Isotonic Energy Gels: What the Science Says About Fueling Endurance Performance

For endurance athletes, fatigue is often less about motivation and more about physiology. As muscle glycogen declines and dehydration progresses, performance drops. Decades of sports nutrition research show that strategic carbohydrate and electrolyte intake during prolonged exercise can significantly improve endurance outcomes. Here's what the science tells us about isotonic energy gels and their key ingredients.

Carbohydrates: The Foundation of Endurance Fueling

Carbohydrates are the primary fuel source during moderate-to-high intensity endurance exercise. Muscle glycogen stores are limited and can become significantly depleted after 60–90 minutes of sustained activity.

Extensive research demonstrates:

• Consuming carbohydrates during prolonged exercise improves endurance performance and delays fatigue (Jeukendrup, 2014).

• Intake of 30–60 g of carbohydrate per hour improves performance during endurance events lasting 1–2.5 hours (Thomas et al., 2016).

• For events longer than 2.5 hours, up to 90 g per hour — especially when using multiple carbohydrate transportable forms (e.g., glucose + fructose) — further enhances performance (Jeukendrup, 2017).

 Carbohydrate ingestion helps maintain blood glucose levels, preserve glycogen stores, and sustain power output — key mechanisms in preventing "hitting the wall." Energy gels provide a compact, rapidly digestible source of carbohydrates that can be consumed without interrupting movement.

Isotonic Formulations and Gastric Emptying

The term isotonic refers to a solution with a similar osmotic concentration to body fluids. This influences gastric emptying and intestinal absorption.

Research shows:

• Gastric emptying is slowed when carbohydrate concentration is too high (hypertonic solutions), which can increase gastrointestinal discomfort (Rehrer et al., 1992).

• Solutions with moderate carbohydrate concentrations empty more efficiently and are associated with improved fluid delivery (Shi et al., 1995).

For athletes prone to stomach upset during competition, isotonic gels may reduce the need for excessive additional water and lower risk of GI distress compared to more concentrated products.

Electrolytes: Supporting Hydration and Performance

Sweat contains water and electrolytes, primarily sodium and smaller amounts of potassium. Replacing these losses supports hydration and physiological stability during prolonged activity.

Sodium

• Sodium enhances fluid retention and stimulates thirst, helping maintain plasma volume (Shirreffs & Sawka, 2011).

• Sodium improves intestinal glucose and water absorption via sodium-glucose co-transport mechanisms (Jeukendrup & Moseley, 2010).

Adequate sodium intake during long-duration exercise can reduce risk of performance decline related to dehydration and electrolyte imbalance.

Potassium

Potassium plays a role in neuromuscular function and cellular fluid balance. While sodium replacement is typically more critical, potassium contributes to overall electrolyte equilibrium (Maughan & Shirreffs, 2008). 

Vitamin C and Exercise Stress

Prolonged high-intensity exercise increases oxidative stress and may temporarily suppress immune function.

Research indicates:

• Vitamin C supplementation may reduce the incidence of upper respiratory tract infections in individuals under heavy physical stress (Hemilä & Chalker, 2013).

• While vitamin C does not directly enhance performance, it may support immune resilience during periods of intense training (Peake et al., 2007).

It is worth noting that chronic high-dose antioxidant supplementation may blunt some training adaptations, but moderate intake during heavy training blocks appears beneficial for immune support.

Practical Evidence-Based Recommendations

Based on consensus guidelines:

• Begin carbohydrate intake within the first hour of prolonged exercise.

• Consume 30–60 g per hour for events lasting 1–2.5 hours. • Increase up to 90 g per hour for ultra-endurance events using multiple carbohydrate sources.

• Include sodium during long sessions, particularly in hot conditions.

• Practice fueling strategies during training to optimize gastrointestinal tolerance.

These recommendations are supported by position stands from leading organizations such as the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine (Thomas et al., 2016). 

 The Bottom Line

Scientific evidence strongly supports carbohydrate and electrolyte intake during prolonged endurance exercise to delay fatigue and improve performance. Isotonic energy gels provide a convenient delivery system for rapidly absorbed carbohydrates and hydration-supporting electrolytes, with potential advantages in gastrointestinal comfort.

For endurance athletes, fueling is not optional — it is a performance strategy grounded in decades of physiology research.