Trail Running Hypoglycemia: Understand & Prevent Energy Bonks

Illustration hypoglycemia and nutrition in trail running

Hypoglycemia in Trail Running: Why You Bonk and How to Prevent It

In trail running, every runner knows that dreaded moment when energy suddenly plummets. Legs feel heavy, the mind gets foggy, and every climb seems insurmountable. We often call it "hitting the wall" or "bonking." But behind this term lies a lot of confusion. Understanding the mechanisms of glycemic regulation during exercise can help prevent these energy crashes and optimize your nutritional strategy.

Diagram of glycemic regulation during effort

Blood Glucose: Your Safety Fuel

Blood glucose refers to the amount of glucose present in the blood. It fuels several essential systems:

  • Muscles for contraction
  • The brain for maintaining cognitive functions
  • Alertness and decision-making ability
  • The capacity to maintain effort intensity

During exercise, your body deploys sophisticated regulatory mechanisms to stabilize blood glucose levels. Insulin decreases, glucagon increases, and the liver releases glucose into the bloodstream. Your body switches to an "internal refueling" mode.

However, during prolonged effort, hepatic reserves gradually deplete. When the liver reaches its capacity limit, an energy bonk occurs.

The Real Risk in Trail Running: Lack of Regular Intake

Reactive hypoglycemia frightens many trail runners. Yet, during exercise, the hormonal context changes radically: insulin is inhibited, while glucagon and catecholamines actively support glucose release.

The real problems stem from three factors:

  • Starting with insufficient glycogen reserves
  • Irregular or insufficient carbohydrate intake
  • Effort that extends beyond what was planned

The risk significantly increases when you wait until you're hungry or feel a dip in energy before eating. In trail running, the winning strategy is to fuel your effort proactively, before your body signals an urgent need.

Glycemic Index: Less Important Than Regularity

For a long time, low glycemic index foods were favored to "avoid hypoglycemia." While the idea seems appealing, in a race situation, other factors prove to be more decisive:

  • The total quantity of carbohydrates ingested
  • The frequency of food intake
  • Individual digestive tolerance
  • The ability to stick to the planned nutrition strategy

A nutritionally "perfect" food on paper is useless if it stays at the bottom of your pack or causes digestive issues. The best carbohydrate is the one you can consistently absorb, not the one with the best theoretical profile.

Trail nutrition strategy and carbohydrate intake

What Science Says About Glycemic Regulation During Effort

Scientific research on blood glucose in endurance

The work of Trefts et al. (2015) demonstrates that exercise profoundly modifies glycemic regulation: insulin decreases while glucagon and catecholamines stimulate glucose release by the liver.

However, hepatic reserves remain limited. According to Borghouts & Keizer (1999), after an overnight fast, the liver contains approximately 75 to 100 g of glycogen. During prolonged effort, this stock gradually decreases, and gluconeogenesis (production of glucose from other substrates) takes over but is often insufficient to maintain stable blood glucose levels.

Scientific Recommendations for Carbohydrate Intake

Current recommendations based on effort duration:

  • Effort less than 1 hour: small amounts of intake possible
  • Between 1 hour and 2 hours 30 minutes: 30 to 60 g of carbohydrates per hour
  • Beyond 2 hours 30 minutes: 60 to 90 g per hour, with multiple carbohydrate sources (glucose + fructose)

The key lies in preserving blood glucose through regular intake, adapted to the duration and intensity of the effort.

Practical trail mountain nutrition application

Practical Strategy to Prevent Energy Bonks

Here are concrete actions to implement to prevent hypoglycemia in trail running:

  • ✔️ Arrive at the start with full glycogen reserves
  • ✔️ Plan a carbohydrate snack if your last meal was several hours ago
  • ✔️ Start fueling early in long runs, with small intakes every 15 to 20 minutes
  • ✔️ Test gels, energy drinks, purees, and bars during training
  • ✔️ Plan a safety margin in case of cold weather, night effort, course errors, or unexpected pace drops
  • ✔️ Seriously refuel with carbohydrates after an ultra to support recovery

Key Takeaways on Hypoglycemia in Trail Running

Hypoglycemia in trail running primarily results from a mismatch between energy expenditure and nutritional intake. At the beginning of the effort, the body effectively regulates blood glucose thanks to its hormonal mechanisms. Over the long term, hepatic reserves reach their limits.

The glycemic index of foods matters less than the regularity of intake, the total amount consumed, and individual digestive tolerance. Nutrition in trail running should be approached like a training session: with method, progressivity, and adaptation to your profile.

— Antoine, Baptiste, and Etienne from TRL