Isotonic Drinks Compared: What Lab Tests Reveal

What Is an Isotonic Drink?

The term "isotonic" means the drink's osmolality (concentration of dissolved particles) matches that of blood — approximately 280–300 mOsm/L. This has practical significance: an isotonic drink is absorbed quickly and efficiently from the digestive tract.

But not all "sports drinks" are actually isotonic, and an isotonic drink is not always the best choice.

Comparing Three Types of Drinks

When to Choose Which?

Hypotonic (low carbohydrate content):

• Intense training over a short period
• Hot conditions where rapid fluid replacement is the priority
• For those following a low-carbohydrate diet

Isotonic (6–8% carbohydrates):

• Endurance training (1+ hours)
• Optimal balance between fluid and energy
• The most versatile choice

Hypertonic (high carbohydrate content):

• Post-workout recovery
• Long endurance events (alongside a hypotonic drink)
• Not suitable as a primary hydration source during training

The Problem with Commercial Sports Drinks

Baker and Jeukendrup (2014) analysed sports drink effectiveness in their review and highlighted an important issue: many commercial drinks do not actually meet scientific criteria.

A typical commercial sports drink contains:

• 30–40 g sugar per bottle (often high-fructose corn syrup)
• Artificial colours
• Minimal electrolytes (often only sodium in small amounts)
• Flavouring, not functionality

A science-based isotonic drink should contain:

• 6–8% carbohydrates (glucose + fructose in optimal ratio)
• 460–1,150 mg sodium per litre
• Potassium and magnesium
• Minimal additives

Natural Alternatives

Coconut Water: The Natural Isotonic Myth

Coconut water is often marketed as a "natural isotonic drink." Let us look at the facts:

Coconut water contains:

• Potassium: ~250 mg/250 ml (good amount)
• Sodium: ~25 mg/250 ml (very low!)
• Carbohydrates: ~6 g/250 ml
• Magnesium: small amounts

Verdict: Coconut water is a good source of potassium, but its sodium content is too low for a sports drink. Athletes should add sodium.

DIY Isotonic Drink

Baker and Jeukendrup (2014) note that a simple recipe is scientifically sound:

Simple recipe (1 litre):

• 1 litre of water
• 1/4 teaspoon salt (~575 mg sodium)
• 2–3 tablespoons honey or sugar (~30–40 g carbohydrates)
• Optional: lemon juice for flavour

This yields approximately a 6–8% carbohydrate solution with sufficient sodium — scientifically appropriate.

What Lab Tests Reveal

Independent laboratories have tested several popular sports drink brands. Key findings:

1. Osmolality discrepancies — some "isotonic" products are actually hypertonic
2. Sodium content varies — many products contain too little sodium for effective hydration
3. Artificial additives — several products contain more additives than active ingredients
4. Price-to-value ratio — powder products are usually far more affordable than ready-to-drink bottles

Practical Summary

An isotonic drink is a scientifically justified way to ensure hydration and energy during training. But not all products bearing the label meet the criteria.

What to look for:

• Carbohydrate content 6–8%
• Sodium at least 460 mg/L
• Minimal artificial additives
• Consider powder format (customisable, more affordable)

References

• Baker, L.B. & Jeukendrup, A.E. (2014). Optimal composition of fluid-replacement beverages. Comprehensive Physiology, 4(2), 575–620.
• Shirreffs, S.M. & Sawka, M.N. (2011). Fluid and electrolyte needs for training, competition, and recovery. Journal of Sports Sciences, 29(sup1), S39–S46.
• Maughan, R.J. & Shirreffs, S.M. (2010). Development of individual hydration strategies for athletes. International Journal of Sport Nutrition and Exercise Metabolism, 20(6), 457–472.

Read more:

• Electrolyte products at MaxFit

Disclaimer

Dietary supplements are not a substitute for a varied, balanced diet and healthy lifestyle.