Substrate Utilization

Find out how your body fuels performance! Identify how your body uses fat and carbohydrates at different intensities with a Substrate Utilization test at the CHAMP Center!

Substrate Utilization (SU): What It Is

Substrate utilization refers to the degree the body relies on fat and carbohydrates for fuel at different exercise intensities. Understanding this balance provides insight into endurance, training efficiency, and metabolic flexibility.

Testing involves progressively increasing intensity on a treadmill, cycle ergometer, client provided rowing machine, or personal bike. Oxygen consumption (VO₂) and carbon dioxide production (VCO₂) are measured through a metabolic mask, while heart rate is continuously monitored. This data identifies the proportion of energy derived from fat versus carbohydrates at each workload, including peak fat-oxidation intensity (Fatmax) and the intensity at which carbohydrate becomes the primary fuel (Crossover Point). Assessments typically last 20–30 minutes, not including warm-up and cool-down. Further preparation details can be found on the CHAMP Center Prepare Page.

All assessments use the TrueOne® Parvo Medics metabolic cart, trusted by NASA, U.S. Olympic Training Centers, and leading research institutions, ensuring precise measurement of fuel use and respiratory exchange for highly accurate results.

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SU: Why It's Important

Efficient fuel use is a key factor in performance, endurance, and metabolic health. During exercise, the body shifts between fat and carbohydrate use depending on intensity. Knowing how and when these shifts occur allows training and nutrition strategies to align with individual physiology rather than generic guidelines.

Understanding substrate utilization allows participants to:
• Optimize exercise intensity to sustain effort longer and reduce premature fatigue
• Target training zones that improve fat oxidation or support high-intensity work
• Personalize fueling, recovery, and nutrition strategies
• Monitor changes in metabolic flexibility and efficiency over time

Whether improving endurance, supporting metabolic health, managing body composition, or training more efficiently, Substrate Utilization testing provides clear insight into how the body fuels exercise across intensities.

At the CHAMP Center, we can customize the SU testing protocol to match each individual’s fitness level, ensuring accurate results that align with personal goals so that training and health strategies are meaningful.

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SU: Who Should Do It

Substrate Utilization testing measures how the body uses fat and carbohydrate for fuel across different exercise intensities. This test benefits anyone seeking insight into metabolic efficiency and fuel use, including:

Energy Efficiency Focused: Enhance fat oxidation, metabolic flexibility, and overall energy efficiency.
Endurance Athletes: Optimize fat and carbohydrate use to improve performance and delay fatigue.
Field and Court Athletes: Efficiently manage energy during repeated high-intensity efforts.
Recreational Exercisers: Guide training, nutrition, and weight management with personalized metabolic insights.
High-Intensity Athletes: Monitor metabolic flexibility and improve recovery between efforts

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SU: Interpreting Results

Your Substrate Utilization results provide a window into how your body selects and shifts between fat and carbohydrate as exercise intensity increases. This helps you understand metabolic efficiency, endurance potential, and how your body adapts to training and nutrition strategies.

Rather than labeling fuels as “good” or “bad,” these results show when and how each fuel source is used and how that pattern supports performance, recovery, and long-term metabolic health. After testing, CHAMP Center staff interpret your results in the context of key metrics such as heart rate, pace, or power to provide a personalized picture of how your body manages energy across different intensities and modalities.

Key Metrics Include:

Fatmax
Crossover Point
Respiratory Quotient
Heart Rate at Fatmax & Crossover Point
Pace/Power at Fatmax & Crossover Point
Ratings of Perceived Exertion (RPE)

View the other tabs to read more on each metric.

Fatmax:

Fatmax represents the exercise intensity at which fat oxidation is highest right before carbohydrate use begins to dominate. Improvements in Fatmax intensity, occurring at higher speeds, power, or heart rates, reflect better metabolic flexibility, enhanced endurance potential, and an increased ability to sustain longer aerobic efforts. Tracking Fatmax over time helps you structure steady-state aerobic training to maximize fat oxidation and mitochondrial efficiency.

Heart Rate at Fatmax & Crossover Point:

These metrics show how your cardiovascular system responds at key metabolic transitions. Using the heart rate measured during the test, rather than a generic prediction formula, allows for precise, individualized training prescriptions.

At Fatmax:
The heart rate is corresponding to the intensity where fat oxidation is highest right before carbohydrate use dominates. A lower heart rate at this point over time, or the ability to sustain higher workloads at the same heart rate, indicates improved fat utilization, metabolic efficiency, and aerobic endurance.

At the Crossover Point:
The heart rate is corresponding with the intensity where carbohydrate becomes the predominant fuel. Tracking changes here can show enhanced endurance capacity and delayed reliance on carbohydrate, meaning your body can sustain higher intensities before switching fuels.

Pace or Power at LT1:

These values represent the speed or power you can sustain at Fatmax and Crossover. Tracking these metrics over time provides a clear picture of how your body adapts to training, improves fuel efficiency, and enhances endurance potential. They are essential for structuring aerobic, tempo, and interval sessions using your individualized workload targets.

At Fatmax:
A higher pace or power reflects improved fat utilization efficiency, allowing longer efforts at moderate intensity with minimal fatigue.

At Crossover Point:
A higher pace or power indicates delayed carbohydrate dependence, showing better metabolic flexibility and the ability to sustain higher intensities while preserving energy stores.

Ratings of Perceived Exertion (RPE):

RPE links subjective effort with physiological responses. When a given workload feels easier over time despite similar physiological markers, it reflects improved efficiency and conditioning. At the CHAMP Center we use the Borg scale which rates perceived exertion from 6 (no exertion) to 20 (maximum exertion). As your aerobic capacity improves the same workload should feel easier. You can view an example Borg scale here.

How to Use Your Results:

Use heart rate and pace/power at Fatmax and Crossover Point to guide aerobic, tempo, and interval session workloads
Monitor metabolic adaptations over time to assess improvements in fat oxidation and endurance efficiency
Personalize fueling and recovery strategies to match training intensity and metabolic demands
Identify whether training is supporting metabolic flexibility or creating early carbohydrate dependence
Pair Substrate Utilization results with VO₂ max and Lactate Threshold testing for a complete picture of aerobic performance and metabolic efficiency. Check out our Aerobic Performance Package which can combine all three!

Crossover Point:

The crossover point marks the intensity at which carbohydrate becomes the primary fuel source. A later crossover indicates greater reliance on fat at moderate intensities, reflecting improved metabolic efficiency and endurance capacity. An earlier crossover suggests a quicker shift to carbohydrate, which may increase fatigue during longer efforts. This metric guides pacing, fueling strategies, and interval training prescriptions.

Respiratory Quotient:

RQ reflects the relative contribution of fat versus carbohydrate used for energy at different exercise intensities. It is calculated by dividing carbon dioxide production (VCO₂) by oxygen consumption (VO₂).

Lower values (~0.70–0.80): indicate greater fat utilization.
Mixed fuel (~0.85): represents a balance of fat and carbohydrate use.
Higher values (~0.90–1.00): indicate increasing reliance on carbohydrate.

Lower RQ at submaximal intensities suggests strong fat oxidation and metabolic efficiency, while higher values indicate a shift toward carbohydrate use. Tracking RQ across intensities helps contextualize energy demands, fuel selection, and endurance efficiency for different training zones.