What Is An Isotonic Sports Drink? – The Complete Guide

What Is An Isotonic Drink?

Isotonic drinks, sometimes called sports drinks or electrolyte drinks, are beverages that aim to assist the performance and recovery of athletes by providing a rapid replenishment of water, energy, and electrolytes that are expended during exercise. Isotonic drinks mainly consist of water, with added minerals such as sodium and potassium, and carbohydrates which are usually in the form of simple sugars. Isotonic drinks can be used before, during, or after a period of exercise, but appear to offer the best benefit to the athlete when consumed during the period of exercise. [1] [2]

An isotonic solution is one that has the same concentration of solutes as another solution across a semipermeable membrane. In the case of Isotonic drinks, the aim is to emulate the concentration of sodium chloride as cellular fluids found in the human body under normal circumstances .

In more common cultural usage, the term isotonic sports drink covers a wide range of beverages. Many companies offer ready made sports drinks for sale in supermarkets, with popular brand names such as Lucozade, Gatorade, and Powerdade, and Prime Hydration being marketed at athletes.

Isotonic drinks can also be made at home using fresh fruit juice and salt to create a solution that contains fluid, natural sugars and sodium. The hydration potential of such homemade drinks depends on the ratios of each component being correct. For best results, the amount of salt in a home made isotonic drink should not exceed 250mg per 500ml of fluid.

For a quick 60 second summary of this article, you can see our video on the topic here:

We will now take a closer look at how isotonic drinks work, and explain the science behind their action.

What Does An Isotonic Drink Do?

When humans exercise and partake in strenuous or extended physical activity, water and electrolytes are lost in sweat. This effect is magnified further by hot weather. When dehydration reaches 2% or more of the body mass, issues can arise such as elevated cardiovascular strain, impaired thermoregulatory function, and impaired aerobic exercise performance. Isotonic sports drinks can help to relieve dehydration at a cellular level due to their balance of carbohydrates and electrolytes, which align with the natural concentrations of fluids in the body. [3] [4]

A semipermeable membrane separates two solutions with different concentrations of solute. However, nature seeks an equilibrium, and water molecules seek to move from the dilute side (less solute) to the concentrated side (more solute) until the concentrations of water and solute are equalised on both sides. The semipermeable membrane in question when it comes to rehydration is found in the cell walls. An isotonic drink aims to utilise the natural principles of this concentration gradient to stimulate the movement of water across the semipermeable membrane and into cells. Correctly made isotonic solutions enable hydration at a rate that is safer and more efficient than drinking water by itself.

During endurance exercise, both dehydration and overhydration can disrupt a person’s fluid-electrolyte balance. Exertion induced dehydration is caused by water and sodium losses in sweat and urine that are incompletely replaced. Overhydration during or after exercise is also a risk however, and is caused by excessive consumption and retention of dilute fluids.

When fluid intake exceeds that of water lost via urine, sweat, and insensible respiratory and gastrointestinal losses (often greater than 1.5 litres), athletes may retain free water, leading to a condition called Exercise-associated hyponatraemia (EAH). EAH is not common amongst the general public, but can be a significant health risk for endurance athletes. One of the main dangers of hypoatraemia is that because of the osmotic gradient, free water is drawn from the vascular space into the extravascular and intracellular spaces. This can lead to excess water being drawn into tissues, including the brain and lungs, which swell with the water. The swelling of major organs is responsible for the most severe symptoms of EAH. [5] [6]

Because the cells have the same concentration of minerals such as sodium as the isotonic solution, they do not shrink by losing more water, nor do they risk oedema (swelling due to fluid retention) with excess water. This cellular consistency is what enables an isotonic drink to helps cells recover from dehydration quickly, and reduces the risk of cellular swelling or damage that can be caused by extreme rehydration.

Isotonic drinks can help provide a hydration middle ground to optimize physical performance, and reduce the risk of exertion related illness. [7]

Types Of Isotonic Sports Drinks

An isotonic solution therefore must replicate the natural balance of water and electrolytes found in the body. However, not every sports drink contains the same volumes of minerals and carbohydrates, and different formulas will produce different type of solutions. Isotonic sports drinks range from volumes of 4-8% carbohydrates, and the variation in this range determines if the drink is isotonic, hypotonic, or hypertonic.

