GLOSSARY

Hydrometry uses the dilution of a tracer in a defined distribution space, commonly stable-isotope dilution for total body water. Appropriate mixing, sampling and corrections are needed before deriving further composition estimates.

Isotope dilution estimates a distribution space after an administered tracer mixes with body water, with appropriate sampling and corrections. Converting total body water to fat-free mass requires assumptions about hydration of the fat-free compartment. This differs from doubly labeled water, which estimates expenditure from isotope elimination over time. Confusing these methods can turn a water measurement into an unsupported calorie estimate. Testing conditions, tracer handling and the population being studied determine how the result should be interpreted.

Fat-free mass includes water, protein, minerals and other nonfat constituents. It therefore includes organs and bone as well as skeletal muscle. Lean body mass, fat-free mass and lean soft tissue can have different technical meanings, although terms are sometimes used loosely. A short-term rise in a device's lean-mass estimate can reflect hydration or glycogen rather than new contractile protein. Check the device definition and testing conditions before calling a change muscle growth.

For example, an isotope-based estimate of total body water may be used within a body-composition model. Its conversion to fat-free mass depends on a hydration assumption. Check that assumption and distinguish the procedure from an expenditure measurement before deciding what the result says about an athlete's energy needs or muscle mass.

Bioelectrical impedance methods measure the body's opposition to an applied electrical current and use models to estimate water and body-composition compartments. Hydration, recent food, exercise, skin temperature and electrode placement can change the result. Extra frequencies or additional displayed compartments do not eliminate modeling assumptions. Compare measurements under standardized conditions on the same validated system. A consumer scale's apparent daily change in muscle or body-fat percentage should not be read as a direct tissue measurement.