Phun Physiology: Hydration Science for Endurance Athletes-The Hydration Triangle

It’s that time of year when the thoughts of endurance athletes turn to fluid intake, a topic motivated by the knowledge that too little or too much water is bad. Even slight dehydration amounting to just 1% of body weight loss adversely affects athletic performance. On the other side of the continuum, overhydration can easily lead to life-threatening hyponatremia.

As my title suggests, the emphasis here will be more on hydration science and less on its application. But why would anyone want to write about—much less read—something as esoteric as the science of hydration? Although application may be at the top of the list of things endurance athletes want to know, and although hydration science does not pretend to provide answers for all the specifics of application, it is nonetheless useful for a few good reasons.

First, hydration science may provide a means for adjudicating between some of the opposing claims we endurance athletes encounter on the topic of hydration. Secondly, it serves both as an excellent starting point and solid foundation for developing personal hydration strategies. Third, some of us might well enjoy learning about science and the history and philosophy of science as it relates to hydration. While we might be inclined to agree with this last reason, if only for pure entertainment value, there is more to it than meets the eye.

The fact of the matter is that most of the resources to which endurance athletes turn to obtain advice on hydration omit the greatest discovery of hydration science of the last century. Obviously, lost too is the importance of this discovery for endurance athletes.

Let’s begin with a “fun” one-question, multiple-choice test to see how much you really know about hydration. After reading the question, choose the one best answer from those listed after the question:

What is the most efficient way for an athlete to stay hydrated during an endurance sporting event?

A. Drink water
B. Consume electrolytes
C. Consume carbohydrates
D. Answers A and B
E. Answers A, B, and C

According to hydration science, answer “E” is by far the best answer, although endurance athletes will rarely if ever encounter this direct advice for hydrating. Read on if you are interested in learning why “E” is by far the best answer.

You may already know that answer “A” is wrong, since drinking water without consuming electrolytes may prove fatal by causing hyponatremia. But were you also aware that drinking popular sugar-based sports drinks during endurance events can also lead to hyponatremia? The reason is that the solute concentration they contain is too dilute. What about the practice of athletes diluting these sports drinks simply because they taste too sweet? Isn’t this even worse? Well, of course it is.

If you picked answer “D,” at least you are doing a little better than if you’d picked answer “A.” You’ve avoided hyponatremia. But the bottom line: it’s still a wrong answer, since the question deals with the best way to stay hydrated, not how to avoid hyponatremia. The science of hydration tells us how to do both. But back to answer “D,” which is the advice you are most likely to encounter on most athletic, health related, and nutritional web sites. This mainstream advice on hydration usually assigns one of two roles to electrolytes. You may already know, for example, that endurance athletes must replace electrolytes lost in sweat in order to maintain proper nervous and muscle function. Obviously a good reason to take electrolytes! Additionally, we are advised—rightly so—to ingest sodium (salt, sodium chloride, electrolytes) in order to maintain proper osmotic balance in the blood and extracellular fluid. A few sources may go a step further in suggesting that the electrolyte, sodium, plays an enormous role in maintaining water balance. But a crucial element has been omitted. Without it, hydration suffers.

You may still be wondering how, as the correct answer “E” suggests, the consumption of carbohydrates relates to efficient hydration. The answer is not simply that carbohydrates fuel the hydration process, something they accomplish long after they’ve been absorbed. Carbohydrates in the form of glucose play a more immediate role in hydration, along with sodium, as an obligatory co-transported item. In other words, a sugar-saline solution in the small intestine is absorbed much, much faster than a saline solution alone. Whether the science interests you or not, you can simply memorize what I coin here as the “hydration triangle,” the fact that there are three ingredients that must be present in the small intestine at the same time to ensure efficient hydration: salt, water, and sugar. If you take away the sugar from this mix, then water absorption (i.e., hydration) slows fourfold. Yet, this scientific fact is rarely, if ever, part of the advice given to endurance athletes on hydration. In what follows, I shall ask the reader for indulgence as I present a little of the science of hydration after which I shall finally raise a few important considerations and challenges on the side of application as a reward for that indulgence.

I shall start with a history lesson. Different people in different places independently discovered that drinking a solution of water, salt, and sugar was an excellent antidote for water-loss associated with diarrhea. Almost three millennia ago, for example, Susruta, the father of Ayurvedic medicine in India, advised that warm water into which rock salt and molasses had been mixed was effective for treating diarrhea. Similar recipes were tried over the centuries for combating the life-threatening dehydration associated with cholera. But without the science providing an underlying causal mechanism, successful oral rehydration therapies consisting of water, salt, and sugar never entered mainstream medicine and remained mere historical anecdotes. Once the science became known, however, it provided focus for life-saving treatment. One hydration scientist notes:

There are few more telling examples in the history of medicine where the credibility provided by a firm basic science foundation played a greater role in translational research.