Hypotonic Drinks

Hypotonic drinks are a more dilute solution that contains a 4% carbohydrate solution or less, which equates to 40g or carbohydrate per litre of fluid. Hypotonic drinks offer a lower concentration of salt and sugar than the normal circulating values in the fluids found in the human body. There is evidence that despite being lower in carbohydrates and salt than other forms of sports drinks, hypotonic drinks may be the most effective option for rehydration. [1]

Hypertonic Drinks

Hypertonic drinks are ones that contain greater than 8% carbohydrate, which is equatable to 80g of carbohydrate per litre of fluid. Due to their high carbohydrate and salt content, hypertonic drinks are best consumed slowly throughout a period of extended exercise. Excessive carbohydrate content in a sports drink may have an adverse effect, especially in cases when the drinks are consumed when cellular dehydration is not present.

Similarly, the amount of sodium present in sports drinks should aim to emulate normal human ranges, and increasing the sodium content beyond this limit will eventually decrease the hydration capacity and start to produce a dehydrating effect.

With this in mind, let’s take a closer look at why the sodium balance in an isotonic drink plays such a crucial role in the drinks hydration capacity.

Why Is There Salt In Isotonic Drinks?

Sodium is the most abundant electrolyte in sweat, with a typical range of 20–80 mmol per litre of sweat. To achieve effective restoration of body water and retain ingested water, electrolytes lost in sweat must also be replaced. Sodium in isotonic sports drinks helps the body to retain fluid, reducing fluid lost in urine and sweat. It has been noted that as the sodium concentration in a drink increases, urine output decreases. Studies indicate that isotonic drinks with sodium solutions greater than 30mM offer the best rehydration, as it appears that the addition of sodium at a concentration equal to or greater than that of the sweat lost is required to maintain a positive fluid balance. [8] [9] [10]

Sodium in sports drinks often takes the form of salt. Salt molecules are a combination of sodium atoms (Na) bound to chloride atoms (Cl) to create sodium chloride (NaCl). Upon ingestion, sodium is absorbed in the gastrointestinal tract, and the levels of sodium in the body are regulated by the kidneys.

Sodium is found as a positively charged ion (Na+ cation) in the fluid outside of cells called the extracellular fluid. Sodium is largely responsible for maintaining the volume of the extracellular fluid, and also regulates the membrane potential of cells.

The extracellular fluid is high in sodium, and contains low levels of potassium. The fluid on the inside of cells is called the intracellular fluid, has the inverse ratio – it is low in sodium, but high in potassium. This imbalance is held in place across the semipermeable membranes of the cells, and helps regulate the flow of water in and out of the cell.

When sodium levels are restored through consumed fluids containing salt, the plasma Na+ cations remain outside the cells in the extracellular fluid, but their presence assists water molecules to enter cells which are suffering the most from dehydration. The presence of sodium cations also acts as a preventative measure to stop excess water flooding and swelling a cell. [11]

It is important to remember that the sodium present in an isotonic drink is there to restore the natural balance of water and electrolytes, and help draw water into dehydrated cells. Adding too much salt to an isotonic drink however, has the opposite effect to this, and creates a hypertonic solution. When sodium levels in the extracellular fluid becomes too high, the imbalance created will start drawing water out of cells in osmosis, leading to an increase in cellular dehydration. This is the same mechanism by which drinking seawater leads to further dehydration the more an individual drinks of it.

Those making isotonic drinks at home should be cautious about adding excessive sodium to the solution. The sodium content of the solution is best kept around 5-6%, which is why the amount of salt in a home made isotonic drink should not exceed 250mg per 500ml of fluid.

Why Is There Sugar In Isotonic Drinks?

The sugars in isotonic drinks have a synergistic effect with sodium, allowing for more efficient replacement of both water and salt losses. Carbohydrates such as glucose are able to improve both water and sodium absorption in the small intestine. Isotonic sports drinks are typically made with a solution of 4-8% carbohydrates, equivalent to 40-80g of carbohydrate per litre. Zero sugar isotonic drinks are also sold by many companies in which the sweetness of carbohydrates is replaced with artificial sweeteners. Sugar free sports drinks however, may not provide the same support to an athlete, as the sugars play several key roles in assisting exercise performance and recovery. [12] [13] [14]

As well as assisting the absorption of water, the carbohydrates in sports drinks may assist in replenishing energy expended during physical activity. Exercise metabolism requires carbohydrates, with the primary sources being muscle glycogen, and circulating volumes of glucose and lactate. Exercise increases whole-body glycogen utilisation, and the breakdown of liver glycogen to maintain blood glucose concentrations becomes increasingly important as exercise intensity increases. [15] [16]

