Why Carbs Rule When Pace or Power Gets Serious

Introduction

Picture a hard progression run or a hill rep session. The first few minutes feel smooth, breathing steady, legs light. Then the pace edges closer to threshold, heart rate climbs, and suddenly you can feel the engine demanding more rapid energy. That sensation is your metabolism switching fuel priority. At low intensities most energy can be supplied by fat, but as intensity and heart rate rise toward VO₂max, carbohydrate becomes the dominant substrate. Understanding why this switch occurs helps athletes plan training, fuelling, and gut strategies for both performance and comfort. In this post we unpack the physiology, show how to infer it from practical metrics like heart rate, and offer ways to support carbohydrate use when you need it most, including where a product like Carb Accelerator fits.

The Problem 

Many athletes hear “fat burns in the flame of carbohydrate,” or that Zone 2 maximises fat use, but then wonder why they still blow up late in a race or why high-intensity training feels so glycogen-limited. The gap is not just knowing that a shift happens, but understanding the mechanisms, approximate intensity landmarks, and how training and nutrition alter where the crossover occurs. Without that framework, fuelling becomes guesswork and GI distress more likely.

Science Deep Dive

The crossover concept. As exercise intensity increases, there is a progressive shift from fat toward carbohydrate oxidation. This pattern, described in the crossover concept, reflects both cellular regulation and whole-body responses. At low intensities, fat provides most ATP; as intensity rises, carbohydrate contribution increases and predominates near and above the lactate threshold. Endurance training can shift the crossover rightward, meaning a greater reliance on fat at a given submaximal workload, but near VO₂max carbohydrate still dominates [1,2].

Quantitative landmarks. In controlled studies using tracers and indirect calorimetry, moving from about 25% to 65% to 85% VO₂max progressively increases carbohydrate use via greater muscle glycogen oxidation and glucose uptake, with concurrent reductions in fat oxidation at the highest intensity [3]. Maximal fat oxidation (Fatmax) typically occurs around 55–72% VO₂max, and the contribution of fat becomes small above roughly 85–90% VO₂max, which often corresponds to the upper end of Zone 4 toward Zone 5 in trained athletes [4].

Why carbohydrate wins at high intensity. Several rate-limiting steps in fat use become constrained as intensity climbs: limited fatty-acid release from adipose tissue, finite transport into muscle and mitochondria, and slower β-oxidation flux compared with glycolysis. Carbohydrate oxidation offers higher maximal rates of ATP production and a slightly greater energy yield per litre of oxygen, which matters when cardiorespiratory capacity is the bottleneck near VO₂max. Sympathetic activation and rising intracellular calcium and AMP also stimulate glycogen phosphorylase and glycolysis. Collectively these mechanisms favour carbohydrate at high workloads [5,6].

Measurement notes. Substrate use is often estimated from respiratory exchange ratio during graded exercise. Equations refined by Jeukendrup and Wallis help differentiate glycogen versus glucose contributions across intensities, improving estimates of carbohydrate and fat oxidation. Nonetheless, context such as diet, training status, sex and prior exercise affects results and the precise intensity at which substrate crossover occurs [7–9].

Training and diet effects. Endurance training increases mitochondrial density and fat-oxidation capacity, shifting Fatmax to a slightly higher percentage of VO₂max. Conversely, acute high carbohydrate availability and pre-exercise carbohydrate feeding tend to depress fat oxidation and may move crossover to lower intensities, while short-term high-fat diets can transiently reduce RER at a given workload. These effects are plastic but do not negate the fundamental dominance of carbohydrate near VO₂max [8,10].

Practical Application

1. Anchor intensities to your physiology. Expect maximal fat oxidation roughly around 55–72% VO₂max and diminishing fat contribution above about 85–90% VO₂max. In heart-rate terms this often aligns with mid-Zone 2 for Fatmax and high Zone 4 to Zone 5 for near-VO₂max work, though individual testing is best. Lab testing with gas analysis gives the clearest picture, but field data from long steady efforts and graded ramps can still inform your fuelling.

2. Fuel to match the session goal.

   • Low to moderate intensity endurance (≤ Fatmax to LT1): prioritise training availability over aggressive carb feeding. A modest intake may be adequate, especially on shorter sessions.

