- - Hydration Science and Practice

Hydration Science and Practice


Hydration Science and Practice 

article by Allen Lim, Ph.D of Skratch 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.


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 your 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, flavoring agents, and even food colorings. 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 favors the passive movement of water into the body, the favorable 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 liter 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 (as a side note, our Rescue Hydration Mix meets those oral rehydration solution standards).

Because our exercise hydration drink contains significantly more sodium (310 mg per ½ liter 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 favorable 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 diarrhea 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 liter 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 hyponatremia, 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.

Eat Real Food:

This of course, leaves us with the final question – if our goal is not hydration, but calorie replacement during heavy exercise, how do we get enough calories in our body if we don’t drink those calories? While the simple answer is to eat solid and real food, the type of food matters and there is, of course, a lot of individual difference in how people process and deal with different foods. So despite this general piece of advice, it’s still important to experiment and stick with foods with minimal ingredients and that are low in fiber during exercise.

Still, the basic concept is this – that when we chew something and it forms a bolus that enters our stomach, as that bolus or mass is broken down and liquefied by acid in our stomach, the entrance of those calories and nutrients into the small intestine, where nutrients are absorbed into the body, is paced. So instead of having a huge influx of a highly concentrated fluid with a high osmolality that can disrupt water absorption into our small intestine, we have a slower but steady influx of reduced chyme (partially digested food, water, acid, and digestive enzymes) that the small intestine can then actively transport into the body at a rate that doesn’t overwhelm the gut. The simple analogy is instead of trying to get 200 cars onto a highway all at once and causing a big traffic jam, eating solid food helps to allow a steady stream of cars (calories) onto the highway (small intestine) in a way that keeps traffic flowing.

That all said, the addition of any calorie source whether it is in liquid or in solid form creates a net osmotic pressure once it arrives in the small intestine. Managing this flow of both energy and hydration is ultimately the key. With that in mind, the real question to ask yourself is how quickly are you losing sweat and valuable water versus how quickly are you burning through available fuel?

Hydrate First & You May Not Need to Eat:

The reality is that in the heat, we can lose fluid at a much faster rate than we burn through fuel. And in the same way that we can live for weeks without food but only days without water, our physical performance can drop immediately and appreciably when we become dehydrated despite having ample untapped fuel supplies in the form of glycogen and fat stores on board.

To put this into some real numbers, let’s do a rough comparison of a very fit “amateur” cyclist riding at 150 watts versus a “professional” riding at 250 watts for 5 hours. At these intensities, the rider pedaling at 150 watts is burning about 594 Calories per hour while the one pedaling at 250 watts is burning about 990 Calories per hour. At these intensities, a very conservative sweat rate at a temperature of 75 to 80 degrees Fahrenheit might be about 1 liter per hour at 150 watts or 2 liters per hour at 250 watts. Because sweat rates can vary greatly depending upon the temperature, humidity, cloud cover, exercise intensity, fitness level, and body size and shape the best way to measure sweat rate is to weigh yourself before and after exercise, realizing that a 1 kg drop in body weight is equal to about 1 liter of sweat loss (1 pound loss is equal to 15.3 ounces of sweat). So these sweat figures, while a guess, are in the range of what I’ve measured in the field.

With that disclaimer on sweat rate, if the first priority for these riders is to hydrate, then the rider pedaling at 150 watts would need to drink 1 liter each hour while the rider pedaling at 250 watts would need to drink 2 liters an hour. Assuming that they are using our 80 Calorie Exercise Hydration mix in each ½ liter or 500 ml bottle (though I realize that most cycling water bottles are about 650 ml or 22 ounces) then over the course of 5 hours, if a rider were to actually drink to match their sweat rate, then the rider at 150 watts would be taking in 160 Calories an hour from their drink mix, while burning 594 Calories an hour. In contrast, the rider at 250 watts would be taking in 320 Calories from their drink mix while burning 990 Calories each hour.

At first glance it’s obvious that the calories coming from the drink mix aren’t enough to match the rate of energy expenditure. But this simple glance doesn’t account for the carbohydrate we have stored in our muscles and liver or our ample stores of fat. Assuming that for each rider 20% of the calories are derived from fat and the rest from carbohydrate and also assuming that the amateur has 1500 Calories and the professional cyclist 2500 Calories stored as glycogen, then the requirements each hour for a 5-hour ride change. Doing the math, if we account for stored glycogen and fat, the amount of energy needed per hour for the amateur cyclist would be 175 Calories per hour while the professional would need 292 Calories per hour. While the amateur would be a little off consuming only 160 Calories an hour from their drink mix alone to meet their 175 Calorie need, the professional would actually be fine consuming 320 Calories an hour from their drink mix alone to meet their 292 Calorie need.

While I’m not advocating that our drink be used as the sole source of energy, in many cases if our drink at 80 Calories per bottle is used to replenish all of our actual hydration requirements then our energy needs would come pretty close to being met depending upon the situation. In effect, because a lower calorie solution has the potential to empty at a much faster rate than a high calorie drink, energy needs can be made up for by the ability to drink a higher volume of fluid, which ultimately ensures proper hydration.

