How to Prepare for an RMR Test
An RMR test takes about 15 minutes and you spend most of it lying still, breathing normally. The test itself is easy. Getting a result that means something depends almost entirely on what you did in the hours before you arrived.
Every rule below exists because a published study found that breaking it changes the number.
The short version first
Requirement | Precision Body Lab protocol | Why |
Fast | 4 hours, water only | Food raises metabolic rate for a variable number of hours |
Caffeine | None on the day of the test | Stimulant effect on resting rate |
Alcohol | None for 24 hours | Alters resting metabolic rate and fuel use |
Nicotine | None on the day of the test | Stimulant effect on resting rate |
Exercise | None for 12 hours before | Longer carryover effect than food |
Sleep | Normal night, no all-nighters | Sleep restriction affects resting rate and hormones |
Arrival | Arrive with time to rest, do not rush in | 10 to 20 minutes of rest is required before measurement |
Clothing | Comfortable, layers | You will lie still and may get cool |
If you follow only those, you will get a usable result.
Why each one matters
The published best practice review for measuring resting metabolic rate in adults sets out the evidence for the protocol (Compher, Frankenfield, Keim and Roth-Yousey, Journal of the American Dietetic Association).
Food, and why 4 hours rather than 12
Digesting, absorbing and storing food costs energy. That cost is called the thermic effect of food and it raises your measured metabolic rate while it is happening. The review found that food affects resting metabolic rate for a variable number of hours after consumption, which is why intake needs to be controlled before measurement.
Four hours is the canonical fast at Precision Body Lab. Twelve hours is a requirement for basal metabolic rate, which is a different measurement taken under laboratory conditions with an overnight stay. A four hour fast clears the bulk of the thermic effect of a normal meal while remaining practical and, importantly, while avoiding the confounding effect of being genuinely hungry and uncomfortable during the test.
Water is fine and encouraged. Being dehydrated is its own problem.
Caffeine, alcohol and nicotine
The same review grouped these with food: all of them affect resting metabolic rate for a variable number of hours after consumption and all of them therefore need controlling.
Caffeine is a stimulant and raises resting metabolic rate. A morning coffee before an RMR test will inflate your result, and an inflated RMR means an inflated calorie target, which means a deficit that is not a deficit.
This is the rule people break most often, usually by accident, and usually with pre-workout, an energy drink, or the assumption that a small coffee does not count. It counts.
Alcohol affects both metabolic rate and substrate use, so it distorts your respiratory exchange ratio as well as your energy figure. Leave 24 hours.
Exercise, and why it needs longer
This is the rule most people underestimate.
The review found that ordinary activities of daily living raise metabolic rate but that a short rest of 20 minutes or less is sufficient for that effect to dissipate. Moderate or vigorous physical activity is different: it has a longer carryover effect and must be controlled in the hours before measurement.
Twelve hours is the practical rule. A hard session the evening before a morning test will still be influencing your result. If you train in the mornings, book your test on a rest day or before you train, not after.
Sleep
An abnormal night before the test introduces a variable you cannot quantify. Sleep restriction affects hormonal regulation and resting energy expenditure. Aim for your normal night rather than an unusually good or unusually bad one.
Arriving in a rush
The protocol requires a 10 to 20 minute rest period before measurement begins, precisely because getting to the clinic raises your metabolic rate. Sprinting from a parking spot, carrying bags up a flight of stairs, or arriving stressed and out of breath undoes the rest period you are about to take.
Arrive with a few minutes spare. Sit down. This costs you nothing and protects the result.
What happens during the test
Useful to know so nothing surprises you.
Pre-test rest. You lie or recline quietly for 10 to 20 minutes. No phone scrolling is ideal, though not mandatory.
The measurement. A hood or mask collects your expired air. You breathe normally. Do not try to control your breathing, do not talk, do not sleep.
Duration. The measurement window is around 10 minutes. The first 5 minutes of data are discarded, because that period captures your settling in rather than your resting state.
The validity check. The remaining data must show a coefficient of variation under 10 per cent. If your breathing was unstable, the test can be repeated. That criterion is what separates a measurement from a reading.
You will not be uncomfortable and nothing is invasive. Most people find it relaxing.
What you get
Your resting metabolic rate in kilocalories per day, measured rather than estimated.
Your respiratory exchange ratio, which shows your resting fuel mix. A value near 0.7 indicates predominantly fat oxidation, near 1.0 predominantly carbohydrate, and around 0.8 a mixed substrate use.
Your measured figure compared against equation predictions, which tells you whether calorie calculators work for you personally.
If you book a Metabolic Baseline, the DEXA scan in the same visit gives you fat mass and lean mass, which is what your resting rate is mostly driven by and what makes the RMR number interpretable.
Common questions
Can I take my medication? Take prescribed medication as normal and tell the technician what you have taken. Do not stop prescribed medication for a test.
What if I accidentally had coffee? Tell us. It is better to reschedule than to build a calorie target on an inflated number. There is no charge for being honest about it.
Can I have the test in the afternoon? Yes, provided the 4 hour fast and the 12 hour exercise window are respected. Morning is simpler for most people because the fast happens overnight.
Should I do the DEXA scan and the RMR test on the same day? Yes, and that is exactly what the Metabolic Baseline is designed for. Both tests are done in the one visit. The booking confirmation sets out the preparation for both so you only have to do it once.
What if I am unwell on the day? Reschedule. Illness raises resting metabolic rate and the result will not represent your normal.
How often should I retest? Before and after a significant fat loss or muscle gain phase, or every six months if you are actively manipulating intake. A retest is $99.
The short version
Fast 4 hours, no caffeine or nicotine on the day, no alcohol for 24 hours, no training for 12 hours, sleep normally, and arrive with time to sit down. Each rule maps to a published finding about what shifts the measurement. Break the caffeine rule and you will get a higher number than your real one, which is the worst possible error because it leads to a calorie target that is too high.
An RMR test at Precision Body Lab is $149, or $279 with a DEXA scan as a Metabolic Baseline. Book online or call 1300 910 163 if you are unsure whether your preparation was adequate.
Related reading
Sources
Compher C, Frankenfield D, Keim N, Roth-Yousey L; Evidence Analysis Working Group. Best practice methods to apply to measurement of resting metabolic rate in adults: a systematic review. Journal of the American Dietetic Association, 2006. https://pubmed.ncbi.nlm.nih.gov/16720129/
Respiratory Quotient. StatPearls, NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK531494/
Frankenfield DC. Bias and accuracy of resting metabolic rate equations in non-obese and obese adults. Clinical Nutrition, 2013. https://pubmed.ncbi.nlm.nih.gov/23631843/
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