This is in reference to the fact that focused treatment based on the science has saved countless lives from cholera. This lead an editor to write in the prestigious medical journal Lancet that . . .:

The discovery that sodium transport and glucose transport are coupled in the small intestine so that glucose accelerates absorption of solute and water was potentially the most important medical advance this century.

Stanley Schultz illustrates the problem posed by water loss due to cholera and the miraculous role played by the applied science:

Death is the result of dehydration and circulatory collapse complicated by metabolic acidosis and can occur within hours of the onset of symptoms. Indeed, one of the terrifying aspects of the disease is that an individual in the pink of health in the morning might turn into a shriveled corpse by the evening.

The point I’m trying to make here regarding the effectiveness of hydration in the presence of sugar is illustrated by this last example. Still used in the treatment of cholera, oral rehydration therapy is sufficiently effective not only to catch up with but get ahead of and finally reverse the effects of even extreme dehydration. The small intestine is capable of absorbing 8L of fluid in a single day!

Now that the important medical history lesson is behind us, let’s wade a little bit into the history of science, the philosophy of science and, while we’re at it, why not a little bit of science, itself?

First up is a lesson in the history of science. Scientists have known for some time that living organisms have no direct way of pumping water into and out of cells in spite of the fact that such water movement is vital to life. Additionally, scientists knew that water moves across cell membranes by following solutes, a process called “osmosis.” But osmosis is a slow, passive process. By the early 1960s, scientists became aware of sodium “pumps,” which we now know occur in great numbers in all cells of all organisms. These pumps actively move ions across cell membranes, creating ionic gradients which water then follows. Because this sodium-potassium ATPase pump is able to create ionic gradients, it uses energy—ATP—which is obtained by the oxidation of glucose inside cellular organelles called “mitochondria.” While the presence of these sodium pumps accounts for much of the water movement at the cellular level, they cannot account for the much more rapid water movement that occurs in the small intestine in the presence of sugar-saline solutions. Could this mean that another type of pump exists, one that can account for the rapid water movement (i.e., hydration) that occurs in the presence of a sugar-saline solution in the small intestine?

Now it’s time for the philosophy of science lesson. This allows an explanation of what I mean by the term hydration “science.” Isn’t science based on observation? If so, isn’t knowledge of the observed fact that water absorption increases fourfold in the small intestine in the presence of sugar enough to satisfy scientists? Isn’t this the scientific discovery alluded to above by Shultz that placed oral rehydration therapies on solid footing? Not really. This is not science in the manner scientists understand science.

Science seeks to explain how the world works. It does so by developing and testing explanatory theories to describe visible events. Here science sought to learn what mechanism was behind, or causing, the observed fact that sugar speeds the rate of water absorption in the small intestine. Science advances by proposing the existence of novel entities and process in order to explain a particular event. Such descriptions are called “theories.” By their very nature, scientific theories deal in speculation. This entails postulating hypothetical—or, yet to be discovered—entities responsible for visible events. Hydration science took a huge leap in 1964 when Schultz proposed the existence of a co-transporter molecule in the small intestine to explain how sugar might be involved in rapid intestinal water absorption. The hypothetical model proposed a new type of transporter molecule, one that simultaneously ferries sodium and glucose. Experiments designed to rule out alternative explanations—such as glucose being used as an immediate energy source, rather than acting simply as an obligatory co-transported item—provided strong evidence for the existence of the sodium-glucose co-transporter. Eventually, the actual existence of the co-transporter was established and its molecular nature was determined. In other words, the sodium-sugar co-transporter had been discovered by science. Here Schultz explains the science behind the discovery:

. . . our model could readily explain the observation that fluid absorption is increased by glucose. We now know that for every glucose molecule absorbed, two sodium ions and two counterions (mainly chloride) must also be absorbed; thus, glucose augments total solute (and, therefore, water) absorption approximately fourfold.

Furthermore, he explains corroboration of the discovery, the evidence that the sodium-sugar co-transporter actually exists . . .

. . . the Na-coupled sugar carrier . . . first cloned and sequenced by Ernst Wright and coworkers, is [now] referred to as [the] Na-coupled glucose transporter (SGLT1).

