Respiratory exchange ratio

The respiratory exchange ratio is how researchers see which fuel you are burning: the carbon dioxide you breathe out divided by the oxygen you take in, which sits near 1.0 on almost pure carbohydrate and near 0.7 on almost pure fat. Most lab measures of metabolic flexibility are built on it — but those measures disagree with one another, and some of their summary scores barely repeat from one day to the next.

You don't need this test. To track your metabolic health, use the standard blood markers — fasting insulin, HbA1c, fasting glucose and the triglyceride-to-HDL ratio. If you do get a fat-burning or breath test, read the number as rough, and only compare it with a repeat done the same way.

What the number means

The respiratory exchange ratio (RER) is measured by indirect calorimetry, which collects your breath and divides the carbon dioxide produced by the oxygen consumed. Clamp studies report a closely related reading, the respiratory quotient (RQ).

Metabolic flexibility — the ability to switch between fuels — shows up as the size of the swing. A flexible person moves widely between a low ratio when fasted and a high one after a meal; an inflexible person swings narrowly, stuck on glucose whatever they have eaten. A narrow swing travels with insulin resistance and type 2 diabetes, but no long-term study has yet shown that a measured flexibility score predicts diabetes, heart disease or fatty liver.^[1]

Four ways to read it, measuring four different things

Method	What it captures	Key limitation
Insulin clamp (fasting ratio vs. ratio under a steady insulin infusion)	The fasting-to-insulin fuel switch; the original gold standard	Invasive and not scalable; the swing largely tracks how much glucose the body takes up
Glucose drink or mixed meal (the swing after an oral glucose tolerance test or meal)	Switching after eating	The single reading repeats well day to day; the area-under-the-curve summary doesn't
Graded exercise test (peak fat burning, "FATmax", crossover point, lactate)	Fat-burning capacity during exercise	A different thing from resting flexibility; no agreed protocol yet
24-hour whole-room calorimetry	The real-world fuel rhythm across sleep, meals and exercise	Needs a specialised chamber, and the answer depends on the challenge imposed inside it

The clamp is the classic method, but its swing is largely a reflection of glucose uptake. In 59 people with type 2 diabetes and 42 controls matched for obesity, sex and race, the diabetic group's swing was smaller (0.06 against 0.10) — but the gap disappeared once their poorer glucose uptake was accounted for, and glucose uptake alone explained 46% of the variation.^[2]

The methods don't agree with one another

Two resting measures in the same people need not match. In eight healthy adults, flexibility measured overnight in a sealed chamber after a high-fat dinner repeated well across a week — yet showed no correlation at all with flexibility measured by the clamp.^[3]

Even inside the chamber, the result depends on what you ask of the body. A high-fat day shifted the day–night swing, while a bout of exercise with energy intake kept in balance moved no flexibility measure at all.^[4]

Some scores barely repeat

Measured twice, 48 hours apart, the summary score for the breath response to a glucose drink varied by about 22% from one day to the next, and the matching energy-expenditure score by about 56%. The authors concluded that studies "would need large sample sizes" to detect a change.^[5] The instantaneous ratio itself repeated well; it is the summary that wobbles.

Exercise fat-burning tests have their own problems

Exercise tests measure how much fat you can burn as effort rises, not how well you switch at rest. Even at easy effort the mix never becomes pure fat: in trained cyclists, fat supplied about 55% of energy at 40% of maximal workload and only 24% at 75%.^[6] The intensity of peak fat burning doesn't sit at a fixed lactate number either: in studies of people with obesity, measured blood lactate there spans roughly 1.3 to 2.7 mmol/L.^[7] See Zone 2 training for how this relates to training zones.

The calculation matters as well. The choice of equation used to turn breath gases into fat burning alone shifts the measured peak fat-burning rate by up to nearly 7%, though not the intensity at which the peak occurs.^[8] And much of the reported day-to-day variation may simply be the measurement error of the gas analysers themselves.^[9] A 2025 proposal to standardise the protocol calls for formal agreement on how to run the test.^[10]

What to do

Skip the breath test as a health check. You don't need a metabolic lab to track your own trajectory.
Track the blood markers instead. Fasting insulin, HbA1c, fasting glucose and the triglyceride-to-HDL ratio show whether insulin resistance — flexibility's closest clinical companion — is developing. Thresholds are on Metabolic flexibility.
Don't trust everyday signs as a substitute. Training without carbohydrate beforehand, skipping a meal without a crash, no post-lunch slump: none of these is validated against any lab measure.
If you do test, compare like with like. Same lab, same protocol, same preparation — and treat a small change as noise.