• What is assessed?
• How is the measurement conducted?
• When is this method used?
• How are estimates of body composition derived?
• Strength and limitations
• Populations
• Further considerations
• Resources required
• References

Fat-free mass (FFM) and fat mass (FM) normalized for 'size': 

1. Fat free mass index (FFMI) = FFM (kg) / height (m)²
2. Fat mass index (FMI) = FM (kg) / height (m)² 

FMI and FFMI require information on body weight, body height, and fat mass content, in contrast with body mass index (BMI) which takes into account only body weight and body height. Height is positively correlated with weight, although for calculation of BMI, FMI and FFMI, this confounding variable is removed. Therefore, the difference between BMI and FMI is directly influenced by fat mass content, which is important when studying obesity.

Fat mass index (FMI) is able to identify individuals with elevated BMI but without excess fat mass (FM). Conversely, fat mass index (FMI) can also identify individulas with 'normal' BMI, but who are at potential risk because of elevated fat mass (FM). These indices can distinguish tall individuals with chronic energy deficiency from shorter well-nourished individuals who can exhibit similar values for FFM and FM.

In children, when comparing body composition values, normalisation for body size is required. BMI and percentage body fat (%BF) fail to account for independent contributions of fat mass (FM) and lean body mass (LBM) to body weight, which vary according to age, sex, pubertal status, and population ancestry in the pediatric population. In addition, children who became obese in infancy tend to subsequentally become taller than average; this difference needs to be taken into account when evaluating their body composition. Low birth weight individuals tend to have subsequent abdominal obesity. Low birth weight also predisposes individuals to reduced FFM in childhood, therefore, the expression of childhood fatness as percentage fat is predicted to be misleading in such individuals when comparing them to their controls.

These estimates can be derived using any body composition methods that assess fat-free mass or fat mass and the measurement of height.

In clinical settings, field surveys, and large scale population studies for comparative purposes in the evaluation of the nutritional status and body composition of individuals with excess energy stores (such as obesity), or deficit of muscle mass (such as in wasting disease e.g. sarcopenia and chronic energy deficiency).

Typically used in epidemiological studies in combination with bioelectrical impedance analysis (BIA) instruments. Useful when investigators are interested in determining ranking of body composition traits rather than absolute values, which avoids the need for BIA population specific validation equations.

This approach involves adjusting both FFM and FM for height:

1. FFM index (FFMI) = FFM (kg) / height (m)²
2. FM index (FMI) = FM (kg) / height (m)²

These terms provide discrete indices of relative fat and lean masses, each expressed in the same kg/m² units as BMI.

Note that, mathematically, BMI (kg/m²) = FFMI (kg/m²) + FMI (kg/m²).

These indices are determined when comparing groups or individuals of similar height. If height differs significantly (e.g. different age groups of children), then expression of FFM data as FFMI is appropriate, but multiple regression analysis is to be used to adjust FM for height. Alternatively, if there is a need to express fatness in independent units, log-log regression analysis could be undertaken to determine the power P by which to raise height to calculate the index FM/height^p. This approach assumes there are sufficient individuals available to minimise the confidence limits for the resulting value of P (further information).

Strengths

1. Cut off points for FFMI and FMI corresponding to the classical BMI cut off values set out by WHO are available for certain populations.
2. FMI has shown superiority over % body fat as this variable is not corrected for height.
3. Because FMI takes height into account, it reduces the bias associated with % body fat.
4. Ability to identify sarcopenic obesity (low fat mass index associated with high fat mass index) typically found in older populations.
5. FFMI is useful for calculating the relative muscle hypertrophy in sports where heavy muscular body build needs to be measured quantitatively in order to exclude false diagnosis of excess body fat based on single BMI measurements. 

Limitations

1. Appropriate reference standards not available in large groups of individuals of different ethnic groups and in certain age categories (e.g. young children).

Considerations relating to the use of fat mass and fat-free mass indices in specific populations are described in Table 1.

Table 1 Application of fat mass and fat-free mass indices in different populations.

Refer to section: practical considerations for objective anthropometry

Resources are dependent on the instruments/methods used to derive the raw data of fat-free mass and fat mass.

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7. Wells, J. C. (2014). Toward body composition reference data for infants, children, and adolescents. Adv Nutr, 5(3), 320s-329s.
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9. World Health Organization Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies [published correction appears in Lancet. 2004;363:902]. Lancet. 2004;363: 157–163