The Simple Reason Your Performance Fades — and the Fuel Strategy That Keeps You Going

You’ve been training hard. You paced yourself. You might even feel “in shape.”
But halfway through your event — whether it’s a long workout, a race, or a tournament — your performance collapses:

  • Legs feel heavy
  • Pace drops sharply
  • Breathing feels harder
  • Mental effort increases
  • Motivation evaporates

This phenomenon has a name every endurance athlete dreads:

Bonking.

If you want to perform consistently — not just explosively at the start — you need to understand the physiology of fuel, stress, and metabolism.

The good news? This is one of the few areas where evidence-based nutrition can meaningfully change outcomes.

What “Bonking” Really Means

Bonking is not just “feeling tired.” It is the moment your glycogen stores are depleted to the point that your muscles and brain lack the fuel needed for continued high-intensity work.

Carbohydrates are stored in the body as glycogen — roughly:

  • ~400–500 g in muscle
  • ~80–120 g in liver

That provides ~1,600–2,400 kcal maximum available energy from glycogen — but at high intensity, you can burn through those stores within 90–120 minutes of hard effort.

Once muscle glycogen drops:

  • Fat oxidation becomes the dominant fuel source, but fat cannot support high-intensity output
  • Power production drops sharply
  • Perceived exertion increases

In practical terms: your body runs out of its best fuel source.

This is metabolic, not mental. One solution is to “carb load”.

How Carbohydrate Loading Works

Carbohydrate loading is a strategy to maximize glycogen stores before an event, delaying the point at which you run out of fuel.

Classic carb-loading protocols (e.g., tapering training while increasing carbohydrate intake to ~8–12 g/kg/day 36–48 hours before an event) consistently show higher muscle glycogen levels and improved time to exhaustion in trained athletes.

What this means for you:

  • More fuel on the start line
  • Higher available energy at mile 50, 100, or minutes 60+
  • Slower depletion curve
  • Sustained pace with less dramatic drops

Why Carbs With Electrolytes Are Better Than Carbs Alone

Carb loading alone may not work as well for you as combining carbs with electrolytes. During exercise, carbohydrate absorption depends on intestinal transporters and hydration status. Without adequate fluids and minerals:

  • Absorption can be slower
  • GI distress can occur
  • Hydration status can lag
  • Muscle cramps become more likely

Electrolytes (sodium, potassium, magnesium, calcium) play key roles in:

  • Fluid balance
  • Nerve conduction
  • Muscle contraction
  • Preventing cramps

Drinks that combine carbohydrate with electrolytes outperform plain water and many sports drinks in maintaining performance and comfort.

By managing your carbohydrate intake, balancing electrolytes, and focusing on a holistic fueling strategy, you can ensure your race day results aren’t impacted by a bonk.

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References 

  1. Bergström J, Hermansen L, Hultman E, Saltin B. Diet, muscle glycogen and physical performance. Acta Physiologica Scandinavica. 1967;71(2–3):140–150. PMID: 5644767.

  2. Coyle EF. Substrate utilization during exercise in active people. American Journal of Clinical Nutrition. 1995;61(4 Suppl):968S–979S. PMID: 7699970.

  3. Jeukendrup A, Gleeson M. Sport Nutrition: An Introduction to Energy Production and Performance. Human Kinetics; 2010. (Foundational text summarizing carbohydrate fueling science.)

  4. Jeukendrup AE, et al. Effect of carbohydrate electrolyte solutions on gastric emptying and performance. Medicine & Science in Sports & Exercise. 1997;29(4):605–612. PMID: 9142996.

  5. Burke LM, Hawley JA. Swifter, higher, stronger: What’s on the menu? Science of performance nutrition. 2018; Sports Medicine. PMID: 29463001.