What is 'tonicity' and why does it matter?
The term 'isotonic' describes a solution that is of a similar ‘thickness’ or concentration (tonicity) as another solution. In this case it refers to a drink being of similar concentration to human blood (~285 to 295mOsm/kg)
This matters in the world of sports drinks because whether a drink is hypotonic (lower concentration that blood), isotonic (about the same concentration) or hypertonic (higher concentration) affects how much energy (carbohydrate) it can deliver and how quickly you can absorb it into your blood stream to replace the fluid you're losing in your sweat.
As with most things in life this all boils down to a couple of key trade offs...
How Isotonic sports drinks work
Isotonic = similar concentration to blood
Most traditional sports drinks (like Gatorade, Powerade, Lucozade Sport and so on) technically fall into the ‘isotonic’ category in that they're supposedly similar in concentration to human blood. Theoretically they deliver a reasonable amount of energy and clear the gut promptly too, if not quite as fast a hypotonic solutions.
They're usually around 6-8% carbohydrate and so deliver 1.5 to 2x the amount of carbs that most hypotonic drinks do. This can be useful for shorter duration, high intensity exercise where getting carbs in quickly can be more important than warding off dehydration.
Whilst isotonic drinks tend to sound great on paper, something of a ‘jack of all trades’, in the real world they can cause quite a lot of gastrointestinal upset - i.e. a sickly, bloated stomach - when consumed in large quantities (as is often the case in long endurance events). That's especially the case in hotter conditions, where high sweat rates drive a faster rate of drinking, or when combined with carb snacks like gels and energy bars.
Lab studies tend to show that fluids from truly isotonic drinks do a reasonably good job of being absorbed into the bloodstream. They're slower than hypotonic drinks, but not necessarily by a huge amount. But it turns out it's questionable whether many commercially available isotonic drinks are, well, actually isotonic after all! The findings of a 2008 paper published in a Swiss sports medicine journal raised this alarming question when they measured the tonicity of a range of commercially available sports drinks.
The researchers found that quite a lot of ‘isotonic’ drinks were actually coming in at well over 300mOsm/kg when measured in their lab and some were up as high as 348mOsm/kg!. This meant that they were actually quite a lot more concentrated than the blood streams of the people drinking them.
In a way this is not massively surprising as the rules (in Europe at least) allow drink manufacturers to claim products are ‘isotonic’ if they are between 250 and 350mOsm/kg. However what this means in reality is that many ‘isotonic’ drinks on the market today actually tend to behave more like hypertonic drinks when you pour them down your throat. That's why a lot of athletes tend to find them easier to stomach by watering them down considerably from their initial concentrations.
Hypertonic drinks and when to use them
Hypertonic = higher concentration than blood
Hypertonic drinks are more concentrated than your blood. That's usually because they're formulated with lots of carbohydrates in order to maximise energy delivery as a way to fuel high intensity activities. Most recovery drinks also fall into this category, with the addition of protein as another major ingredient.
There's no problem with drinks being hypertonic per se. In fact, it can be a very good thing when what you want is to deliver large amounts of calories or certain macronutrients quickly and efficiently into the blood stream. Liquid calories tend to be more readily absorbed than solids, which need more work from the body to break them down first. Where they can cause issues though is if you’re trying to use a hypertonic drink at times when fluid intake is the priority and dehydration is a concern; such as during long duration activities where you’re sweating a lot.
This, ironically, originates from the same mechanism that makes hypotonic drinks so good for fluid delivery; osmosis.
When a hypertonic drink lands in your gut from your stomach, the concentration of fluid in your intestine itself tends to becomes hypertonic. Your body then has to first move water out of the bloodstream back into the intestine to dilute the solutes in there down to a level that allows absorption of nutrients and fluids back across the gut wall into your body.
This net movement of water from the blood into the intestine is therefore technically ‘dehydrating’ you. It's moving water out of the blood into the gut when what you actually want to do is increase your blood fluid levels. From personal experience, this is something that can make you feel a bit sick and even more thirsty when what you intended by drinking is exactly the opposite!
It’s therefore best to stick to using hypertonic drinks exclusively at times when you are looking to get calories in as fast as possible, but when hydration is not a major priority.
Hypotonic drinks and when to use them
Hypotonic = lower concentration than blood
In simple terms, the fluids in hypotonic drinks tend to be absorbed into the bloodstream the fastest, but they deliver the least amount of carbohydrate per unit volume.
This is because hypotonic drinks create something called a ‘favourable osmotic gradient’ in that they are of a lower concentration than blood, so the water in them flows naturally across the gut wall into blood vessels, moving from an area of lower solute concentration (the gut) to an area of higher concentration (the blood) via osmosis.
Hypotonic drinks are usually of a low concentration because they are formulated using small amounts of carbohydrate (less than a 6% solution). That's because carbohydrates tend to make up the majority of the stuff diluted into most sports drinks.
Hypotonic drinks should therefore be your preferred option if the primary goal of your drink is hydration rather than delivery of large amounts of energy.
This is a major reason why my company - Precision Hydration’s - drinks are all hypotonic. Their main aim is to help replace the fluids (and electrolytes) lost in your sweat, not to provide large amounts of carbohydrate energy that are better taken from more solid sources of food/fuel as required. It’s also a why mixing them up as directed (one packet or tablet into 500ml/16oz of water) is important if you want them to perform as intended.
A spoonful of sugar (and sodium) helps the medicine go down.
There's a lot of research out there pointing to the fact that having some glucose and plenty of sodium in a drink can facilitate faster absorption of the fluids. This is because as well as via osmosis (a passive process), water can be moved across the wall of the small intestine along with sodium and glucose via a method called active transport.
Active transport is best thought of as a 2nd, separate 'doorway' from the gut into the bloodstream which can only be accessed with the right key, that being the right combination of sodium and glucose. This is in fact the basis on which ORS (Oral Rehydration Salts) - used to treat very dehydrated people in a medical setting - are formulated, with great success.
Because stronger electrolyte drinks contain much larger amounts of sodium than traditional sports drinks, this gives them the potential to use this active transport route to increase the amount of fluid moved into the blood when compared with more traditional sports drinks or plain water.
This is, of course, in addition to the fact that the sodium ends up replacing that which is lost in sweat during prolonged exercise, so it's something of a double win at times when sweat losses are very high.
Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.