Isotonic Drinks & Gels: What the Science Says
Isotonic drinks and energy gels are widely used in endurance sport. Their basic idea is simple: replace fluids and carbohydrates lost during exercise to maintain performance. But do the formulations actually deliver on this promise? Here is what the evidence from randomised controlled trials and meta-analyses shows.
What They Are and How They Work
An isotonic drink has an osmolality roughly matching blood plasma — approximately 270–330 mOsm/kg — which in theory allows rapid gastric emptying and intestinal absorption compared with hypertonic fluids. Energy gels typically deliver a concentrated carbohydrate load (often around 20–25 g per sachet) that athletes consume with water during prolonged exercise.
The primary mechanism is carbohydrate oxidation: exogenous carbohydrate (glucose, fructose, or maltodextrin) supplies fuel to working muscles when glycogen stores begin to deplete. A secondary mechanism involves electrolyte replacement (sodium, potassium), which supports fluid retention and nerve–muscle signalling.
What RCT and Meta-Analysis Evidence Shows
The most robust evidence concerns carbohydrate ingestion during exercise lasting more than 60–75 minutes. Jeukendrup (2014) reviewed the dose–response relationship and concluded that for events lasting 1–2.5 hours, carbohydrate ingestion at rates of around 30–60 g/hour provides a meaningful performance benefit, and that for events exceeding 2.5 hours, dual-source carbohydrates (glucose + fructose) at up to 90 g/hour can be oxidised and used more effectively than glucose alone.
Fluid replacement also has a well-supported evidence base. Shirreffs et al. (2004) demonstrated in a controlled trial that sodium-containing rehydration solutions improve fluid retention compared with plain water after exercise-induced dehydration, which is relevant to electrolyte-containing isotonic formulations.
Gel studies generally show equivalent carbohydrate delivery to liquid forms when consumed with adequate water. A practical consideration is palatability and timing: gels are faster to consume during racing but require co-ingestion of water to prevent gastrointestinal discomfort.
Effect Sizes and Who Benefits Most
Benefits are most clearly demonstrated for:
- Endurance events lasting more than 60–75 minutes — where glycogen depletion becomes a limiting factor.
- High-intensity intermittent sport (football, rugby) — where carbohydrate availability supports repeated sprint performance.
- Situations involving meaningful sweat loss — hot or humid environments where sodium replacement aids rehydration.
For exercise sessions under 45–60 minutes at moderate intensity, evidence for performance benefit from carbohydrate ingestion is weaker; plain water is generally sufficient for hydration.
EFSA-Approved Claims
The European Food Safety Authority has approved specific health claims for isotonic drinks and their components:
- Carbohydrate-electrolyte solutions (isotonic drinks): EFSA authorises the claim that these solutions enhance the absorption of water during exercise and contribute to the maintenance of endurance performance. This is one of the few sports-nutrition claims that has passed EFSA's scientific scrutiny.
- Sodium contributes to the maintenance of normal muscle function (EFSA Regulation 432/2012).
No EFSA-approved claim exists for energy gels as a category. However, the underlying ingredient claims for carbohydrates (which contribute to the recovery of normal muscle function after highly intensive and/or long-lasting physical exercise) apply to their carbohydrate content.
Honest Verdict
Isotonic drinks and gels are among the more evidence-backed categories in sports nutrition for their target use case: sustained endurance exercise lasting more than an hour. The carbohydrate and electrolyte replacement mechanisms are well understood, and the performance effects are replicated across multiple trials. The key caveats are dose (too little has no effect; too much causes gastrointestinal distress), timing (carbohydrates are most useful from around 45–60 minutes in), and context (short workouts don't need them).
For athletes training or competing in Estonia, isotonic products available at maxfit.ee cover a range of formats — ready-to-drink, powder, and gel — allowing matching to individual preference and exercise type.
References
Jeukendrup, A. E. (2014). A step towards personalized sports nutrition: carbohydrates during exercise. Sports Medicine, 44(Suppl 1), 25–33. PMID: 24791914
Shirreffs, S. M., Watson, P., & Maughan, R. J. (2007). Milk as an effective post-exercise rehydration drink. British Journal of Nutrition, 98(1), 173–180. PMID: 17459189 https://pubmed.ncbi.nlm.nih.gov/17459189/
EFSA Panel on Dietetic Products, Nutrition and Allergies. (2011). Scientific Opinion on the substantiation of health claims related to carbohydrate-electrolyte solutions and enhancement of water absorption during exercise. EFSA Journal, 9(6), 2211. DOI: 10.2903/j.efsa.2011.2211 https://doi.org/10.2903/j.efsa.2011.2269
FAQ
Are isotonic drinks necessary for every workout?
No. For sessions under 45–60 minutes at moderate intensity, water is generally sufficient. Isotonic drinks and gels are most useful for endurance exercise lasting more than an hour, particularly in hot conditions or when glycogen depletion is a real concern.
Can I use an energy gel without water?
It is not recommended. Energy gels are typically hypertonic (higher osmolality than blood), so ingesting them without water can slow gastric emptying and increase the risk of gastrointestinal discomfort. Always follow the manufacturer's instructions, which typically advise consuming a gel with water.
What is the difference between isotonic, hypotonic, and hypertonic drinks?
Isotonic drinks match blood plasma osmolality (~270–330 mOsm/kg), allowing relatively rapid absorption. Hypotonic drinks have lower osmolality (less solute than blood) and absorb fastest — ideal for rapid hydration. Hypertonic drinks have higher osmolality (more solute) — these slow fluid absorption and are better suited for post-exercise carbohydrate recovery than in-exercise hydration.
