Fat-Free Mass Index (FFMI) Calculator

Key Reference

Mittal, R. , Goyal, M. , Dasude, R. , Quazi, S. and Basak, A. (2011) Measuring obesity: results are poles apart obtained by BMI and bio-electrical impedance analysis. Journal of Biomedical Science and Engineering, 4, 677-683. doi: 10.4236/jbise.2011.411084.

Key Reference

Schutz, Y., Kyle, U. & Pichard, C.. Fat-free mass index and fat mass index percentiles in Caucasians aged 18-98 y. Int J Obes Relat Metab Disord. 2002; 26(7):953-60. https://doi.org/10.1038/sj.ijo.0802037

• Objective: To determine reference values for fat-free mass index (FFMI) and fat mass index (FMI) in a large Caucasian group of apparently healthy subjects, as a function of age and gender and to develop percentile distribution for these two parameters.
  
• Results: The median FFMI (18-34 y) were 18.9 kg/m2 in young males and 15.4 kg/m2 in young females. No difference with age in males and a modest increase in females were observed. The median FMI was 4.0 kg/m2 in males and 5.5 kg/m2 in females. From young to elderly age categories, FMI progressively rose by an average of 55% in males and 62% in females, compared to an increase in body mass index (BMI) of 9 and 19% respectively.
  
• Conclusions: Reference intervals for FFMI and FMI could be of practical value for the clinical evaluation of a deficit in fat-free mass with or without excess fat mass (sarcopenic obesity) for a given age category, complementing the classical concept of body mass index (BMI) in a more qualitative manner. In contrast to BMI, similar reference ranges seems to be utilizable for FFMI with advancing age, in particular in men.
  
• The major shortcoming of the BMI is that the actual composition of body weight is not taken into account: excess body weight may be made up of adipose tissue or conversely muscle hypertrophy, both of which will be judged as ‘excess mass’. On the other hand, a deficit of BMI may be due to a fat-free mass (FFM) deficit (sarcopenia) or a mobilization of adipose tissue or both combined.
  
• The partitioning of BMI into FFMI and FMI is obviously not possible without associated measurements of body composition. Note that the original idea of calculating the FFM and fat mass (FM) indexes, in analogy to the BMI, was proposed several years ago. The potential advantage is that only one component of body weight, ie FFM or FM, is related to the height squared. Surprisingly, these indexes have not found a wide application yet, probably because appropriate reference standards have yet to be defined. By determining these indexes, quantification of the amount of excess (or deficit) of FFM, respectively FM, can be calculated for each individual.
  
  
• Calculation of FFM and FM indexes
  The FFM and FM indexes are equivalent concepts to the BMI,
  as shown in the following definition:
  
  FFMI  (Fat-free mass index) = fat-free mass [kg]/height[m]²
  FMI (Fat mass index) = fat mass [kg] / height[m]²
• BMI (kg/m²)=  FFMI (kg/m²) + FMI (kg/m²)
  
• Expression of FFM:
  An issue which has plagued nutritionists and body composition specialists is the expression of body composition results when inter-individual comparison are made: comparison in absolute value (kg) vs in relative value (ie percentage of body weight) or normalized value for ‘size’ (ie typically height squared in FFMI concept or occasionally adjustment for body surface area). Since FFM is related to height, it seems inappropriate to give, for any individual, a cut-off point of FFM in absolute value (kg) below which FFM is judged as ‘low’. For example, a short individual would be penalized since his absolute FFM is expected to be lower than that of a tall individual. Indeed a healthy and well-nourished young man would have a FFM expressed in absolute terms virtually the same as that of a similarly aged but taller individual suffering from proteinenergy malnutrition.
• Age related issues:
  When the BMI increases with age, it is expected that an increase fat storage would specifically affect FMI and very little FFMI. The impact of the weight gain in the percentile distribution cannot be assessed without carrying out a prospective study. The fact that we used a standardized BMI range at all ages is evidence that the rise in BMI with age is not characteristic of all populations and is not something desirable.
  

Key Reference

Kyle UG, Schutz Y, Dupertuis YM, Pichard C. Body composition interpretation. Contributions of the fat-free mass index and the body fat mass index. Nutrition. 2003 Jul-Aug;19(7-8):597-604. doi: 10.1016/s0899-9007(03)00061-3. PMID: 12831945.

• Effect of Aging on FFMI and BFMI
  Mean FFMI was higher in men and women older than 60 y than in those 18 to 39 y (Table I). The relation between FFMI and age was curvilinear, with the highest predicted values in the age category of 30 to 59 y and lower values noted in younger and older subjects (Fig. 1). Forbes proposed that a 2.3 kg/decade weight gain (or 0.9 kg/m2 BMI increase/decade) is required to counteract the loss of FFM with aging. Our study suggested that mean BMI increases of 1.9 kg/m2 in men and 4.0 kg/m2 in women older than 60 y is sufficient to maintain FFMI at or above the levels of those 18 to 39 y. Thus, small BMI increases would prevent FFMI from decreasing in older subjects. The differences noted between the study by Forbes and the current study are likely due to methodologic differences (n = 75 subjects evaluated longitudinally by Forbes versus n = 5629 subjects evaluated cross-sectionally in our study).
• Interpretation of FFMI and BFMI
  The purpose of using FFMI and BFMI is to facilitate interpretation of body composition parameters regardless of height. FFMI and BFMI values corresponding to low, normal, high, and very high BMI categories allow the classification of subjects in low, normal, high, and very high FFMI and FMI categories.
• OBJECTIVE: Low and high body mass index (BMI) values have been shown to increase health risks and mortality and result in variations in fat-free mass (FFM) and body fat mass (BF). Currently, there are no published ranges for a fat-free mass index (FFMI; kg/m2), a body fat mass index (BFMI; kg/m2), and percentage of body fat (%BF). The purpose of this population study was to determine predicted FFMI and BFMI values in subjects with low, normal, overweight, and obese BMI.
• METHODS: FFM and BF were determined in 2986 healthy white men and 2649 white women, age 15 to 98 y, by a previously validated 50-kHz bioelectrical impedance analysis equation. FFMI, BFMI, and %BF were calculated.
• RESULTS: FFMI values were 16.7 to 19.8 kg/m2 for men and 14.6 to 16.8 kg/m2 for women within the normal BMI ranges. BFMI values were 1.8 to 5.2 kg/m2 for men and 3.9 to 8.2 kg/m2 for women within the normal BMI ranges. BFMI values were 8.3 and 11.8 kg/m2 in men and women, respectively, for obese BMI (>30 kg/m2). Normal ranges for %BF were 13.4 to 21.7 and 24.6 to 33.2 for men and women, respectively.
• CONCLUSION BMI alone cannot provide information about the respective contributions of FFM and FM to body weight. This study presented the FFMI, BFMI, and %BF values that correspond to low, normal, overweight, and obese BMIs. FFMI and BFMI can provide meaningful information about body compartments, regardless of height.