# Blood ketones and the breath reading (BHB)

The main blood ketone, a fat-derived fuel that rises in fasting and on very-low-carbohydrate diets; the breath meter gives a coarse estimate.

Type: biomarker · Unit: mmol/L; 1–5 · Domain: Metabolic Health · Accuracy: Measured · Importance: Context

Also called: beta-hydroxybutyrate, ketones, ketone meter, ketosis, breath acetone, Lumen, fat burning reading, Lumen score

## 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.

## Method

BHB: each reading in mmol/L with its method, sample type and timing; the day's value is the mean of its readings. Breath: the device's 1–5 scale stored as given; the wake-up breath is the day's value.

## Sources

1. Anderson JC. Measuring breath acetone for monitoring fat loss: review. Obesity 2015;23:2327–34. [doi:10.1002/oby.21242](https://doi.org/10.1002/oby.21242)
2. 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](https://doi.org/10.2196/25371)
3. Vitality Tracker, What VT measures (catalogue of 15 October 2026).

## Related

- [Use the fasting timer even if you don't fast](https://vitalitytracker.app/knowledge/use-the-fasting-timer)
- [Fasting glucose](https://vitalitytracker.app/knowledge/fasting-glucose)
- [Energy sources in a session](https://vitalitytracker.app/knowledge/energy-sources-in-a-session)
- [Why log meals even if you are not trying to lose weight](https://vitalitytracker.app/knowledge/why-log-meals)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/blood-ketones-and-breath-reading