Lumen is an Israeli company that makes a handheld breath based metabolism tracker. Users take guided measurements and receive an estimate of whether the body is using carbohydrate or fat. The app connects these readings with nutrition, exercise, sleep and daily guidance.

How it works
A carbon dioxide sensor measures the concentration of carbon dioxide in an exhaled breath, while a flow meter checks breathing consistency. Pressure and temperature sensors aid calibration. Because carbohydrate use generally produces more carbon dioxide than fat use, software converts the reading into a fuel score and follows changes over time.

Key considerations
Lumen measures carbon dioxide but does not directly measure oxygen consumption like a full metabolic cart. Results depend on breathing, timing, food and exercise. A membership is required for many coaching features.