Fat-free mass (FFM) is everything in the body except fat: muscle, bone, water and internal organs. It is the "working" part of the body and the second half of body composition.
About half of fat-free mass is skeletal muscle. That is precisely why preserving FFM with age is the key to strength, metabolism and longevity, and its loss leads to sarcopenia.
Reference ranges
FFM is assessed through an index (FFMI = FFM / height²); guides:
| Share of skeletal muscle in FFM | ≈ 50% |
| FFMI men | ≈ 18 – 22 kg/m² |
| FFMI women | ≈ 15 – 18 kg/m² |
| Low muscle mass | FFMI < 18 (men) / < 15 (women) |
What an elevated level means
High fat-free mass (a lot of muscle) is, as a rule, a favorable sign: greater strength, better metabolism and insulin sensitivity, stronger bones. It is precisely a rise in FFM that is sought with strength training.
Very high FFMI values occur in athletes; they are not harmful in themselves if achieved through natural training.
What a low level means
Low fat-free mass is an unfavorable sign. A deficit of muscle reduces strength, slows metabolism and raises the risk of falls, fractures and disease; in old age this is sarcopenia.
Especially insidious is the combination of low muscle and high fat mass — sarcopenic obesity: the weight is normal, but the body composition is unhealthy.
How the test is done
Fat-free mass is assessed by bioimpedance or more accurately — DEXA; from it FFMI is calculated (FFM divided by height squared). Home scales give a rough guide and the trend.
FFM is looked at together with fat mass and the muscle index: the goal of healthy aging is to preserve fat-free mass while losing fat.
What affects the value
- Strength training and protein — build fat-free mass.
- Age — after 30–40 muscle is gradually lost.
- Protein and calorie deficit — reduces FFM.
- Hormones (testosterone, growth hormone) — affect muscle.
- Hydration — affects the bioimpedance estimate (water is part of FFM).
- Bed rest and immobility — quickly reduce muscle.
Assess body composition
Assess your fat-free and muscle mass within your body composition:
Frequently asked questions
What is included in fat-free mass?
Everything except fat: skeletal muscle, bone, water and internal organs. About half of fat-free mass is skeletal muscle, so it is often used as a guide to muscle health.
Why is fat-free mass normalized to height?
Because in a tall person the absolute mass is greater simply because of size. The FFMI index (mass divided by height squared) allows people of different heights to be compared and whether a person has enough muscle to be assessed.
Why does preserving FFM matter with age?
Fat-free (mainly muscle) mass determines strength, metabolism and resistance to disease. With age it naturally declines; its loss is sarcopenia, leading to weakness, falls and worsening metabolism. Strength exercise and protein help preserve it.
Related terms
Sources
- Schutz Y. et al. Fat-free mass index (review). PubMed
- Fat-Free Mass and metabolic health (review). NCBI/PMC
- MedlinePlus (NIH) — Body Composition