In one sentence
Two ways of asking what your body is burning: a blood meter that measures beta-hydroxybutyrate, the main ketone, and a breath device that estimates from your exhaled air whether you are running mainly on carbohydrate or on fat.
What ketones are
When carbohydrate is scarce, in a long fast or on a very-low-carbohydrate diet, the liver turns fat into ketones, which the brain and muscles can burn in glucose's place. Beta-hydroxybutyrate is the one a blood meter measures, with a finger-prick like a glucose meter's. The level rises with fasting length and carbohydrate restriction and falls within hours of a carbohydrate meal; the ranges people quote for "nutritional ketosis" are practitioners' conventions, not consensus figures, and VT's chart shades 0.5 to 3.0 mmol/L, where mild fasting or low-carbohydrate ketosis usually sits, as a guide, not a target. Very high ketones with high glucose are a different matter entirely, a medical emergency for people with type 1 diabetes, and the meter's manual and your doctor, not this page, say what to do.
The breath reading
Acetone, a ketone the body cannot use, leaves in the breath, and its concentration tracks the rate of fat loss closely enough that a review of the field concluded breath acetone is a usable non-invasive measure of it when breathing and diet are controlled, with values from about 1 part per million in a non-dieting person to a thousand times that in diabetic ketoacidosis (Anderson 2015). Consumer breath devices read a different signal: the Lumen measures the carbon dioxide in a held breath as a stand-in for the respiratory exchange ratio, the laboratory measure of fuel mix. In 33 healthy young adults the Lumen's reading and a metabolic cart's ratio both rose after 150 grams of glucose, and the two agreed statistically (Lorenz 2021). VT stores the device's 1-to-5 reading as a coarse estimate of carbohydrate-versus-fat burning, not as a ketone level.
What moves it
Blood ketones: fasting length, carbohydrate intake, exercise, and, upward, the first weeks of a ketogenic diet before the body adapts. The breath reading: the previous meal's carbohydrate, the fast since it, exercise, and how the breath was taken, which the device's own instructions govern.
How to measure it well
Ketones: the same meter, the same time of day, usually before breakfast; the strips are expensive, so measure when you are testing something. Breath: follow the device's breath protocol exactly, and take it at the same time each day.
How Vitality Tracker uses it
Both are stored by day and have their own charts in the app's glucose section; neither feeds a score or an engine. They are here because members who fast or eat low-carbohydrate asked for them, and because a ketone series beside a glucose series makes a fast legible.
How far to trust the number
Blood ketones: Measured, by a meter accurate to a few tenths of a mmol/L. The breath reading: Estimate, of a fuel mix rather than of a ketone, validated at rest in young adults and sensitive to how you breathe.
Sources
- Anderson JC. Measuring breath acetone for monitoring fat loss: review. Obesity 2015;23:2327–34. doi:10.1002/oby.21242
- Lorenz KA, et al. A handheld metabolic device (Lumen) to measure fuel utilization in healthy young adults: device validation study. Interact J Med Res 2021;10:e25371. doi:10.2196/25371
- Vitality Tracker, What VT measures (catalogue of 15 October 2026).