The result of this is that prolonged exercise at moderate-to-high exercise intensity results in depletion of liver glycogen stores unless carbohydrates are ingested during exercise. Low carbohydrate (glycogen) availability in muscles and the liver is known to be a cause of fatigue during periods of prolonged physical activity. The sugars provided in isotonic drinks may therefore play a role in replenishing the spent carbohydrates, and reduce exertion related fatigue. Indeed, this was the conventional logic behind the long time Sunday League Football tradition of eating fruit at half-time. [17] [18]

Glucose is the carbohydrate perhaps most often associated with isotonic drinks and so called ‘energy tablets’, as revealed by naming conventions such as Lucozade being a compound word of Glucose Aid. Evidence is emerging that fructose from fruit can assist glycogen repletion. Fructose is a natural fruit sugar, and it is the most water soluble of all sugars. Fruits naturally high in fructose include apples, grapes, and watermelons. [19]

Excess fructose consumption has been linked with an increased risk of negative health outcomes including diabetes and obesity for those with sedentary lifestyles. Despite this, in situations of intense physical exertion, the script is flipped on fructose, and it can offer multiple benefits to an athlete. It appears that fructose and exercise have a synergistic relationship, with fructose providing support to exercise, and exercise mitigating against fructose’s negative health impacts. [17] [19] [20]

Fructose stimulates rapid fluid and solute absorption in the small intestine, and helps increase exogenous carbohydrate oxidation during exercise, an important response for improving exercise performance. Upon digestion, fructose is absorbed by the gut directly into the blood of the portal vein, before being converted by the liver into glucose. [19]

Evidence consistently demonstrates that fructose-containing sugars accelerate post-exercise liver glycogen repletion and could reduce recovery time by as much as half when compared to ingestion of glucose polymers only. Therefore, athletes aiming for rapid recovery in multi-stage events should consider ingesting fructose-containing sugars to accelerate recovery.

Javier T. Gonzalez, James A. Betts – Dietary Sugars, Exercise & Hepatic Carbohydrate Metabolism (2018) [17]

This aspect of fructose is a welcome benefit for those making isotonic drinks at home using fresh fruit, as the natural blend of sugars in many fruits seem to offer support in exercise recovery. As with sodium levels however, simply increasing the carbohydrate and sugar content of an isotonic drink does not make it ‘better’, and the correct balance is required to replenish expended glycogen without causing blood sugar spikes. Drinks with carbohydrate contents over 8% (80g carohydrate per Litre) appear to offer less benefit as a sports hydration option. [1]

When Should You Have An Isotonic Drink?

Due to their ability to safely and efficiently rehydrate athletes in a state of cellular dehydration, isotonic sports drinks are best consumed before, during, or after extended periods of physical exercise. Research suggests that isotonic drinks can provide the best hydration for an athlete when consumed gradually throughout the exercise. However, whilst isotonic drinks are mainly used in the context of exercise, their ability to provide rapid hydration has led them to be used in situations outside of a sporting or athletic environment. [1] [21]

It is known that young adults in many Western countries use sports drinks as a hangover cure, with the rapid hydration potentially able to relieve alcohol induced dehydration. Alcohol is a diuretic, and triggers the renal system to remove of fluids from the blood. It is perhaps therefore no surprise that the rehydration qualities of isotonic drinks seem to be able to reduce the severity of headaches associated with alcohol use. [22]

The beneficial effects of an isotonic sports drink can help to combat dehydration caused by diarrhoea, which can cause a high rate of fluid loss for the individual. This ability to offer rapid and thorough rehydration is why solutions of water, sugar, and electrolytes are used to combat the dehydration caused by chronic, severe diarrhoea, which is responsible for a significant number of infant deaths in countries such as Ethiopia. Whilst isotonic formulas do not treat the underlying cause of chronic diarrhoea, the quick hydration they offer can mitigate against the worst effects of dehydration. [23]

Are Isotonic Drinks Bad For You?

Due to their ability to combat cases of moderate to severe dehydration and electrolyte loss resulting from physical exercise, isotonic drinks can play a role in supporting athletes partaking in high intensity exercise. Their rehydration properties may also be useful for those suffering from severe diarrhoea. Whilst isotonic sports drinks make the perfect remedy in such situations, outside of a these contexts, isotonic drinks are simply a flavoured beverage high in sugar and salt. Under normal circumstances, water provides a better option than an isotonic drink for rehydration.

