Electrolytes or Isotonic Drink: What Is the Difference

On the shelves of sports nutrition, «electrolytes» in tablets or powder stand side by side with «isotonic drinks» in bottles or sachets. Both products are associated with hydration, so they are often considered interchangeable. In reality these are different categories: an electrolyte supplement replenishes the minerals lost with sweat, while an isotonic drink simultaneously supplies water, sodium and carbohydrates in a set concentration. Our editorial team has examined how they differ in composition, in the physiology of their action and in purpose.

What electrolytes are and what an isotonic drink is

In physiology, electrolytes are substances that in an aqueous solution break down into charged ions. For humans the key ones are sodium, potassium, chloride, magnesium and calcium, as well as bicarbonate. They maintain the volume of extracellular fluid, the membrane potential of nerve and muscle cells, muscle contraction and acid-base balance. In sports nutrition marketing, «electrolytes» refers to supplements that contain these minerals in the form of salts: tablets, effervescent tablets, capsules, powders and concentrated drops.

An isotonic drink is a ready-made beverage (or a powder for preparing it) whose concentration of dissolved particles is close to that of blood plasma. A classic sports drink contains water, carbohydrates (glucose, sucrose, maltodextrin), sodium, a little potassium, flavorings and acids. The purpose of such a composition is to simultaneously replace fluid, supply energy to working muscles and stimulate the absorption and retention of water.

So the main difference is not in the presence of minerals, they are in both products, but in the presence of carbohydrates and in the fact that an isotonic drink is a balanced «turnkey» solution. An electrolyte supplement usually contains no calories or minimal ones, is dissolved in an arbitrary volume of water or swallowed with water, so the final concentration depends on the user.

Between these poles there are intermediate products: hypotonic drinks with fewer carbohydrates, sugar-free «electrolyte drinks» with flavor enhancers, and medical oral rehydration solutions. Precisely because of the variety of names, the buyer should look not at the category label but at the composition table.

Osmolality: why the word «isotonic» matters

Osmolality is the number of osmotically active particles per kilogram of solvent. For blood plasma it is approximately 285–295 mOsm/kg. A drink whose osmolality is close to this range is called isotonic; lower is hypotonic; higher is hypertonic. This parameter determines how quickly fluid leaves the stomach and is absorbed in the small intestine.

Hypertonic solutions (for example, juices or carbonated drinks with 10–12% sugar) first «draw» water into the intestinal lumen, which slows net hydration and can cause gastrointestinal discomfort while running. Hypotonic and isotonic solutions are absorbed faster. At the same time, the presence of glucose and sodium accelerates water absorption thanks to the SGLT1 cotransporter, which carries sodium and glucose together, and water moves after them osmotically.

It is precisely on this mechanism that oral rehydration solutions recommended by the WHO for diarrhea are built: in the reduced-osmolarity formula they contain 75 mmol/L each of sodium and glucose at a total osmolarity of about 245 mOsm/L. Sports isotonic drinks use the same principle but with a higher carbohydrate content for energy and a lower sodium content for taste.

An electrolyte tablet dissolved in water usually gives a hypotonic solution, since there is not much salt in it and no carbohydrates. For replenishing fluid at rest this is acceptable, but the glucose-sodium cotransport mechanism is practically not engaged in such a drink.

Composition: sodium, potassium, carbohydrates

Sodium is the main electrolyte of sweat and the primary ion of extracellular fluid. The sodium concentration in sweat is highly individual: according to an ACSM review, on average it is about 35 mmol/L with a range of approximately 10 to 70 mmol/L. That is, two athletes with the same sweating intensity may lose several times different amounts of salt.

Potassium is present in sweat at a much lower concentration (a few mmol/L), so replacing it during a workout is of secondary importance, although it is part of most products. Magnesium and calcium are also lost with sweat in small amounts; their presence in «electrolytes» is more often marketing than physiologically necessary for short-term exertion.

