Soft Lean Mass (SLM)

• A calculated value under the relationship SLM = fat free mass − bone mineral content.
• Fat free mass itself arises as the sum of body water, protein and minerals derived from impedance. Bone minerals are an estimate calibrated against DEXA.
• It follows that the uncertainty of two different steps feeds into soft lean mass: the estimate of body water and the estimate of bone minerals.
• Because it contains all of the body water, it is one of the values on the sheet most sensitive to hydration.
• The reference range in brackets is a comparison against a reference; the manual does not publish the formula for it.

• Read the value against the reference range in brackets. Absolute kilograms cannot be compared between people.
• The difference between fat free mass and soft lean mass equals the bone mineral estimate. In an adult that is a relatively small part of the whole.
• If muscle is what interests you, read skeletal muscle mass. Soft lean mass is a broader category and the manual separates the two.
• A high value alongside a raised ECW/TBW ratio may be shifted by retained fluid, because all of the body water is part of it.

• It responds fast. Because it contains all of the body water, every change in hydration propagates into it in full.
• A rise over months alongside a rise in muscle mass, protein and intracellular water corresponds to a real gain in tissue.
• An abrupt change between two visits is almost always explainable by measurement conditions rather than by tissue gained or lost.
• During weight reduction, watch whether soft lean mass falls only slightly. A marked fall together with muscle mass means the loss is coming from active tissue too.

General principles: how to read change between measurements →

• Fat free mass – the parent quantity soft lean mass is produced from.
• Bone mineral content – the difference between the two quantities.
• Skeletal Muscle Mass – the narrower indicator covering muscle alone; the manual separates the two.
• Total body water and the ECW/TBW ratio – show whether the value has been shifted by retained fluid.
• Protein – the other main component of soft lean tissue alongside water.

• The name invites reading the value as muscle. It is not — it includes all of the body water, organs and other soft non-fat tissue.
• Soft lean mass is not skeletal muscle mass. The manual says so explicitly and gives no relationship between the two.
• A higher value does not automatically mean more muscle. It may mean more water.
• The gap against fat free mass is not a measurement error. It is precisely the bone mineral estimate.

• Hydration status, because the value contains all of the body water.
• Carbohydrate and salt intake over the preceding days, which affect the amount of retained water.
• Training, sauna and showering shortly before the measurement.
• Food and drink before the measurement; the manual recommends measuring fasted.
• Time of day, body position before the measurement and the menstrual cycle.

• If you are following muscle, use skeletal muscle mass. Soft lean mass is not built for that.
• Use it as supplementary information about the volume of soft non-fat tissue and always read it alongside body water.
• Measure under the same conditions. Of all values on the sheet it is among the most sensitive to hydration.
• When the value changes unexpectedly, check the ECW/TBW ratio and total body water first.

• It is not identical to skeletal muscle mass; the manual gives no relationship between the two.
• It contains all of the body water, so it reacts strongly to hydration.
• It does not measure muscle strength or function.
• It does not diagnose any medical condition.
• It shares the uncertainty of both the body water estimate and the bone mineral estimate.