Review: Hypotonic, Isotonic and Hypertonic Sports Drinks Compared

HOW DOES THE ‘TONICITY’ OF SPORTS DRINKS AFFECT HYDRATION?

The ‘tonicity’ of a drink is a measure of the relative concentration of sugars and salts compared with the concentration found in blood.  The difference between the two determines the direction and extent of fluid absorption after being consumed. A low tonicity (or hypotonic) drink will be absorbed quicker than a high tonicity (or hypertonic) drink. The science of sports drink ‘tonicity’ can make a big difference to your hydration in training and competition. Most sports drinks are marketed as being isotonic which means they should be of similar concentration (or tonicity) to your blood (~285 to 295mmol/kg).

The most effective sports drinks for rehydration are hypotonic, because their low tonicity drives absorption of fluid. These drinks contain less than 6% carbohydrates. Low levels of sugar is ideal, because fluid absorption in the body primarily occurs in the small intestine, and requires some glucose.

HOW TO BOOST THE IMPACT OF SPORTS DRINKS

Despite dehydration remaining a significant barrier to peak endurance performance, sports drink formulations haven’t substantially changed since they were invented over 50 years ago, and have two key shortcomings:

  • Tonicity – Most sports drinks have too much sugar for optimal hydration in the small intestine.
  • Gut hydration – All sports drinks ignore the hydration potential of the large intestine, which can absorb up to 5L of fluid per day.

Backed by over 20 years of medical research, PREPD is a hydration enhancer designed to address these two shortcomings to boost the impact of sports drinks and water alike. A two-step hydration system consumed pre- and post-exercise, the PREPD drinks feature a unique resistant starch, clinically proven to enhance fluid uptake in the large intestine.

In a recently published clinical trial, PREPD was able to demonstrate significantly better hydration before, during and after exertion in elite AFL football players compared with consuming leading sports drinks only. The full case study from the PREPD hydration trial can be viewed here.

ISOTONIC SPORTS DRINKS

Most sports drinks are isotonic and typically contain between 6-8% carbohydrates, providing athletes with a balance of energy, electrolytes and fluids.

However, according to Mettler et al 2006, many sports drinks marketed as being isotonic are actually slightly hypertonic in formulation such as Gatorade Orange (osmolarity: 350mmol/kg) and Powerade Mountain Blast (osmolarity: 391mmol/kg). Hypertonicity actually slows hydration by delaying gastric emptying of fluid from the stomach into the small intestine where it is absorbed. This can often cause gastrointestinal upset and leads to the feeling of fluid ‘sloshing in the stomach’ after consumption.

Isotonic sports drinks are therefore best used for short, high intensity exertion where energy is the priority rather than hydration. Popular sports and electrolyte drinks currently marketed as being isotonic include:

  • Gatorade
  • Gatorade Endurance
  • Powerade ION4
  • Staminade
  • Maximus
  • Science in Sport – GO Electrolyte
  • Pure Sports Nutrition – Pure Electrolyte Hydration
  • Endura Rehydration Performance Fuel
  • GU Hydration Drink Mix
  • Nuun Performance
  • Pro4mance Produrance
  • Isowhey Sports Electrolyte Formula
  • Bindi Sports Hydration

HYPERTONIC SPORTS DRINKS

Only a few sports drinks are marketed as being hypertonic – having a higher concentration than the blood – and typically have over 10% carbohydrate composition. The goal of these drinks is to rapidly provide a significant source of energy. This additional sugar content comes at the expense of hydration and can become especially problematic for athletes exerting for long periods or in the heat.

Hypertonic sports and recovery drinks are therefore most appropriate in scenarios where taking in energy and nutrients is the main priority and where dehydration is insignificant.

Popular sports and electrolyte drinks currently marketed as being hypertonic include:

  • GU Roctane Energy Drink Mix
  • Lucozade Energy

HYPOTONIC SPORTS DRINKS

Hypotonic sports drinks have less than 6% carbohydrate content and fluid is absorbed faster than isotonic drinks (Rowlands et al 2011). While providing less energy than isotonic drinks, the low concentration of hypotonic drinks relative to the blood allows for faster fluid uptake through the natural process of osmosis.

Hypotonic sports drinks are therefore recommended as being ideal for any exertion lasting over an hour or where hydration is a priority. Energy requirements can be better topped up through other food and fuel sources such as gels, bars and snacks, which are ideally consumed with some separation from substantial fluid intake.

Popular sports and electrolyte drinks currently marketed as being hypotonic include:

  • Hydralyte Sports
  • Mizone
  • SOS Hydration Drink Mix
  • G Active and G2
  • Powerade Zero
  • Endura Rehydration Low Carb Fuel
  • Nuun Electrolytes
  • Shotz Electrolyte tablets
  • Skratch Labs Sport Hydration Drink Mix
  • Aqualyte Solution
  • High 5 Zero
  • Pro4mance Prodrate
  • PREPD Recover (Post-Workout Hydration Enhancer)

SUMMARY

Choosing the right sports drink depends on your individual needs and involves trade-offs between fluid absorption and energy requirements. Given many isotonic sports drinks are actually slightly hypertonic in formulation, they can really be considered as ‘energy’ drinks rather than ‘hydration’ drinks. Where hydration is a priority, we recommend hypotonic sports drinks should be consumed during exertion to promote faster absorption.

Whether your favourite electrolyte drink to meet your performance needs is hypotonic, isotonic or hypertonic, PREPD can boost their hydration effectiveness and can even enhance the body’s absorption of water. PREPD Prime should be consumed 6-18 hours before intense exertion for better hydration when you need to perform and PREPD Recover should be consumed immediately after exertion for better rehydration in recovery.

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