One of the most valuable tests I run in my practice is something most people have never heard of: a resting respiratory quotient measurement. It tells me, with precision, whether your body is burning fat or carbohydrates as its primary fuel source at rest. And in a surprising number of patients, the answer is carbohydrates, even in people who exercise regularly, eat well, and have no obvious metabolic disease.
Here is what that means, why it happens, and what we can do about it.
What I Am Actually Measuring
When I perform a resting metabolic assessment using the PNOĒ analyzer, I measure two gases: the oxygen you breathe in and the carbon dioxide you breathe out. The ratio between them is called the Respiratory Quotient, or RQ.
The math is straightforward. Fat requires a lot of oxygen to combust, so burning fat produces a relatively low amount of CO2 per unit of oxygen consumed. The RQ for pure fat oxidation is approximately 0.70. Carbohydrates, on the other hand, burn more cleanly and produce CO2 and oxygen in nearly equal amounts, giving an RQ close to 1.00.
A metabolically healthy person at rest typically falls between 0.70 and 0.80, meaning their body is drawing predominantly from fat stores as a background fuel. When I see an RQ above 0.90 in a fasted patient, that tells me fat oxidation is impaired. The body is defaulting to glucose even when it should have no reason to.
Why This Matters Beyond the Number
Fat oxidation at rest is not just a curiosity. It is a window into how well your metabolism is functioning at a fundamental level.
Your body stores roughly 2,000 calories of carbohydrate as glycogen. It stores tens of thousands of calories as fat. A metabolism that cannot access fat efficiently is running on a small, volatile fuel tank. These are the patients who feel energy crashes between meals, who struggle to lose body fat despite clean eating, who hit walls in their endurance training, and who wake up tired no matter how much sleep they get. The symptoms are non-specific, which is exactly why so many of these patients spend years being told their bloodwork is normal and there is nothing wrong.
There is something wrong. It just requires the right test to find it.
What Drives a High Resting RQ
In my clinical experience, low fat oxidation at rest is almost never caused by one thing. It is the result of several systems converging on the same outcome. The most common drivers I see are:
Insulin resistance or hyperinsulinemia. Insulin is the primary fuel-switching hormone, and even mildly elevated fasting insulin suppresses fat mobilization and directs the body toward glucose. This is the single most common driver I find, and it is frequently present in lean, active patients with no diabetes diagnosis.
HPA axis dysregulation. Chronic stress keeps cortisol elevated, which drives blood sugar up through gluconeogenesis and keeps insulin tonically elevated in response. The body interprets chronic stress as a reason to hold onto glucose as a fuel source.
Low free T3. Thyroid hormones regulate the speed of mitochondrial function. Low T3, even with a normal TSH, slows the entire fat-burning machinery. I find this in a significant number of patients who have been told their thyroid is fine based on TSH alone.
Low mitochondrial density. Mitochondria are where fat is actually combusted. They multiply in response to sustained low-intensity aerobic training and atrophy without it. Patients who train hard but exclusively at high intensity often have surprisingly poor fat oxidation because they never built the aerobic base.
Nutrient depletion. Carnitine, CoQ10, magnesium, B vitamins, and iron are all required for fat to move into and through the mitochondria. Deficiencies in any of these act as a bottleneck in the fat-burning pathway.
Poor sleep and circadian misalignment. The body’s primary fat-burning window is overnight during deep sleep. Late eating, low deep sleep, and irregular sleep timing all compress or eliminate this window.
What I Do With This Information
A high resting RQ gives me a concrete starting point. From there, I order a targeted investigation: fasting insulin and glucose, a full thyroid panel including free T3, morning cortisol, sex hormones, inflammatory markers, and ferritin. In patients where I suspect adrenal or hormone involvement, I add a DUTCH complete hormone test for a detailed picture of the full cortisol curve and hormone metabolism.
I also review body composition with InBody, sleep quality data, and a food log focused on carbohydrate load and meal timing. Within a few weeks, the pattern becomes clear and the treatment plan writes itself.
Most patients who address their root causes see meaningful improvement in their RQ within 8 to 12 weeks. Fat oxidation is trainable. The metabolism is responsive. It just needs the right inputs.
If you have been struggling with energy, body composition, or performance despite doing the right things, a resting metabolic assessment may be the most useful test you have not yet done. I offer this testing at Noble Naturopathic alongside a full metabolic workup tailored to your presentation.
You can book a metabolic assessment at any of our Vancouver locations. I would be glad to take a look at what your metabolism is actually doing.
Dr. David Duizer ND
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