Hydration for Endurance Sports

I found this interesting. An article by Scratch Labs

Less is More:

    Two frequently asked questions that we get are 1) why our Exercise Hydration Mix is only 80 Calories per 500 ml (16.9 ounce) serving and 2) how to get more calories in during long endurance events with our relatively low calorie sports drink.

    The short answer to the first question is that our Exercise Hydration Mix is only 80 Calories per serving because it is our experience that a 4% carbohydrate solution (4 grams of carbohydrate per 100 ml or 20 grams per 500 ml at 4 Calories per gram) is the highest concentration of carbohydrate that we can have in our drink while still optimizing water or fluid transport across the small intestine.

    I’ll get to the science of it all below for those who are interested, but for those who aren’t, the bottom line is that in practice, when athletes I’ve worked with used solutions that were concentrated with too much carbohydrate (regardless of type of carbohydrate), most experienced gut rot – that bloated, sick, less than fresh, I don’t want to drink anymore, stomach upset that is a problem common with many sugary sports drinks and gels – a problem that motivated the development of our line of anti-gut rot hydration products.  

    The answer to the second question is that if you need more calories to meet your energy demands during a long workout, what I’ve observed amongst Grand Tour riders, who require and consume more calories while competing than almost any other group of athletes in the world, is this - that for the same amount of energy, eating real food that forms a bolus in the stomach and slowly trickles into the body always works better than trying to drink that fuel or energy in a solution.

The Details:

    So how does a lower calorie drink help to prevent gut rot? To understand that, you need to know a little bit about how water gets transported into our body. In short, water can be transported across the small intestine passively through a process called osmosis or co-transported with sodium and glucose.

Osmosis & Semi-permeable Membranes: 

    Osmosis is the movement of a particular fluid from an area of low concentration to an area of high concentration across a semi-permeable membrane. A semi-permeable membrane allows the fluid but not certain molecules to pass through it.  When a membrane keeps certain molecules from crossing it, differences in concentration can exist on either side of the membrane. As a result, any fluid that can pass through the membrane will move from the side with the lower concentration of molecules to the side with the higher concentration to create equilibrium through the process of osmosis. Examples of semi-permeable membranes include the walls that form each of our body’s cells, the thin layer of film that is visible when you crack an egg, and our small intestine, which is the primary gateway that water uses to enter our body.

    With this in mind, one very important idea is that the inside of our belly or gastrointestinal tract, where we stuff our food and drink into via our mouths, is not the inside of our body. In fact, the GI-tract is simply a tube within our body that is open to the exterior world at our mouth and anus. The GI-tract, not only digests and processes the food we eat, it acts to selectively transport fluid and nutrients from the outside world (which is the inside of the belly) into our body primarily at the small intestine.

    While the small intestine can actively transport nutrients like sugars, amino acids, and electrolytes, it also acts as a semi-permeable membrane where water flux is strongly influenced by osmosis. This means that if you drink a solution with a greater concentration than your blood or bodily fluids, water will flow out of your body into your belly through osmosis to dilute that concentrated solution unless the molecules or ingredients in that solution are permeable to your small intestine or unless those molecules can be quickly and actively transported to the other side to help pull water along.

    Said another way, if your sports drink is “thicker” than blood, then water will flow out of your blood stream into your gut, effectively dehydrating and bloating you, especially if the concentration is so high that active transport of solutes or particles in the solution can’t keep up with the initial water flux. Ultimately, drinking a solution with a very high concentration of anything (e.g., gels) is like throwing a lot of junk down your sink’s garbage disposal and not having either enough water or a strong enough motor to keep the drain open.  

Osmolality:

    Given that we don’t want to clog our drain, it’s critical that we also understand what determines the osmotic pressure or force that a solution exerts. Simply, osmotic pressure is a function of the total number of molecules (solute) that end up dissolving into a fluid (solvent) to form a mixed solution. This osmotic force or “thickness” can be measured as that solution’s osmolality.  Thus, if you want to ensure that what you drink is easily absorbed into your body, then in theory the osmolality of that drink needs to be less than the osmolality of blood or plasma, assuming that osmosis is the only mechanism for water transport (more on this in a bit). 

    Depending on one’s hydration state, blood osmolality can range anywhere from 275 to 295 milliosmoles per kg of water. Our exercise hydration drink has an osmolality of 280 to 285 milliosmoles per kg of water, primarily because of the lower concentration of carbohydrate that we use, which is our main ingredient.

    It’s important, however, to realize that a solution’s osmolality is affected by all of the molecules that enter into solution. This means that the osmolality of a sports drink is determined not just by the amount and type of carbohydrate in the solution but by all of the ingredients in that solution, from the electrolytes to ingredients like preservatives, artificial sweeteners, flavouring agents, and even food colourings. This is a key reason why we do not add superfluous ingredients to our hydration products. This is also why gels and heavily concentrated carbohydrate solutions that are already dissolved in water can exert a greater osmotic force than a bolus of real food even when matched for calories. And while we intentionally designed a drink with an osmolality that favours the passive movement of water into the body, the favourable osmotic gradient of our drink relative to the body is not the only factor that helps to help optimize hydration and prevent bloating.  

Co-Transport of Water with Sodium and Glucose:

    Water can also move into the body through channels known as SGLT1 transporters that actively transport sodium and glucose across the small intestine. These channels use energy to move 2 sodium ions and one glucose molecule into the body. As this happens, 210 molecules of water also move across, effectively getting a free ride into the body as sodium and glucose pay a toll to gain entry. While this seems like a lot of water relative to sodium and glucose, this gateway is rate limited or locked by the availability of sodium and glucose. Crunching the numbers, to move 1 litre of water across the gut through this mechanism, just over 12 grams of sodium and close to 48 grams of glucose (a 4.8% glucose concentration) would be needed.  This is one reason why oral rehydration solutions used to treat diarrheal diseases contain grams, not milligrams of sodium in them along with some sugar or glucose to help take advantage of this route.

    Because our exercise hydration drink contains significantly more sodium (310 mg per ½ litre or 16.9 ounces) than other sports drinks to help replace the sodium we lose in our sweat as well as plenty of glucose, a relatively greater, albeit still very small, amount of water (25 ml) can be theoretically co-transported through the active transport of sodium and glucose. In theory, the active transport of sodium itself along with other molecules like glucose also creates a more favourable concentration difference for the flow of water into the body by osmosis. That all said, starting with a sports drink with a concentration that is too high and without enough sodium or salt makes this a real uphill battle despite active transport systems that might help to facilitate water transport. This significantly increases the risk of stomach problems especially over the course of a really long day since water flow is still primarily dependent on osmosis.  

Water Alone Can Kill You:

    All of this may lead people to think that if hydration were the primary goal then just drinking water would be the quickest and most effective way to hydrate. In fact, because water can be transported passively along its osmotic gradient and also co-transported with sodium and glucose, having some salt and sugar in a drink solution that is hypotonic (less concentrated) or even isotonic (same concentration) compared to blood would actually be the fastest way to hydrate. While drinking water alone is just fine if you’re sitting around or having dinner at home, drinking water alone is not fine if you’re trying to rehydrate when you’re sweating or have some illness or hangover that results in diarrhoea or vomiting. In fact, drinking water alone when we are exercising can be risky since we can lose an appreciable amount of sodium in our sweat (400 to 800 mg per litre of sweat) and if we don’t replace that sodium then an influx of just water can dilute the sodium in our body.  This can lead to a scenario called hypernatremia, which include symptoms like headache, confusion, a drop in performance, fatigue, nausea, vomiting, irritability, muscle spasms, seizures, coma, incontinence, and in some very rare cases death.