Clubhouse #18 | The Truth About Training Low (Carbs) and Racing High 🍞🏃‍♀️

Imagine training your body to tap into endless energy reserves—not by adding more fuel, but by using less. That’s the premise behind the increasingly popular strategy known as “Train Low, Race High.” It’s a concept rooted in metabolic science: train with low carbohydrate availability to enhance fat oxidation, then race with high carbohydrate stores to maximize performance.

But how does it work in practice? What’s the evidence? And more importantly, is it something you should be doing?

In today’s Clubhouse, we’ll unpack the full science behind this approach, explore how to implement it safely and effectively, and help you understand whether training low and racing high could be your next breakthrough..

So why is altitude training so important?

The Science of Fuel Use: Carbohydrates vs. Fat

During endurance exercise, the human body primarily relies on two distinct fuel sources: carbohydrates, which include glycogen stored in muscles and glucose circulating in the bloodstream, and fats, primarily in the form of free fatty acids and intramuscular triglycerides. These fuels are utilized through different metabolic pathways depending on the intensity and duration of the activity.

Carbohydrates are the most accessible and efficient form of energy, especially during moderate to high-intensity exercise. They can be rapidly broken down via glycolysis to generate ATP, the cellular energy currency, even in the absence of oxygen during anaerobic conditions. However, the limitation of this system is its capacity—muscle glycogen stores are finite, typically lasting 90 to 120 minutes of continuous high-intensity effort before they are depleted, leading to the familiar experience of 'hitting the wall.'

Fat, by contrast, is an abundant and long-lasting fuel source. Even the leanest athletes possess tens of thousands of kilocalories stored as fat. The catch is that fat metabolism, while highly efficient in terms of energy yield per gram, is considerably slower than carbohydrate metabolism. It requires more oxygen and time to oxidize fat molecules and generate ATP. Therefore, it is predominantly utilized during lower-intensity, longer-duration efforts where oxygen availability is not a limiting factor.

This balance between fat and carbohydrate use is governed by what exercise scientists refer to as metabolic flexibility—the body's ability to adaptively shift between fuel sources depending on the physiological demands. An athlete with high metabolic flexibility can efficiently burn fat at low intensities, preserving glycogen stores for when they are needed most—during surges, climbs, final kicks, or sustained race paces.

Training with low carbohydrate availability aims to challenge and enhance this flexibility. By deliberately depriving the body of its preferred fuel, it forces a metabolic adaptation: upregulating fat oxidation enzymes, increasing mitochondrial content and efficiency, and promoting a shift toward a more aerobic, fuel-conserving energy system. This adaptation has the potential to make an endurance athlete more efficient, resilient, and capable of maintaining performance deeper into long events.

However, these adaptations must be weighed against potential trade-offs. With lower glycogen, the body also reduces capacity for high-intensity output, neural drive, and recovery. That’s why 'train low' isn’t a universal fix—it’s a tool, and like any tool, its effectiveness lies in how and when it’s applied.

What Happens When You Train Low?

Training with low carbohydrate availability involves intentionally starting a workout with reduced or depleted glycogen stores, or refraining from consuming carbohydrates prior to the session. This state challenges the body to adapt by increasing its reliance on fat oxidation for energy. There are several ways to induce low carbohydrate availability:

• Fasted training: Often performed in the morning before eating, when glycogen stores are already partially depleted from the overnight fast.

• Two-a-day training: The first session (typically higher intensity) is used to deplete glycogen stores. The second session, performed later the same day without carbohydrate intake in between, forces the body to train in a low-glycogen state.

• Sleep low strategy: Athletes perform a high-intensity evening workout, then restrict carbohydrate intake post-exercise and overnight. The following morning, they complete a low-intensity session in a fasted or low-carb state.

These protocols aim to stimulate specific metabolic adaptations by exposing the body to repeated bouts of carbohydrate scarcity. Over time, several physiological changes occur:

• Increased mitochondrial biogenesis: The body produces more mitochondria—its energy powerhouses—resulting in improved aerobic metabolism and energy production.

• Enhanced fat oxidation capacity: The body becomes more efficient at breaking down and utilizing fatty acids as a primary fuel source, especially during prolonged submaximal efforts.

• Upregulation of fat metabolism enzymes: Enzymes involved in lipolysis and beta-oxidation become more active, enabling faster and more efficient fat breakdown.

• Glycogen sparing: With better fat adaptation, athletes rely less on glycogen during lower intensity work, preserving it for moments of peak intensity.

• Potential improvements in metabolic flexibility: The body's ability to switch between carbohydrate and fat metabolism becomes more refined, contributing to more efficient fueling under different demands.

These adaptations are particularly valuable in long-distance events, where energy conservation and endurance efficiency are crucial. However, training low is not without its trade-offs.

Training with reduced glycogen can significantly limit high-intensity performance, diminish central nervous system drive, and blunt neuromuscular output. It also increases perceived exertion, and can elevate cortisol levels, which may interfere with recovery and adaptation over time. Additionally, if not managed carefully, it can lead to insufficient energy availability, compromising immune health, hormone balance, and muscle repair.

That’s why “train low” should not be viewed as an everyday approach, but rather a targeted training strategy. When integrated selectively into a periodized training plan—particularly during base or aerobic-focused phases—it can provide long-term benefits without sacrificing performance or recovery.

The Case for Racing High

Carbohydrates remain the most efficient and powerful energy source for sustaining moderate to high-intensity performance. This is due to their rapid digestibility, high yield of ATP per unit of oxygen, and central role in fueling both the muscles and brain during prolonged exertion. In competition scenarios where every second counts and intensity often spikes unpredictably, glycogen availability becomes a key performance determinant.

