What does hypotonic, isotonic and hypertonic mean?
The tonicity of a drink determines how concentrated it is compared to body fluids (approx. 285–295 mOsmol/kg):
| Drink type | Osmolarity | Characteristics |
|---|---|---|
| Hypotonic | < 275 mOsmol/kg | Less concentrated than body fluids |
| Isotonic | 275–300 mOsmol/kg | Approximately the same concentration as blood plasma |
| Hypertonic | > 300 mOsmol/kg | More concentrated than body fluids |
The difference in concentration determines how quickly the liquid is absorbed from the intestines into the blood - that is, how effectively the drink hydrates you.
What the meta-analysis of Rowlands et al. (2021)
The authors analyzed 28 studies, 68 drinking protocols, and 258 measurements of blood plasma volume change (ΔPV) during continuous exercise.
| Drink type | Average change ΔPV | Effect summary |
|---|---|---|
| Hypotonic | − 6.3% | 🏆 Best hydration |
| Hypertonic | − 7.4% | Better than water, but worse than hypo |
| Isotonic | − 8.7% | The worst of electrolyte drinks |
| Water | − 7.5% | Similar effect to hypertonic, without energy |
Hypotonic carbohydrate-electrolyte drinks consumed regularly during exercise provide the greatest hydration benefits.
Hypertonic vs. isotonic: small difference, but no advantage
At first glance, it might seem that the higher carbohydrate content in hypertonic drinks would improve hydration, but according to a meta-analysis, this is not the case.
The change in blood plasma volume was −7.4% for hypertonic drinks, while −8.7% for isotonic drinks. The difference exists, but it is small and statistically uncertain (probability of superiority p⁺ = 0.74) .
Additionally, hypertonic solutions can cause water to move in the opposite direction – from the tissues back to the intestine , which actually worsens hydration.
Therefore, a hypertonic drink is more suitable after exercise (e.g. to replenish energy and glycogen), while a hypotonic drink provides the best effect during exercise .
Why a hypotonic drink works best
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Faster absorption: lower concentration promotes the passage of water from the digestive tract into the blood.
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Less burden on digestion: it does not linger in the stomach, does not cause a feeling of heaviness.
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More efficient energy intake: the combination of glucose, fructose and maltodextrin allows for better absorption.
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Better thermoregulation: higher plasma volume improves blood flow to muscles and body cooling.
When to use each type of drink
| Drink type | Suitable use | Main purpose |
|---|---|---|
| Hypotonic (< 275 mOsm/kg) | During training or competition > 45 min, in heat, at high intensity | Fastest hydration, electrolyte replenishment and partial energy |
| Isotonic (275–300 mOsm/kg) | Short workouts (up to 1 hour) or when you want more energy than fluids | Balanced compromise, but slower absorption |
| Hypertonic (> 300 mOsm/kg) | After exercise or when replenishing glycogen | Rehydration and energy restoration (high carbohydrate content) |
| Water | Normal drinking regimen outside of exercise or during light activity | No energy or electrolytes – can dilute blood sodium levels during exercise |
Summary
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Hypotonic drinks best support hydration and maintain performance during exercise.
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Isotonic is more suitable for shorter workouts or because of the taste.
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Hypertonic benefits after exercise – for energy and glycogen recovery.
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Water is sufficient outside of sports, but it will not replace an ionic drink during exercise.
An ideal hypotonic drink has an osmolarity of 200–260 mOsmol/kg , contains multiple types of carbohydrates (e.g. glucose and fructose in a 2:1 ratio) and about 15–20 mmol/l of sodium .
Reference:
Rowlands, DS, Kopetschny, BH, & Badenhorst, CE (2021). The hydrating effects of hypertonic, isotonic and hypotonic sports drinks and waters on central hydration during continuous exercise: A systematic meta-analysis and perspective. Sports Medicine, 52 (2), 349–375. https://doi.org/10.1007/s40279-021-01558-y
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