Why Cyclists Don’t Need Isotonic Sports Drinks?

When it comes to sports drinks, it is likely that at some point, you’ve heard of the phrase Isotonic, or been told that it’s really important that you have an isotonic sports drink. But, what does it actually mean and does it even matter?

What Is Osmolality?

Before we get into it, it’s essential to understand a few key terms. One of these is osmolality, which is the scientific term for measuring the number of particles that have been dissolving within a solution. Distilled water, which has all the minerals removed, will have an osmolality of 0 milliosmoles per litre (mOsm/L), whilst a carbohydrate-rich sports drink with lots of carbohydrate and electrolyte particles dissolved within the water will have a high osmolality. Whether or not a sports drink is isotonic is dictated by its osmolality.

What Is Tonicity?

Tonicity (the tonic part of isotonic) refers to the osmolality of the sports drink relative to the osmolality of your blood, which is believed to be around 290 milliosmoles per litre. The significance of this is around osmotic pressure. When we want to absorb a sports drink into the body so we get a rapid delivery of energy and water, we want to try and create an osmotic gradient across the stomach wall into the blood to help support this movement. If the gradient isn’t optimal, it may slow down that absorption or, worse still, result in stomach problems as the solution is absorbed slowly and end's up sloshing around in the gut. This is where the terms isotonic, hypotonic and hypertonic come fromt. 

Hypotonic – This refers to a solution that has a lower osmolality than the blood. Typically, hypotonic drinks have a low amount of carbohydrate (i.e. less than around 6% total carbohydrate and often some electrolytes like sodium.  This means that the water moves from the lower concentration in the stomach to the higher concentration in the body rapdily, allowing rapid rehydration. Hypotonic drinks are things like Dyralight or oral rehydration solutions (ORS); these typically have high electrolytes but low carbohydrates and are generally seen as the fastest way in which to get fluid into the body via the gut, so they are commonly recommended when looking to rehydrate. However, due to the low amount of carbohydrates, they may not be great when it comes to fuelling, as it would require intakes of fluid in excess of what is practically possible in order to consume enough to fuel well (i.e. 60-90grams of carbohydrate an hour for rides longer than 2-3 hours).

Isotonic – This refers to when a solution is a close match for the osmolality of the blood. Typically, isotonic drinks have around a 6% carbohydrate content along with some electrolytes. They are typically seen as the best solution for balancing both the delivery of carbohydrates and fluids. Isotonic is a common term on many sports drinks and is often used to promote the benefits of a certain drink.

Hypertonic – This refers to when a solution has a higher osmolality than that of the blood. This is typical of many modern sports drinks or an energy gel, where a large amount of carbohydrate is put into a fairly small amount of fluid. For example, Science in Sport, Beta Fuel or Maurten's sports drink mix, all pack a lot of carbohydrates for a relatively modest amount of fluid. With the osmolality of these solutions being higher than that of the blood, it is thought that these types of solutions require the stomach to draw in fluid from the body to level the concentration gradient (i.e. effectively making the solution in the stomach isotonic) before it can be absorbed and this can potentially cause stomach problems as it takes time before it can be absorbed into the body. It is also thought these solutions can dehydrate you, owing to the movement of fluid out of the blood and into the stomach, albeit this is temporary.

Why Doesn’t Tonicity Really Matter?

For a long time, tonicity was seen as an absolutely vital component of any sports drink. It was thought that if you consumed a hypertonic solution, it would be a guarantee of performance-wrecking stomach upset, leaving you with debilitating stomach cramps, but with modern sports drinks, this is highly unlikely to be the case.

We now know that tonicity is just one factor that influences the absorption of a sports drink and there are a number of other factors to consider.

In particular, many modern sports drinks use a combination of glucose and fructose; this allows the rapid absorption of carbohydrates in the gut through the use of two independent transporters. This means that if a solution is hypertonic (i..e very concentrated) as is the case for many modern sports drink, the absorbtion of the carbohydrate is so  fast that the solution in the stomach will be reduced down to a lower osmolality solution very quickly. This means that there is less likelihood of stomach problems and faster fluid absorbtion with these solutions even though they can be very hypertonic.

Mixing Things Up

One key thing to bear in mind that is often forgotten by many sports drink manufacturers is that, whilst your drink may be isotonic when it’s consumed, when this combines with anything else in your stomach or if you combine an isotonic sports drink with a gel or any food, as this all combines within the stomach it will adjust the osmolality anyway. It is highly unlikely that you'll have a precisely isotonic solution within the gut, even if that's all you drink. 

In conclusion.

As with any drink or on-the-bike fuelling option, it’s important to try it in training before using it in races and ensuring it doesn’t cause any stomach upset. But largely, if you are getting enough fluid and carbohydrates for your needs, the tonicity of your drink is largely irrelevant. 

References

 Leiper J. B. (2015). Fate of ingested fluids: factors affecting gastric emptying and intestinal absorption of beverages in humans. Nutrition reviews, 73 Suppl 2, 57–72. https://doi.org/10.1093/nutrit/nuv032