Why High-Carb Performance Science Is Being Misapplied to the Wrong Athletes
In recent years, sports nutrition has entered an arms race.
Carbohydrate recommendations for endurance athletes have climbed from 60 g per hour to 90 g, and now up to 120 g per hour — sometimes even higher. Elite marathoners and cyclists are experimenting with increasingly aggressive glucose–fructose protocols in pursuit of marginal gains. Research laboratories are publishing studies showing improved exogenous carbohydrate oxidation rates at these doses. The message being amplified across social media is clear: more carbohydrate equals more performance.
But performance for whom?
And performance in which physiological system?
What is often missing from these conversations is a critical distinction: metabolic health is not the same thing as high-intensity glycolytic performance. These systems overlap, but they are not interchangeable. Confusing them leads to nutritional advice that may serve elite competitors while undermining long-term metabolic resilience in the broader athletic population.
At BSE, we believe that before discussing how much carbohydrate to consume at race pace, we must first define which engine we are trying to fuel.
Three Engines, Not One
Human performance is not governed by a single energy system. It operates through interacting but distinct physiological engines.
The first is the fat-dominant oxidative base, what we call the TL1 metabolic engine. This system underpins daily activity, long aerobic sessions, and the majority of recreational endurance training. It is mitochondrial, low-insulin, and durability-driven.
The second is what we define as the Aerobic Glycolytic System (AGS) — the high-flux oxidative state where carbohydrate-derived substrate becomes dominant despite adequate oxygen availability. This is threshold territory. Marathon pace for elites. Sustained tempo efforts. The zone where glycogen turnover and lactate flux matter.
The third is the oxygen ceiling system, where maximal cardiac output becomes the primary limiter. Here, performance is constrained by stroke volume, hemoglobin mass, and oxygen delivery capacity rather than substrate flexibility.
An athlete can have a massive aerobic base but struggle at threshold. Another may possess an impressive VO₂max yet fatigue rapidly in prolonged events. A third may excel in short high-intensity efforts while lacking metabolic durability.
Different engines. Different limiters. Different fueling needs.
Yet modern sports nutrition often treats them as one.
Defining Metabolic Health
For clarity, let us define metabolic health as:
The ability to generate stable, high-output energy across conditions without pathological dependence on glucose, excessive insulin signaling, or chronic stress activation.
This concept extends far beyond sport. It governs how we function in daily life, how we recover, and how we age. It applies to the recreational runner training at 60–70% VO₂max just as much as to the elite ultramarathoner navigating self-supported terrain.
Metabolic health is reflected in low fasting insulin, stable blood glucose under stress, robust fat oxidation capacity, preserved mitochondrial function, hormonal resilience, electrolyte stability, and low chronic inflammation. Markers such as fasting insulin below 6 µIU/mL, a low triglyceride-to-HDL ratio, and minimal glycemic variability provide practical insight into this state.
Research from Volek and Phinney has demonstrated that fat-adapted athletes can achieve peak fat oxidation rates exceeding 1.5 g/min without impairment of aerobic capacity (Volek et al., 2016). Meanwhile, work by Noakes and colleagues has challenged the long-standing glycogen depletion model of fatigue, suggesting that exercise-induced hypoglycemia and central regulation may play a more decisive role in endurance limitation than previously believed (Noakes et al., 2023).
These findings do not negate the importance of carbohydrate. They simply highlight that metabolic stability — particularly the regulation of the small glucose pool comprising blood and liver glycogen — is often more critical than maximal glycogen saturation.
A metabolically healthy human should be able to train at moderate intensities primarily on fat, fast overnight without distress, and avoid catastrophic energy crashes during prolonged effort. If constant carbohydrate feeding is required to prevent collapse at moderate workloads, that reflects metabolic fragility rather than performance sophistication.
The Aerobic Glycolytic System: Where Carbohydrate Matters
The Aerobic Glycolytic System occupies a different physiological territory.