When carbohydrate stores are topped up, athletes are better equipped to respond to tactical surges, hills, sprint finishes, and psychological stressors. Full glycogen availability ensures that the body can:

• Perform at higher intensities for longer durations without premature fatigue, maintaining pace and output under pressure.

• Delay the onset of exhaustion, commonly referred to as “hitting the wall” or “bonking,” which is a hallmark of depleted glycogen.

• Support cognitive sharpness and decision-making, since the brain is a carbohydrate-dependent organ, particularly under competitive stress.

• Enhance buffering capacity, aiding in lactate clearance and maintaining acid-base balance in the muscles during maximal efforts.

Importantly, carbohydrate intake before and during competition also mitigates the catabolic effects of prolonged exercise, helping preserve lean muscle mass and supporting hormonal stability.

This dual strategy—adapting to fat use through low-carb training while maximizing carbohydrate availability during races—is what makes the “train low, race high” model so compelling. It blends metabolic economy with performance potential. The body learns to operate efficiently under restriction, but is then unleashed at full throttle when fully fueled, delivering optimal performance on race day.

Strategies for Training Low

There are several ways to structure low-carb training, depending on your goals, schedule, and recovery capacity:

1. Fasted Training (Morning Sessions)

Performing aerobic workouts in a fasted state (after an overnight fast) can enhance fat oxidation and adaptation. Best used for low- to moderate-intensity efforts under 90 minutes.

2. Two-A-Day (Deplete and Train)

The first session is used to deplete glycogen (e.g., intervals or a long run). The second session—done without carb replenishment—stimulates adaptation under low-carb conditions. Requires careful monitoring of fatigue and recovery.

3. Sleep Low Strategy

Complete a high-intensity evening session with carbs. Then avoid carbohydrates afterward and perform a low-intensity session the next morning in a fasted state. Combines intensity and adaptation in a single cycle.

4. Low-Carb Days

Deliberately plan some training days with reduced carb intake to stimulate adaptation, paired with high-carb days to support harder sessions.

5. Long Slow Distance (LSD) Fuel Restriction

During long steady runs or rides, restrict carbohydrate intake to force greater fat oxidation. Often done early in the season or during base training.

When It Backfires: Risks and Limitations

While training low can enhance endurance metabolism and improve fat adaptation, it is not a one-size-fits-all solution. Implemented poorly or too frequently, it can introduce significant physiological and psychological stressors that outweigh its benefits—particularly in athletes with high training loads, compromised recovery, or inadequate nutritional support.

One of the primary concerns is impaired recovery. Glycogen plays a crucial role in muscle repair, immune function, and restoring hormonal balance after training. Without adequate glycogen stores, the body may struggle to regenerate damaged muscle fibers, replenish intramuscular energy reserves, and maintain robust immunity. This can result in extended recovery times, suppressed training gains, and increased susceptibility to illness.

Chronic low-carb exposure can also lead to suppressed adaptation. While occasional training in a glycogen-depleted state can trigger beneficial stress responses, long-term adherence may blunt anabolic signaling pathways such as mTOR, which are essential for muscle growth, repair, and adaptation. The consequence is a potential plateau or regression in overall performance, particularly if low-carb training is not alternated with periods of glycogen replenishment.

Another key risk is blunted training quality. High-intensity workouts, technical drills, and neuromuscular conditioning require immediate energy availability—something carbohydrates deliver far more effectively than fats. Training with depleted glycogen can lead to reduced output, compromised form, and increased injury risk due to neuromuscular fatigue and diminished cognitive focus.

Further, immune system suppression can occur if athletes fail to adequately fuel and recover between sessions. Prolonged low energy availability can depress white blood cell count, increase inflammation, and elevate cortisol levels, impairing the body’s ability to fight off infections and heal from stress.

One of the most concerning risks, particularly in female athletes, is Low Energy Availability (LEA), a state in which caloric intake is insufficient to support both exercise expenditure and basic physiological function. LEA can lead to Relative Energy Deficiency in Sport (RED-S), a syndrome associated with hormonal disruption, decreased bone mineral density, menstrual dysfunction, and impaired metabolic health. Even in male athletes, LEA can result in low testosterone, fatigue, and compromised bone and immune health.

That’s why the key to effective implementation is periodization. Athletes and coaches should strategically incorporate train-low sessions within a broader framework that prioritizes high-quality training, adequate fueling, and individualized recovery. This might involve scheduling low-glycogen sessions during lower intensity days or base-building phases, while ensuring high-carbohydrate availability supports race-specific intervals, competition prep, and strength work.

Monitoring metrics like heart rate variability (HRV), mood, sleep quality, and training performance can help assess whether the approach is yielding benefits or veering into overreaching territory. Ultimately, 'train low' works best not as a daily routine, but as a tool to sharpen metabolic resilience in the hands of a smart, well-fueled athlete.

Practical Framework: Fuel for the Work Required

A modern approach to endurance nutrition is to “fuel for the work required.” This means matching your carbohydrate intake to the purpose and intensity of the workout:

• Easy aerobic sessions: Low or no carb.

• Threshold or tempo runs: Moderate carb before and during.

• VO2 max intervals or race simulations: High carb intake before, during, and after.

This approach balances adaptation with performance, allowing you to train smarter and recover faster.

Conclusion

“Train low, race high” is not a gimmick—it’s a powerful tool rooted in metabolic science. Used strategically, it can improve fat adaptation, increase mitochondrial efficiency, and teach your body to burn fuel more effectively over long durations.

But it’s not for every workout. High-intensity efforts, key sessions, and races demand glycogen. That’s why the best athletes don’t just train hard—they train smart, periodizing their nutrition as carefully as their intervals.

Fuel wisely. Train deliberately. Race with power.

Robert