Substrate Utilization:

Find out how your body fuels performance! Measure how your body utilizes fat & carbohydrates at different intensities so you can optimize training and use the insights to train smarter with a Substrate Utilization test at the CHAMP Center!

Substrate Utilization (SU): What It Is

Substrate utilization refers to the degree our bodies rely on fuel sources (fat & carbohydrates) at different exercise intensities.

A substrate utilization (SU) test typically lasts 20–45 minutes (including warm-up) and is performed on a treadmill or cycle ergometer. The individual exercises through progressively increasing intensities (speed or power) using a stepwise protocol, with each stage lasting 4-6 minutes to allow physiological steady state to be reached.

Throughout the test, the participant wears a mask connected to a metabolic cart that measures oxygen consumption (VO₂) and carbon dioxide production (VCO₂). A high-precision, Polar heart rate monitor records cardiovascular responses at each workload. Using the gas-exchange data, the following variables are calculated:

Respiratory Exchange Ratio (RER)
Rates of fat and carbohydrate oxidation
Peak fat-oxidation intensity (Fatmax)
The intensity at which carbohydrate becomes the dominant fuel source (Crossover Point)

The test is concluded once the participant approaches volitional fatigue or once fuel utilization clearly shifts toward predominant carbohydrate reliance. Substrate utilization testing pairs well with Lactate Threshold (LT) testing and can be performed during the same graded exercise protocol (see the Aerobic Performance Package).

At the CHAMP Center we offer SU testing on a cycle ergometer (stationary bike), treadmill, rowing machine (client provided), or personal bike (client provided). We use the TrueOne® Parvo Medics metabolic cart, the same system trusted by NASA, US Olympic Training Centers, and leading research centers worldwide. Testing with the CHAMP Center always ensures access to professional-grade equipment, precise gas-exchange analytics, and industry-leading software for highly accurate SU results.

SU: Why It's Important

Performance / Athletic Benefits

Optimizes fuel use for endurance. Knowing how your body shifts between fat and carbohydrate at different intensities helps you train smarter and sustain effort longer without hitting the wall.
Identifies your Fatmax and crossover points. Athletes can target training zones that maximize fat oxidation or prepare for high-intensity efforts relying on carbohydrates.
Guides personalized training. Substrate utilization data allows coaches to tailor workouts, recovery, and nutrition strategies to each athlete’s metabolism and energy profile.
Monitors metabolic adaptations. Shifts in fuel use over time indicate improvements in mitochondrial efficiency, aerobic capacity, and metabolic flexibility.

General Health Benefits

Improves metabolic flexibility. Training in the right intensity zones can enhance the body’s ability to switch efficiently between fat and carbohydrate as fuel—a key factor in long-term metabolic health.
Supports weight management and body composition. Knowing your fat-oxidation profile helps optimize training and nutrition strategies for fat loss and lean-mass preservation.
Personalizes nutrition and recovery. Understanding how your body uses energy allows for smarter pre- and post-workout fueling, supporting performance and overall well-being.

At the CHAMP Center, we can customize the SU testing protocol to match each individual’s fitness level, ensuring accurate results that align with their personal goals.

SU: Who Should Do It

A substrate utilization test is ideal for anyone looking to understand how their body uses fuel during exercise and optimize training or health outcomes. This includes:

Endurance athletes: Runners, cyclists, swimmers, rowers, triathletes, and others who want precise insights into fat and carbohydrate use to improve performance and delay fatigue.
Field or court athletes: Soccer, basketball, hockey, and similar sport participants who need to optimize energy management and conditioning for intermittent high-intensity efforts.
Recreational exercisers: Hikers, kayakers, and anyone wanting a clear picture of their metabolic profile to guide training, nutrition, or weight management.
Individuals focused on health optimization: Those interested in improving metabolic flexibility, fat oxidation, and overall energy efficiency for long-term health and fitness.

Special Considerations for Women

Substrate utilization can vary across the menstrual cycle due to hormonal influences on fat and carbohydrate metabolism. For the most accurate longitudinal tracking, scheduling tests in the same menstrual phase can help, though it is not strictly necessary.

SU: Interpreting Results

What the Data Means

Higher Fatmax and later carbohydrate crossover = greater metabolic efficiency and endurance potential. Your body can sustain moderate-to-high intensities using fat & conserving carbohydrates for high-intensity efforts.

Respiratory Exchange Ratio (RER) → indicates the balance of fat vs. carbohydrate use at each intensity.
Fat and carbohydrate oxidation rates → quantify fuel contribution across workloads.
Fatmax → The exercise intensity where your body uses fat most efficiently (fat oxidation peaks). A higher Fatmax (fat use at a higher workload) often indicates your body can rely on fat for energy at more challenging intensities.
Crossover Point → the intensity where carbohydrate becomes the dominant fuel source. A later crossover (at a higher workload) suggests improved fat-use efficiency and endurance potential.
Heart rate at Fatmax and crossover → helps set precise training zones for fat-usage as fuel, endurance, and high-intensity workouts.

These data points are often more actionable for training than a single peak value, as they show how your metabolic fuel use responds across intensities.

How to Use Your Results

Set individualized training zones based on fat-oxidation peaks and carbohydrate crossover for steady-state, tempo, and interval workouts.
Track metabolic adaptations over time to see improvements in fat utilization, endurance efficiency, and metabolic flexibility.
Pair with Lactate Threshold & VO₂ max testing for a comprehensive view of aerobic capacity, fuel management, and performance potential (see our Aerobic Performance Package).