Body Fat Mass (BFM)

• A calculated value, and a residual one: fat mass = measured weight − estimated fat free mass.
• Fat free mass is derived from total body water measured by impedance, assuming water makes up roughly 73 percent of fat free mass.
• An important property follows: fat is never measured directly. Anything that shifts the estimate of body water or the measured weight propagates straight into fat mass.
• Percent body fat is a step further downstream: fat mass ÷ weight × 100.
• The reference range in brackets is a comparison against a reference; the manual does not publish the formula for it.

• Always read fat mass together with percent body fat and weight. On its own it does not say whether the amount is high or low for your build.
• The manual gives a reference percent body fat of 15 percent for adult men and 23 percent for adult women, with a recommended range of 10 to 20 percent for men and 18 to 28 percent for women. It gives no reference value for fat mass in kilograms — the sheet instead shows an individual range in brackets.
• In the Muscle-Fat Analysis the sheet compares the length of three bars: weight, skeletal muscle mass and fat mass, with length relative to ideal weight. The manual does not give numeric boundaries for the Under, Normal and Over bands in this section.
• The manual also gives health context: it associates excess fat with a higher risk of hypertension, hyperlipidaemia, arteriosclerosis, fatty liver, diabetes and cancer, and insufficient fat with depression, feeling cold or infertility. The measurement itself establishes none of these states.

• A change in fat mass in kilograms is a more direct indicator during weight reduction than percent body fat. The percentage moves even when only muscle mass changed.
• When weight falls and fat mass falls by roughly the same amount, the loss is coming predominantly from fat. When weight falls markedly faster than fat mass, something else is going too — check muscle mass and body water.
• A difference of a few tenths of a kilogram between two measurements is not a reliable signal. Because fat is calculated as a residual, fluctuation in hydration propagates into it.
• A meaningful interval for tracking fat loss is four to eight weeks under comparable conditions, not a few days.

General principles: how to read change between measurements →

• Percent Body Fat – the share that complements the absolute value.
• Weight – without it a change in fat mass cannot be placed properly.
• Skeletal Muscle Mass – shows whether weight loss came from fat or from muscle as well.
• Fat Mass Index (FMI) – relates fat to height, so it does not swing with changes in muscle mass.
• Visceral Fat Area and segmental fat – add the distribution information that total fat mass does not carry.
• Weight, fat and muscle control – the recommendation derived precisely from fat and muscle mass.

• Fat mass is not percent body fat. The same percentage in two people of different weight means a different amount of fat in kilograms.
• Zero fat is neither a goal nor a possibility. The manual describes essential fat as part of organs, bone marrow and nerve tissue.
• The value does not say where the fat is stored. Subcutaneous and visceral fat have different context and are addressed by other indicators.
• Fat is not measured directly. It arises as a difference, so the uncertainty of the fat free mass estimate feeds into it.
• The manual does describe associations with disease, but those are population associations, not the result of your measurement.

• Hydration status. Because fat is calculated as a residual, every deviation in the estimate of body water shifts it.
• Food and drink shortly before the measurement. The manual recommends measuring fasted, because stomach contents count towards both weight and composition.
• A full bladder. The manual notes that water in the bladder is not counted by the measurement, which leads to overstated fat mass.
• Training, sauna and showering before the measurement.
• Time of day and the menstrual cycle.
• Clothing and objects on the body, because they enter the measured weight.

• It does not diagnose obesity or any disease.
• It cannot separate subcutaneous from visceral fat.
• It is calculated as a residual, so the uncertainty of the fat free mass estimate feeds into it.
• Absolute values are not interchangeable with DEXA results or with other devices.
• It cannot establish whether a change came from a real loss of fat or from a change in hydration.