and have a more accurate picture of success.

In terms of body composition, your total body fat includes essential and storage body fat. Essential fat includes fat in your organs, such as your heart, liver, kidneys, intestines, muscles, and bone marrow.

This type of fat is required for normal physiological functioning. The amount differs greatly between men and women. Women have around 12% essential fat as compared to men, who have about 3% essential fat.

Why do women have more essential fat? Well, mostly due to its biological function for reproduction. Women need extra fat stores during pregnancy.

Whether or not a woman has children, she will most often have slightly more fat than a man. For this reason, if you are a woman, it is important not to compare your body fat percentage to a man’s body fat percentage.

Purpose and Potential Problems of Storage Fat

Storage fat, on the other hand, can be used for energy. It includes the visceral fat, or deep fat, around your organs, and the fat just underneath your skin, which is called subcutaneous fat.

Storage fat insulates your body, helps to maintain body temperature, and provides vital protection for your organs. The amount of storage fat is similar between men and women, usually just 2%–3% percent higher in a woman.

Health issues arise when storage fat becomes excessive. Body fat is important for supporting daily function and health, but excess amounts will contribute to overweight and overfat conditions.

The other component of body mass that is not made up of fat is called fat-free mass or lean body mass. These terms are often used interchangeably, but they are technically different.

Fat-free mass only includes muscle mass, while lean body mass also includes bone mass, connective tissue like ligaments and tendons, internal organs, and any essential fat stored in these tissues.

In practice, body composition is often described as a two-compartment model, meaning fat mass and fat-free mass or lean body mass. If you know these values, you can avoid getting stuck in the rut of only measuring and talking about your weight.

Knowing your body composition will also give you much more information about your overall health.

This article was edited by Kate Findley, Writer for The Great Courses Daily.
Dr. Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University.

Dr. Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his M.S. in Exercise Physiology from South Dakota State University and his Ph.D. in Bioenergetics from East Carolina University.

About Kate Findley 73 Articles
Kate is a freelance writer, novelist, and blogger living in L.A. She has been writing for the Great Courses since 2017. It incorporates her two favorite things: writing and learning.