Consumers should also be aware that many highly processed products sold under this category may have an adverse effect on health when consumed regularly. Many commercially sold products marketed as isotonic drinks are high in free sugars, meaning they are sugars not found naturally inside the cell of a food. Free sugars added to isotonic drinks include dextrose, a glucose enantiomer typically derived from corn or wheat that structurally mirrors glucose. This chemical mirroring gives dextrose the name D-Glucose. [24]

Being labelled as a sports drink, manufacturers claim that these sugars are added to sports drinks as a means of improving water absorption and recovery from exercise. The high sugar content however, constitutes a significant health risk for individuals not participating in any form of exercise.

Isotonic Drinks Sugar Content

Many commercial sports drinks contain similar amounts of free sugars to conventional sweet fizzy drinks and sodas. Indeed, many isotonic sports drinks are produced by the same companies as sodas, with CocaCola and PepsiCo manufacturing Powerade and Gatorade respectively. As a result, when we look at how much sugar is in isotonic drinks regularly sold by these major brands, we find that a typical commercial isotonic sports drink contains sugar levels that range from 17.2g to 22.5g of sugar. These volumes of free sugars are nearly identical levels of added sugar as several well known soft drinks in the United Kingdom. [25]

The excess consumption of sugar sweetened beverages has been linked to a greatly increased risk of the onset of type 2 diabetes and cardiovascular disease. For this reason, the World Health organisation have stated that under ideal circumstances, free sugars should constitute a maximum of 10% of an individuals total energy intake for a day. [26] [27] [28]

Simply switching the drink to a sugar free version of the product may sound like an obvious solution to this issue, but alas, it is not the ideal scenario. Earlier, we discussed the benefits of that carbohydrates provide in a sports drink. For the dehydrated and fatigued athlete, the sugar content in a sports drink is a positive, not a negative. Sugar free sports drinks therefore are a slight contradiction, and appear to be a marketing decision to open up the products to a wider audience.

Individuals who are not engaged in strenuous exercise or activity in hot weather simply do not require the sugar and sodium content provided by isotonic sports drinks. As such, they should not be considered a casual drink to have when ones feels thirsty, but rather as a limited use option for cases of genuine dehydration. Regular consumption of sports drinks without accompanying exercise is likely to be a risk factor in the development of chronic metabolic illnesses. [22]

“There is compelling scientific evidence linking sugar-sweetened beverage consumption to increased risk for obesity, type 2 diabetes and cardiovascular disease in the overall population, and particularly among young adults.”

Nicole Larson , Melissa N Laska , Mary Story and Dianne Neumark-Sztainer – Sports and energy drink consumption are linked to health-risk behaviours among young adults (2015) [22]

Those who are exercising to lose weight and burn fat may also find that drinking isotonic drinks high in carbohydrates may reduce the effectiveness of their weight loss strategy. High intensity exercise allows the body to burn fat for energy, but the consumption of high amounts of carbohydrates and sugars after exercise has been shown to delay and inhibit this process. As a result, maintaining a carbohydrate deficit for the evening and night following exercise may improve the metabolic benefits of the workout. [29]

Conclusion

Isotonic drinks should be consumed only when moderate-high levels of dehydration are apparent, such as in the case of extended physical exercise or chronic diarrhoea. Due to their high sugar content, isotonic drinks should not be consumed outside of these contexts. For the best form of rehydration during or after exercise, individuals should seek out non-carbohydrate-electrolyte hypotonic sports drinks.

Many health conscious athletes create their own isotonic drinks at home to get the benefit of their rapid hydration, but avoid the added sugars and additives found in many commercial products. It is typical for athletes to use combination of Himalayan salt and fresh fruit juice to create a natural and additive free sports drink naturally, and there is evidence that these drinks may even function better than the commercial equivalent when made correctly.

Fresh fruit juice provides peripheral benefits that can offer support to an athlete. Fresh fruit has been shown to enhance vascular function via nitric oxide-mediated mechanisms, and even exhibits anti-inflammatory properties. Most fruits contain a huge range of polyphenols and flavonoids that will not be present in the average sports drink purchased from a store. [30] [31]

Finally, if you don’t fancy getting your hands fruity making your own isotonic solution at home, you may be pleased to learn that cows milk can function as a ready made post-exercise drink. The natural carbohydrates contained in milk such as lactose synergise with the high quality protein and micronutrients to assist rehydration and glycogen resynthesis. Indeed, cows milk naturally contains a balance of carbohydrates and sodium that allow it to function much in the same way that a well made isotonic drink does. Indeed, milk provides an ideal post-exercise recovery drink in many scenarios, and often out performs most commercially available recovery drinks! [32]

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