Isotonic drinks – how to choose an effective rehydration drink and whether it’s worth drinking one every day

Is every ‘isotonic’ drink on the shop shelf actually isotonic? Find out what characteristics distinguish a genuine isotonic drink from an ordinary sweetened drink, and how to read the ingredients list so you don’t end up paying over the odds for marketing.

An isotonic drink is one with an osmotic concentration similar to that of blood plasma (typically 280–330 mOsm/l), designed to replenish fluids, electrolytes and carbohydrates as quickly as possible during and after physical exercise. The word ‘isotonic’ refers precisely to osmolality – that is, the concentration of dissolved substances – which is comparable to that circulating in our blood vessels. This characteristic means that an isotonic drink is absorbed more quickly than plain water, which is of significant importance during intense exercise. Sodium contributes to the body’s normal water and electrolyte balance, whilst carbohydrates help maintain performance during prolonged endurance exercise (when consumed in appropriate amounts during exercise) – both of these claims are authorised by the EFSA.

What exactly is an isotonic drink – a technical definition

From a technical point of view, an isotonic drink should meet several criteria. Firstly, its osmolality should be within the range of 280–330 mOsm/l (similar to that of plasma). Secondly, a carbohydrate content of 4–8 g per 100 ml (usually a mixture of several types of sugars for better absorption). Thirdly, sodium in quantities sufficient to effectively replenish losses through sweat (typically 20–50 mmol of sodium per litre).

Lower carbohydrate concentrations (below 4 g/100 ml) classify the drink as hypotonic – it is absorbed even more quickly, but provides less energy. Higher concentrations (above 8 g/100 ml) result in hypertonic drinks – more energy-dense, but absorbed more slowly. Each of these types has its own use, but isotonic drinks work best as a universal rehydration drink during exercise.

How to read the ingredients list of an isotonic drink – what to look out for

Three elements are key: carbohydrates, electrolytes and additives. Check the packaging:

Carbohydrates – it’s good if the ingredients list includes a mixture of several types of sugars (e.g. glucose, fructose, maltodextrin). A mixture of these sugars utilises different intestinal transporters, which speeds up energy absorption.

Electrolytes – sodium is absolutely essential (usually in the form of sodium chloride or sodium citrate). Many isotonic drinks also contain potassium, magnesium and calcium. Check that the amounts are realistic – if an isotonic drink contains only trace amounts of sodium (less than 200 mg per serving), its role in rehydration is debatable.

Additives – the simpler the ingredients, the better. Artificial colourings, synthetic flavourings and excessive sweeteners reduce the product’s value. A clean formula with natural flavourings is the hallmark of a good isotonic drink.

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Isotonic vs hypotonic vs hypertonic – when to choose which

Hypotonic (low in carbohydrates) is ideal when rapid rehydration is the priority – for example, in extreme heat, during short, less intense sessions, or when you don’t want to consume a large number of calories.

Isotonic is the gold standard for moderate- to high-intensity exercise lasting 60–180 minutes. Running, cycling, team sports, longer gym sessions – wherever both hydration and energy supply are important.

Hypertonic (high in carbohydrates) is best suited mainly for post-exercise, when the priority is to quickly replenish glycogen stores. During exercise, its slower absorption may be a limitation.

When to drink an isotonic drink – during and after exercise

The most obvious use for an isotonic drink is during physical exercise lasting longer than 60 minutes, particularly in hot conditions. Typical recommendations suggest consuming 400–800 ml of an isotonic drink per hour of exercise, divided into smaller portions every 15–20 minutes.

After training, an isotonic drink helps to replenish fluids and electrolytes lost through sweat, whilst the carbohydrates it contains provide the substrate needed to rebuild muscle glycogen stores. This is particularly important when training several times a week or when doing two training sessions in a single day.

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Is it worth drinking an isotonic drink every day?

The short answer is: for most people, no. Drinking an isotonic drink every day without a specific reason (such as intense exercise, hot weather or long hours of physical labour) introduces extra carbohydrates into the diet, which are not needed in the calorie balance of a person with average activity levels. Under such circumstances, plain water and a balanced diet are sufficient to maintain proper water and electrolyte balance.

Situations in which drinking an isotonic drink daily is justified include: regular intensive training, physical work in high temperatures, long journeys, regular visits to the sauna, and periods of recovery following an illness involving dehydration. In such circumstances, an isotonic drink can form part of a sensible daily routine. You can find out more about hydration and electrolytes in the articles isotonic drinks – natural hydration and what are electrolytes.

[tip: It is best to consume isotonic drinks at a temperature of 10–15 degrees Celsius – a cool drink is better tolerated by the digestive tract during exercise. If it is too cold, it may cause discomfort; if too warm, it is absorbed less effectively.]

Powdered isotonic drink vs ready-to-drink in a bottle

From a practical point of view, powdered isotonic drinks have several significant advantages. Firstly, you can prepare exactly the amount you need – without wasting any excess. Secondly, it is easy to adjust the concentration (a milder dilution for those with a sensitive stomach, a stronger one for intense exercise in hot weather). Thirdly, it is simply better value for money per serving.

Ready-made bottled drinks win on convenience – you can take them with you without any preparation. They’re a good solution for trips, journeys or competitions. In everyday training, however, the powder works better. You’ll find the full range of isotonic drinks and other training support products in the Sport and Recovery category.

[note: Vitaler’s isotonic drink has a pure formulation – sodium, potassium, magnesium and a blend of carbohydrates in the right proportions, with no artificial colourings or unnecessary fillers. The powder form allows for flexible dosing and the preparation of a fresh serving every time.]

Isotonic drinks and other training supplements

Isotonic drinks complement other supplementation solutions for active people well. It can be combined with creatine monohydrate (creatine dissolves well in an isotonic solution and can be taken in this form after training), with protein supplements (after training) or with pre-workout amino acids. You can find out more about the role of creatine in training in the article creatine monohydrate.

[Warning: People with diabetes, insulin resistance or metabolic disorders should take the carbohydrate content of isotonic drinks into account in their daily intake. Consulting a doctor or dietitian will help you choose the right hydration strategy.]

Summary: An isotonic drink is a beverage with an osmolality similar to that of blood plasma (280–330 mOsm/l), containing electrolytes (mainly sodium) and carbohydrates (4–8 g/100 ml). Its main advantage over water is the rate of absorption and the simultaneous replenishment of energy and electrolytes. It works best during exercise lasting over 60 minutes, in hot weather and after training. When choosing an isotonic drink, look out for a sufficient sodium content (over 200 mg per serving), a blend of sugars, and a simple, clean formula without artificial additives. The powder form is usually more practical and economical than ready-to-drink bottled drinks.