What a breath test can—and cannot—show
Indirect calorimetry estimates how much energy your body uses by measuring oxygen consumption and carbon dioxide production. A mask or canopy collects your breath while you rest; the instrument compares the gases going in and out. The ratio of carbon dioxide produced to oxygen consumed is called the respiratory exchange ratio, or RER.
When measured under steady conditions, RER can offer a clue about which fuel the body is using. Values nearer 0.7 are associated with greater fat use, while values nearer 1.0 are associated with greater carbohydrate use. In real life, the body generally burns a mix of fuels, not one fuel exclusively. A measured RER is an estimate for that moment, not a permanent label for a person’s metabolism.
Why the reading changes
Food, fasting, recent exercise, and even breathing pattern can affect RER. For that reason, clinicians standardize the test—often by measuring someone at rest after a period without food or exercise—if they want a useful estimate of resting energy expenditure or fuel use. A hard workout or a recent meal can shift the reading without indicating that anything is wrong with the person’s cells.
Indirect calorimetry does not directly inspect enzymes, measure stored body fat, or diagnose a loss of “metabolic flexibility.” That term refers broadly to the ability to adjust fuel use as circumstances change; a single resting measurement cannot establish whether someone can make that adjustment across a full day.
A plateau is not a breath-test diagnosis
Weight-loss plateaus can arise from several factors, including changes in energy needs as body weight falls and ordinary variation in intake, activity, and water balance. An RER reading alone cannot explain a plateau or show that the body is unable to access fat stores. Its practical value is narrower: under controlled conditions, it helps quantify energy use and gives a snapshot of fuel use—not a verdict on why weight loss has stalled.