In reality, I know very few riders who actually keep pace with their sweat rate. At best, if they keep their weight loss due to dehydration to less than 3% of their body weight then they are doing quite well. After a 3% weight loss there are significant performance declines so trying not to lose 4.5 pounds for a 150-pound rider or 6 pounds for a 200-pound rider is critical. Because of this eating some food consistently and from the onset of a ride or competition is critical. As a general rule of thumb, I recommend that beyond a 2 to 3 hour ride that athletes make it a goal to replace at least half the calories they burn each hour. In most cases, if an athlete is staying hydrated (less than a 3% loss in body weight) then about 75% of that energy might come from their energy drink while the rest might come from real food. For a hard working professional this might mean that in a race at an average energy demand of 800 Calories an hour that the athlete needs to eat about 400 Calories each hour or 100 grams of carbohydrate. Typically, 300 of those Calories can be brought in through 3 to 4 bottles an hour, while the final 100 Calories can be eaten. While I’ve had riders try to just drink the entire 400 Calories in one bottle and then take in plain water, this method usually wreaks havoc on their gut and prevents them from getting adequate sodium. Ultimately, the bottom line is that it is as important to think about hydration as it is to think about fuel and that if done right, you don’t need an appreciable amount of calories in each bottle to meet the needs of both.

The Egg Model:

I’ve demonstrated an extremely simplified model of osmosis using a raw egg with its shell removed by soaking the egg in standard cooking vinegar for about a day and a half. The vinegar literally cooks the shell off and stiffens the very thin membrane surrounding the egg leaving a completely raw egg that feels like a delicate water-balloon.

Since that thin membrane is permeable to water but not larger molecules like sugar, it’s, in theory, a great way to demonstrate osmosis. And in theory, if you soak that raw egg in a solution that has a concentration that is greater than the inside of the egg, water will flow out of the egg. Likewise, if you soak that raw egg in a solution that has a concentration that is less than the inside of the egg then water will move into the egg.

So in this particular model, the glass jar is meant to represent the inside of our small intestine while the interior of the egg is meant to represent the inside of our body. Finally, the thin semi-permeable membrane surrounding the egg separates the inside of the body or egg from the solution or outside world that the egg is floating in. If the egg is bathed in a solution that hydrates it, then the egg will gain weight. On the other hand, if the egg is bathed in a solution that dehydrates it, then the egg will lose weight.

With all of this in mind, and despite not knowing the exact osmolality of a raw chicken egg soaked in vinegar, I wanted to see if soaking that raw egg in our Exercise Hydration Mix would hydrate or dehydrate the egg compared to higher calorie solutions. Initially, I wanted to compare our drink at 80 Calories to three other solutions at 160, 280, and 400 Calories. Unfortunately, I busted one of the eggs and had to settle with comparing 80 Calories of Skratch Labs (4% carbohydrate solution) against 160 Calories worth of a gel product (8%), against 400 Calories of a maltodextrin solution (20%), all mixed in 500 ml of water over the course of 2 hours.

To measure the hydration state, I weighed the eggs on a precision digital scale before soaking them and after soaking them to see if there was a positive or negative shift in water. About an hour and a half into the experiment, I got impatient and wanted to see what was happening. At this point, the egg soaked in our 4% Exercise Hydration Mix from Skratch Labs went from an initial weight of 107.52 g to 113.44 g, our egg in the 8% solution went from 94.32 g to 93.75 g, and our egg in the 20% solution went from 92.80 g to 86.15 g in just 1 hour and 35 minutes.

Thus, our Skratch Labs egg gained + 5.5% in water mass, the egg in an 8% solution lost – 0.6% of its water mass, and the egg soaked in the 20% solution lost – 7.1% of its water mass. What’s interesting is that over the course of 24 hours, our Skratch Labs egg stayed at virtually the same weight demonstrating that it hydrated quickly and achieved equilibrium with the solution. In contrast, the egg soaked in the 8% solution continued to lose weight and lost a total of – 13.02% of its weight after 24 hours, while the egg soaked in the 20% solution lost a total of – 36.61% of it’s weight after 24 hours.

Obviously, this is a neat model for osmosis but it by no means represents the truly incredible and complex process of digestion or fluid and nutrient absorption during exercise. That being said, this little demonstration is a great example of the basic core of a scientific experiment – making a guess based on a sound theory and then testing it by making observable measurements that either prove or disprove your original theory.

Despite it’s simplicity, this process is essentially how we developed our Exercise Hydration Mix. We took real athletes preparing for the Tour de France and poured different formulas down their throats as if their bellies were delicate eggs soaking in different solutions. Regardless of the theories or known or unknown physiological mechanisms behind them, we had some very simple bottom lines that we assessed – were the athletes able to get hydrated, did their stomachs bloat when we tried to keep pace with their hydration needs, did they like the taste, did they feel good or bad, and were they able to perform better. Because, available drink mixes did not satisfy our needs, we were strongly motivated to find something better. Through a lot of hard work and time, we ultimately did find something better and we continue to work to learn and improve.

While a myriad of arguments can be made for one idea or theory versus another idea or theory – for one product or philosophy versus another – the bottom line is that we are each our own scientific and real world experiment and that the best way to discover is to try for yourself. In the end, soaking your egg or gut in our drink may just not work for you or it may work better than anything you’ve ever tried. But you’ll never know until you stop reading all these ideas and just try it out! We believe that it’s the best available option, though we realize this will always be a work-in-progress and strive to make our products even better.


Skratch Labs

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