Many people ask me, “How is it possible to quantify fat metabolism by measuring exhaled breath? Are there fat molecules in your breath?” Well, fat metabolism can be quantified by measuring a chemical in your breath and that chemical isn’t fat.

When fat is metabolized, some of the fat is converted into ketone bodies. Ketone bodies are defined by three chemicals: acetoacetate, β-hydroxybutyrate (BOHB) and acetone. The more fat is burned, the greater the concentration of these ketone bodies. (As a scientific side note: You may have heard the term ketosis. Ketosis describes the presence and amount of ketone bodies in the blood. An increase in fat metabolism causes an increase in ketosis.)

Measuring the concentration of any of these ketone bodies can provide an indication of the level of fat burning. Acetoacetate and BOHB are typically measured in urine or blood, respectively, because the large size of these molecules limits their ability to volatilize (think: evaporate) into air and breath. Acetone is a much smaller molecule than the other two. Thus, it can readily volatilize, move into the air spaces of the lung, and end up in exhaled breath where it can be measured.

Why would someone choose to measure fat burning using exhaled breath (acetone) instead of urine (acetoacetate) or blood (BOHB)?

Breath is easy to sample and doesn’t require collection cups or a needle puncture for sampling. Additionally, breath doesn’t require the privacy of urine or sample preparation required for blood. Breath acetone analysis results in numerical values for chemical concentration similar to the analysis of blood BOHB.  Like blood analysis, breath gives an immediate assessment of fat burn. Results from a urine sample represent an average level of fat metabolism between urine samples. All three media (urine, blood, and breath) are affected by biologic factors such as dehydration, acid-base status, breathing pattern, etc. Due to our understanding of how acetone enters the lung from the surrounding body, the biologic factors affecting breath acetone measurement can be minimized.

Quantifying elevated fat burning (i.e., ketosis) via breath sampling is relatively easy, an indicator of your current state of ketosis, quantitative, and can be done in public.