An RER (respiratory exchange ratio) greater than 1.0 indicates that ______ is the primary fuel source.

a) carbohydrates and fat
b) fat
c) protein
d) carbohydrates

Answer: d) carbohydrates.

In exam language, the best answer is carbohydrates. In real-world coaching and sports nutrition, though, that answer needs a little more nuance. 

RER is a useful tool, but it is not a perfect live readout of fuel use. It is a practical, whole-body estimate based on breath-by-breath gas exchange.

RER is calculated as the ratio of carbon dioxide produced to oxygen consumed (VCO₂/VO₂), usually measured through indirect calorimetry. Under steady-state conditions, a value around 0.70 suggests a strong reliance on fat oxidation, while a value near 1.0 points to predominant carbohydrate oxidation. 

In practice, RER is shaped by more than one factor. Exercise intensity, habitual diet, recent carbohydrate intake, training status, age, sex, and glycogen availability can all shift the number, which is why RER works best when it is interpreted in context rather than treated like a single all-purpose verdict.

So why does RER climb as exercise gets harder? The short version is simple: as workload rises, the body leans more heavily on carbohydrate. A large modelling analysis in Sports Medicine found that RER increases with exercise intensity and carbohydrate availability, while longer duration and higher fat intake tend to pull it down. A recent 2026 exercise calorimetry paper also supports the classic crossover idea: as intensity rises, carbohydrate oxidation keeps climbing and eventually becomes the predominant substrate.

That is why d) carbohydrates is the right answer in a multiple-choice question. But once exercise becomes very hard, RER above 1.0 is not just a fuel story. At higher intensities, hydrogen ions associated with rising lactate are buffered mainly by bicarbonate. That buffering process generates additional CO₂, which is then exhaled. As a result, VCO₂ rises faster than VO₂, and RER can move above 1.0. In other words, an RER above 1.0 usually means carbohydrate use is very high, but it also reflects the body’s acid-base response to intense exercise.

This distinction matters, especially when people try to interpret exercise test results too literally. A value close to 1.0 during steady-state work is a reasonable sign that carbohydrate is dominating oxidative metabolism. A value above 1.0 during an incremental or near-maximal test is better understood as a sign of high-intensity effort, respiratory compensation, and excess CO₂ output from buffering, not simply “more carbs.” Recent cardiopulmonary exercise testing research has also challenged the habit of using one fixed RER cutoff for everyone when judging maximal effort, because age, sex, fitness level, and body size can affect what peak RER looks like in practice.

For coaches, clinicians, and practitioners, the practical takeaway is straightforward. 

• First, do not read RER like an on-off switch between fat and carbohydrate. 
• Second, always match the number to the testing situation: rest, steady-state aerobic exercise, or a graded exercise test. 
• Third, remember that a lower RER may indicate greater fat use, but that is not automatically “better.” High RER values are expected when training intensity is high and performance depends more heavily on carbohydrate metabolism. 

Used properly, RER helps connect exercise physiology, nutrition strategy, and training prescription in a way that is far more useful than a single quiz answer.

So the cleanest way to say it is this: if RER is above 1.0, carbohydrates are the main fuel source in practical exam terms, but the number also reflects extra CO₂ released when the body buffers exercise-induced acidosis. That explanation is both more accurate and more useful for real-world sports nutrition.

References

Klein I, Keijer J, van Schothorst EM. Quantitative interpretation and modeling of continuous nonprotein respiratory exchange ratio. American Journal of Physiology-Endocrinology and Metabolism. 2025. doi:10.1152/ajpendo.00459.2024.

Rothschild JA, Kilding AE, Stewart T, Plews DJ. Factors Influencing Substrate Oxidation During Submaximal Cycling: A Modelling Analysis. Sports Medicine. 2022;52:2775-2795. doi:10.1007/s40279-022-01727-7.

Brun JF, Varlet E, Myzia J, Varlet-Marie E, Raynaud de Mauverger E, Mercier J. Carbohydrate and Fat Oxidation in Muscle Assessed with Exercise Calorimetry in 6465 Subjects. Metabolites. 2026;16(2):121. doi:10.3390/metabo16020121.

Beaver WL, Wasserman K, Whipp BJ. Bicarbonate buffering of lactic acid generated during exercise. Journal of Applied Physiology. 1986;60(2):472-478. doi:10.1152/jappl.1986.60.2.472.

Beaver WL, Wasserman K, Whipp BJ. A new method for detecting anaerobic threshold by gas exchange. Journal of Applied Physiology. 1986;60(6):2020-2027. doi:10.1152/jappl.1986.60.6.2020.

Wasserman K, Whipp BJ, Koyl SN, Beaver WL. Anaerobic threshold and respiratory gas exchange during exercise. Journal of Applied Physiology. 1973;35(2):236-243. doi:10.1152/jappl.1973.35.2.236.