Contributed by Dr. Byron Fergerson, Freak Shake Endurance Chief Scientist

Every endurance athlete eventually learns one lesson the hard way: you can’t outrun biochemistry. That hollow-leg feeling mid-race — when your pace drops, your brain fogs, and every mile feels uphill — is glycogen depletion. To avoid “the bonk,” you have to understand how your body manages the constant trade-off between fat and carbohydrate fuel.

The Endurance Engine: Two Fuels, Two Speeds

Your aerobic engine can burn two major fuels: fat and carbohydrates.

• Fat is slow but abundant — even lean athletes carry tens of thousands of calories of it. It’s the diesel of the endurance world: energy-dense but oxygen-hungry, burned best at lower intensities.

• Carbohydrates, stored as glycogen in muscle and liver, are premium octane. They yield ATP (cellular energy) fast, but the tank is small.

Inside your muscle cells, mitochondria oxidize these fuels to create ATP. Fatty acids must be shuttled and processed through multiple steps (β-oxidation → Krebs cycle → electron transport) before energy is released. Glycogen-derived glucose jumps straight into glycolysis and burns quickly — perfect for hills, sprints, or tempo efforts when oxygen delivery can’t keep up.

This trade-off defines endurance performance: at easy paces, you can rely mostly on fat oxidation; as intensity climbs, the body recruits more carbohydrate because it can generate energy three to four times faster than fat can.

The 90-Minute Limit

Here’s the catch: glycogen stores are limited. Even a trained endurance athlete tops out around 400–600 grams in muscle plus 100 grams in the liver — roughly 2,000–2,500 calories, or about 90 minutes of moderate effort.

Once glycogen runs low, your brain and muscles both hit energy conservation mode. Fat alone can’t supply ATP fast enough, so you slow down whether you want to or not. That’s “hitting the wall.”

Without carb resupply, power output falls, perceived exertion spikes, and coordination falters. You don’t bonk because you’re weak — you bonk because you’ve run out of accessible fuel.

Training for Glycogen Sparing

The good news: your metabolism is trainable. Consistent aerobic work — long runs, Zone-2 rides, endurance rows — builds mitochondria, capillaries, and fat-oxidation capacity. This lets your body spare glycogen for when it matters most: climbs, surges, and finishing kicks.

This “glycogen-sparing effect” allows trained athletes to sustain higher paces while relying more on fat and delaying fatigue.

Both base endurance and HIIT sessions stimulate a key cellular adaptation called mitochondrial biogenesis — the creation of new mitochondria inside muscle cells. More mitochondria mean higher capacity for fat and oxygen utilization, and better glycogen preservation. Each method activates different signaling pathways, so mixing steady aerobic work with tempo and VO₂-max intervals helps maximize adaptation over time.

Fueling strategy matters too. Instead of “training low,” most athletes benefit from carb periodization — fueling key sessions generously while letting easy aerobic days encourage fat metabolism.

Finally, incorporating Freak Shake’s endurance-focused flavonoid stack can complement training adaptations. In a clinical study using Grit Fuel VO₂ ax Endurance, two weeks of use increased baseline cell activity by an average of 23%, a strong proxy for improved aerobic efficiency. These natural plant compounds have been shown to stimulate mitochondrial biogenesis — even in the absence of exercise — enhancing the same endurance pathways your training targets. [1-4]

Fueling Strategy: Don’t Wait Until Empty

Because glycogen is finite, early and steady fueling is non-negotiable in endurance sports. Research shows that consuming 30–90 grams of carbohydrate per hour maintains blood glucose and spares muscle glycogen (dependent on athlete muscle mass & intensity of exercise). Ingesting glucose + fructose together (a.k.a. cane sugar) also improves absorption, glycogen replacement rate and comfort over long durations.

Hydration and sodium balance matter too — they affect how efficiently carbs are delivered and oxidized.

Recovery & Refill

Post-exercise, your body’s “glycogen window” opens wide for about two hours. Pairing carbohydrates with protein accelerates glycogen resynthesis and muscle repair. This is where Freak Shake Recovery shines — designed to restore glycogen, rebuild muscle, and reset for the next session.

Meanwhile, Freak Shake Grit Fuel Endurance capsules has been clinically demonstrated to support mitochondrial function — the cellular machinery behind glycogen sparing itself. Together, they help your aerobic engine run longer, cleaner, and more resilient.

The Takeaway

Glycogen sparing isn’t just about maintaining a steady inflow of carbohydrates to sustain effort — it’s about getting the most out of your carb fuel. Endurance performance is a constant negotiation between fat’s abundance and carbohydrate’s speed. Training your metabolism to spare glycogen and your gut through consistent fueling to stem depletion, is the objective of progressively increasing miles and minutes.

And of course don't neglect adequate recovery to completely replenish the tank before your next progressive workout.

References

• Craig DM, Ashcroft SP, Belew MY, et al. Utilizing small nutrient compounds as enhancers of exercise-induced mitochondrial biogenesis. Front Physiol. 2015;6:296. doi:10.3389/fphys.2015.00296

• Davis JM, Murphy EA, Carmichael MD, Davis B. Quercetin increases brain and muscle mitochondrial biogenesis and exercise tolerance. Am J Physiology-regulatory Integr Comp Physiology. 2009;296(4):R1071-R1077. doi:10.1152/ajpregu.90925.2008

• Zbinden-Foncea H, Castro-Sepulveda M, Fuentes J, Speisky H. Effect of Epicatechin on Skeletal Muscle. Curr Med Chem. 2022;29(6):1110-1123. doi: 10.2174/0929867329666211217100020. PMID: 34923936.

• Daussin FN, Heyman E, Burelle Y. Effects of (-)-epicatechin on mitochondria. Nutr Rev. 2021 Jan 1;79(1):25-41. doi: 10.1093/nutrit/nuaa094. PMID: 32989466.