Carbohydrates are the defining difference of an isotonic drink. The ACSM position on fluid replacement notes that sports drinks with about 6–8% carbohydrates and 20–30 mEq/L of sodium (about 460–690 mg/L) can support water-salt balance and performance during prolonged exertion. For comparison, a typical electrolyte tablet in 500 mL of water may provide from several hundred to a thousand milligrams of sodium, but zero carbohydrates.

The practical conclusion from the table: if replacing salt at a low energy need comes first, an electrolyte supplement is more logical; if it is important to simultaneously fuel the muscles with carbohydrates, the isotonic drink does the job. More on the choice for specific situations in the companion article of our editorial team.

What science says about effectiveness

For exertion lasting up to an hour of moderate intensity in cool conditions, ordinary water is enough for most people: sodium losses are still small, and glycogen stores are not depleted. The benefit of an isotonic drink here is weakly proven and, according to sports nutritionists, it shows mainly in very intense workouts.

During prolonged exertion (over 1–2 hours), carbohydrates during work steadily improve endurance. Jeukendrup, in his review, summarizes the reference points: about 30–60 g of carbohydrates per hour for exertion of 1–2.5 hours and up to 90 g per hour for longer work using a mix of glucose and fructose. An isotonic drink is one of the most convenient ways to partly cover this need.

Sodium is important above all for recovery after fluid loss. The classic study by Shirreffs and colleagues showed that after dehydration the retention of drunk fluid increases with a higher sodium content in the drink, and the total volume drunk must exceed the weight loss. That is, water without salt is largely excreted in the urine.

The studies by Maughan and colleagues, who proposed a «beverage hydration index», showed that drinks with a higher electrolyte content (in particular an oral rehydration solution) were retained in the body better than water, whereas an ordinary sports drink had no significant advantage over water at rest. This underscores: for hydration at rest, sodium is more important than carbohydrates.

During exertion itself, neither electrolytes nor an isotonic drink fully return the lost salts to the body. The goal is to prevent excessive dehydration (usually the reference point is a loss of no more than ~2% of body weight) and at the same time not to overdo it with fluid.

Risks, limitations and common mistakes

The most serious risk is associated not with supplements but with excessive drinking. Exercise-associated hyponatremia develops when a person drinks more fluid than they lose and the sodium level in the blood falls. The consensus on this topic emphasizes that the main cause is an excess of fluid; sports drinks contain too little sodium to prevent hyponatremia with excessive drinking. The condition can be life-threatening.

Isotonic drinks contain sugar. For people who train briefly and little, regular consumption of such drinks adds «liquid» calories and can work against a weight-loss goal; the effect of acids and sugar on tooth enamel should also be taken into account.

Electrolyte supplements with a large amount of sodium require caution in people with arterial hypertension, kidney and heart diseases. Preparations with a high potassium content are potentially dangerous in renal failure and while taking certain drugs (ACE inhibitors, potassium-sparing diuretics).

• Do not consider «electrolytes» a source of energy, they usually contain no carbohydrates.
• Do not drink «in reserve» beyond thirst and sweat losses.
• Do not dilute an isotonic drink more concentrated than indicated: a hypertonic solution is absorbed worse.
• Do not go by the product name alone, compare sodium in mg per serving and carbohydrates in g.

Editorial conclusions

An electrolyte supplement is primarily a source of salts without energy, whose final concentration depends on dilution. An isotonic drink is a balanced solution of fluid, sodium and carbohydrates with an osmolality close to plasma.

For short workouts, water is mostly enough. For prolonged and intense exertion, an isotonic drink gains the advantage thanks to carbohydrates, and with significant salt losses or in recovery after dehydration, products with a higher sodium content.

No product protects against hyponatremia if you drink significantly more than you lose. Go by thirst, the change in body weight before and after training, and the composition on the label.

How to choose between the two categories for your sport, our editorial team explains in the article «Electrolytes vs Isotonic Drink: what to choose and for whom». We also recommend materials on carbohydrates during training and on magnesium for athletes.

References

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