Lumen measures the CO2 concentration in your breath to determine whether your body is currently burning primarily carbohydrates or fat for fuel. You breathe into it like a breathalyzer (inhale 10 seconds, hold 10 seconds, exhale 10 seconds), and the app gives you a score from 1-5 plus personalized nutrition recommendations.

Developed by twin Ironman triathletes with PhDs in Physiology, it's validated by peer-reviewed studies from San Francisco State University showing it correlates with gold-standard metabolic cart measurements.

The goal is 'metabolic flexibility'—training your body to efficiently switch between fuel sources for better energy, weight loss, and performance.

Requires daily use and commitment to macro tracking.

Users report significant weight loss when following recommendations consistently. 30-day money-back guarantee.