It emerges when ATP demand exceeds the maximal rate at which fat oxidation alone can supply energy. Glycolytic flux increases. Pyruvate dehydrogenase activity rises. Lactate production accelerates but remains balanced by clearance. Oxygen is available, but substrate flux — not oxygen delivery — becomes the limiting variable.
This is where carbohydrate availability becomes performance-relevant.
Studies led by Louise Burke at the Australian Institute of Sport have demonstrated that elite race walkers adapting to low-carbohydrate high-fat diets significantly increased fat oxidation but exhibited higher oxygen costs at race pace, impairing economy during high-intensity competition (Burke et al., 2017). Carbohydrate oxidation yields more ATP per liter of oxygen than fat, making it advantageous in oxygen-limited high-intensity scenarios.
This does not invalidate metabolic adaptation. It simply clarifies context. At 90% of VO₂max, oxygen efficiency matters. At 60% of VO₂max for six hours, fuel stability may matter more.
The problem arises when fueling strategies designed for Olympic-level threshold performance are exported wholesale to recreational athletes whose primary limiter is not glycolytic flux but metabolic resilience.
The Misapplication of High-Carbohydrate Protocols
When elite cyclists ingest 100–120 g/hour of carbohydrate, they are operating within the Aerobic Glycolytic System for extended periods. Their training density, race intensity, and competitive margins justify aggressive carbohydrate strategies.
But when a weekend runner at 65% VO₂max consumes similar quantities based on influencer advice, the physiological equation changes. At moderate intensities, exogenous carbohydrate often displaces fat oxidation almost isocalorically (Coyle et al., 1986). Performance gains are not always proportional to intake, and minimal doses may suffice to stabilize blood glucose.
Moreover, chronic high carbohydrate exposure in metabolically compromised individuals may exacerbate hyperinsulinemia — a state strongly associated with cardiovascular disease risk and type 2 diabetes (Kraft, 2008).
Even among elite athletes, recent data suggest that a substantial proportion display abnormal glucose tolerance markers despite high training volumes. Performance does not immunize against metabolic dysfunction.
Why Metabolic Performance Comes First
At BSE, we advocate building the metabolic engine before maximizing glycolytic throughput.
This means prioritizing:
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Low-intensity aerobic volume below the first lactate turn point
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Strategic carbohydrate timing rather than constant feeding
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Periodic low-glycogen sessions performed judiciously
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Long-duration aerobic work to enhance hepatic glucose management
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Electrolyte optimization to maintain plasma volume under low-insulin conditions
Low-intensity training robustly stimulates mitochondrial biogenesis through AMPK and PGC-1α signaling pathways. Long aerobic sessions improve liver glucose output stability. Reduced insulin exposure enhances lipolysis and metabolic flexibility.
Once a stable base exists, high-intensity work can be layered strategically, and carbohydrate can be introduced according to intensity demands.
Health first. Performance layered on top.
The Bigger Picture
The question is not whether carbohydrate works. It does.
The question is: for whom, at what intensity, and at what metabolic cost?
A sub-two-hour marathon fueled by maximal carbohydrate intake represents the optimization of the Aerobic Glycolytic System under elite conditions. It does not define the nutritional needs of the broader athletic community.
Metabolic health provides resilience. Glycolytic capacity provides speed. Oxygen delivery provides peak power.
Fueling must match the engine being trained.
Stop searching for magic glucose–fructose ratios. The answer is not in a packet. It lies in understanding your physiology, your metabolic health, and the intensity at which you operate.
Build the base.
Then build the power.
BSE — Fuel Smarter.
Alvaro Madrazo
References
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Burke, L. M., et al. (2017). Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. Journal of Physiology.
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Coyle, E. F., et al. (1986). Carbohydrate feeding during prolonged strenuous exercise can delay fatigue. Journal of Applied Physiology.
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Kraft, J. R. (2008). Diabetes Epidemic & You. Trafford Publishing.
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Noakes, T., Volek, J., D’Agostino, D., et al. (2023). Carbohydrate ingestion and exercise metabolism: A reappraisal of fatigue mechanisms.
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Volek, J. S., et al. (2016). Metabolic characteristics of keto-adapted ultra-endurance runners. Metabolism.
