FFMI The Fat-Free Mass Index

Authors: Eva Heinen, graduate sports scientist (Univ), Prof. Dr. med. Edgar Heinen

Fat-free mass (FFM) is another important parameter when measuring body composition. While fat mass (FM) clearly only refers to the mass of fat, not the mass of adipose tissue, the FFM parameter is much more complex. On the Internet, FFM is often equated with muscle mass, which is logically nonsense. We are not just made up of fat and muscle! We also consist of skin (the fat of the skin is included in the fat mass), bones (fat marrow in the fat mass), internal organs including the vascular system with its contents (blood) and muscles. The fat that is in the muscles is also included in the fat mass.

However, adipose tissue also contributes to FFM. However, the data on the FFM of adipose tissue vary greatly in the literature. For example, a water content of the subcutaneous adipose tissue of around 15 % was measured using magnetic resonance imaging, whereas Webster JD, Hesp R, Garrow JS state that the fat-free mass of the adipose tissue is between 22 and 30 %. Overall, it seems reasonable to assume that adipose tissue consists on average of around 22% fat-free mass. However, if 1 kg of adipose tissue contains only 0.780 g of fat, then the fat-free mass of 1 kg of fat is 0.282 g, which must be taken into account in the corresponding calculations.

If we ignore the fact that adipose tissue always contains fat-free mass, misinterpretations are inevitable, especially when looking at weight progression – e.g. when losing weight. You can find an example of this at the end of the body composition section.

The fat-free mass (FFM) is calculated from: body weight – fat mass. If dual-photon absorptiometry is used, the FFM = BMC (bone mineral content) + LBM (mass of fat-free body soft tissue) is calculated.

Fat-free mass (FFM) shows a highly significant (p<0.001) correlation with height in both women and men (see Fig. 1)

The calculation of the corresponding index is also useful here to enable a size-independent view:

FFMI (Fat Free Mass Index) = FFM / height² (m²).

There is no longer a significant correlation between this parameter and body size. When using the DEXA method, there is therefore no reason to make a body size-dependent correction to the value determined, as required by Kouri et al. 1995 was proposed. The authors used skinfold thickness to measure body fat and examined athletes, more than half of whom were abusing anabolic steroids!

The following mean values ± standard deviations for the FFMI were determined for the collective studied:

Women: 16.4 ± 2.5 kg/m²
Men: 20.9 ± 2.5 kg/m².

As shown above, the FFM and thus the index of the FFM includes the fat-free mass of the adipose tissue. If the fat-free mass of the adipose tissue (= FM + 0.282) is calculated for each person examined, an index for the fat-tissue-freemass can be calculated. The following mean values and standard deviations were found for this parameter:
Women: 12.6 ± 1.9 kg/m²
Men: 18.2 ± 2.2 kg/m².
The differences between men and women in this parameter are even greater, as women have more adipose tissue and therefore more fat-free mass in their adipose tissue.

After eliminating the problem of fat-free mass in adipose tissue, we now approach the second parameter of particular interest alongside fat mass: muscle mass: Muscle mass is highly variable and of particular interest during training and weight loss.