   • Threshold and VO₂max sessions: plan carbohydrate ingestion because oxidation rates are high and endogenous glycogen is finite. Carbohydrate delivery strategies that reach 60–90 g h⁻¹ for long threshold work or races are common in trained athletes, rising further if using multiple transportable carbohydrates [7,8].

3. Protect glycogen for the decisive moves. In long events featuring surges and climbs, preserving glycogen early with sensible pacing and fuelling ensures capacity for high-intensity bursts when fat cannot keep up.

4. Train the gut as you raise carb delivery. The intestine adapts to repeated carbohydrate feeding during training. Progressively increase carbohydrate intake in hard sessions and simulate race fuelling to reduce GI symptoms while hitting higher oxidation rates [8].

5. Remember the ceiling. Even with excellent fat adaptation, the high rates of ATP turnover near VO₂max require carbohydrate. Do not under-fuel high-intensity training or the latter stages of racing where the decisive moves occur.

When intensity climbs and you rely more on carbohydrate, digestion and absorption can become the limiting steps rather than supply on your bike or in your pocket. Carb Accelerator is designed with this physiology in mind. Its digestive-support blend includes enzymes relevant to carbohydrate handling, such as amylase and alpha-galactosidase, alongside proteases and lipase, and botanicals like peppermint, ginger and fennel that are traditionally used to support GI comfort. This profile reflects a science-led approach to supporting carbohydrate tolerance during demanding sessions where carb use is maximal.

In practice, athletes can pair higher carbohydrate intakes in threshold and VO₂max sessions with strategies that support gastric emptying and intestinal handling. A formulation that focuses on supporting carbohydrate absorption and reducing perceived GI burden may help you deliver the carbs your muscles can use when it matters most. A pragmatic approach is to test your race-day carb plan in key workouts while using a GI-supportive strategy like Carb Accelerator for enhancing sustained energy during endurance training on the sessions where intensity is high and the gut is challenged.

Conclusion

As pace, power and heart rate rise toward VO₂max, your muscles increasingly depend on carbohydrate because it can be mobilised, transported and oxidised at higher rates than fat. Training status and diet shift the crossover point, but the hierarchy remains. The practical response is simple: match fuelling to session intensity, protect glycogen for decisive efforts, and train your gut to tolerate the carbohydrate delivery that high-intensity performance demands. Layering science-led GI support, as in Carb Accelerator with progressive fuelling practice helps turn physiology into performance.

References

1. Brooks GA. Balance of carbohydrate and lipid utilization during exercise. Proc Nutr Soc. 1994;53(1):141–148.

2. Brooks GA, Mercier J. The importance of the crossover concept in exercise metabolism. Exerc Sport Sci Rev. 1997;25:67–92.

3. Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E, Wolfe RR. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am J Physiol. 1993;265:E380–E391.

4. Achten J, Jeukendrup AE. Determination of the exercise intensity that elicits maximal fat oxidation. Med Sci Sports Exerc. 2002;34(1):92–97.

5. Spriet LL. Regulation of fat metabolism in exercise. Ann Nutr Metab. 2014;64(3–4):Suppl 1:111–121.

6. Randell RK, Rollo I, Roberts TJ, Dalrymple KJ, Jeukendrup AE, Carter JM, Spriet LL. Nutritional strategies to optimise fat oxidation during exercise. Sports Sci Exch. 2022;No. 218.

7. Jeukendrup AE, Wallis GA. Measurement of substrate oxidation during exercise by means of gas exchange measurements. Int J Sports Med. 2005;26 Suppl 1:S28–S37.

8. Maunder E, Plews DJ, Kilding AE. Contextualising maximal fat oxidation during exercise: determinants and normative values. Front Physiol. 2018;9:599.

9. Muscella A, Stefàno E, Marsigliante S. The regulation of fat metabolism during aerobic exercise: a review. Biomolecules. 2020;10(12):1699.

10. Burke LM. Ketogenic low-carbohydrate, high-fat diet: the future of elite endurance sport? J Physiol. 2021;599(3):819–843.