My “hydration triangle” entails not only the discovery of the co-transport molecule but its successful translation. The stepped-up hydration afforded by the sodium-glucose pump requires a sugar-saline solution in the small intestine. Neither a water solution nor a saline solution is sufficient to activate the sodium-sugar transporter. According to the hydration triangle, the only things an athlete must supply during an endurance event are water, sodium, and glucose, given that the sodium-glucose transporter is already in place and that chloride ions usually accompany sodium ions in most food sources.

One last philosophy of science lesson is needed before I turn to application. There is a reason why scientific laws, principles, theories, even models, are difficult to apply to the real world. Scientific models like the sodium-sugar transporter do not mirror reality. They are simplifications of reality, representing only those aspects of reality we wish to consider at any given time. As such, simplified working models cannot perfectly predict outcomes in the real world, which is incredibly more complex. This implies that prediction is only one of several goals of good theories and models, but certainly not their main goal. In other words, scientific models work perfectly in their perfect worlds, not in the real world where other influences, many of them unaccounted for, influence outcomes. This is one of the paradoxes of scientific understanding regarding the relationship between scientific laws and the observable world which do not always match. Richard Feynman suggested:

There is . . . a rhythm and a pattern between the phenomenon of nature which is not apparent to the eye, but only to the eye of analysis; and it is these rhythms and patterns which we call the Physical Laws . . .

Try to envision the point this way. One may have a good, bad, or indiferent experience with a particular sports drink which is a sugar-saline solution. Either way, the experience neither adds to nor subtracts from the basic science of hydration. Each application of science comes with its own complex set of special circumstances whether known, unknown, or even knowable. I’m ready now to present a shopping list of run-on considerations when applying hydration science.

One must be careful not to attribute too much to or to the exclusivity of the sodium-sugar model. First, there are several means by which water is absorbed in the small intestine, including amino-acid co-transport pumps, although the SGLT1 pump presented here is by far dominant. Second, the model presented here does not account for the fact that glucose may not be the only sugar transported with sodium, although it is the most important sugar. None of this changes the validity of the hydration triangle—water, salt, and sugar—a model that is supported by the following comment by an international team of researchers:

In the human intestine we estimate about half of the 8 liters absorbed each day occurs by cotransport and the other half occurs by osmosis through cotransporters. This would account for the intimate link between sugar, salt and water transport . . .

Further, the fact that the obligatory co-transported sugar is glucose in no way obligates athletes to directly consume glucose. Many types of carbohydrates, including sugar sources like maltodextrin, are enzymatically digested to glucose when they reach the small intestine. The model also does not account for the fact that other sugars besides glucose are absorbed by their own transporters. The claim has been made that a 2:1 ratio of glucose to fructose is absorbed faster than glucose alone. Another study indicates that a glucose-fructose-sucrose sugar combination is absorbed even faster. Critics of simple-sugar formulations contend that while this may be true, the studies which included these findings indicate that exertion was only at 50-55% maximum power. These same critics contend that the absorption rate provided by a blend of sugars is insufficient for either higher energy output or for endurance sports.

Another application consideration has to do with the concentration of sugar solutions in the stomach, which directly affects stomach emptying and the subsequent rate of intestinal absorption. If the sugar solution introduced to the stomach is too concentrated—estimates put the upper limit between 6-8%—gastric emptying is delayed. If the athlete continues drinking overly concentrated sugar solutions, the chances of stomach discomfiture increase, including nausea, and vomiting. If, on the other hand, athletes drink formulations with lower sugar concentrations, they may run up against other problems. One problem is caloric intake. Many sports drinks containing simple sugars do not provide sufficient calories for endurance athletes. Taste is not always a sufficient determiner of sugar concentration. Late in endurance events, the taste threshold for sweets decreases, meaning that sports drinks containing simple sugars taste even sweeter. Athletes who dilute what was already a dilute drink obtain even fewer calories. The practice of athletes diluting sugar-based sports drinks poses a second potential problem. Many popular sugar-based sports drinks do not contain sufficient amounts of sodium for endurance athletes. Diluting them further exacerbates the problem and may even lead to dilutional hyponatremia. According to one source,

The Endurance Formula, introduced in 2004, contains twice the sodium and three times the potassium than the typical Gatorade formula, as well as chloride, magnesium, and calcium to better replace what athletes lose while training and competing.

Some critics argue that in spite of reformulation the sugar concentration issue still lingers. These same critics believe that the only way endurance athletes can get sufficient calories is to consume solutions containing carbohydrates that can pass quickly through the stomach but that do not increase the osmolar concentration in the stomach above the tolerable limit. This means that the upper limits of osmolar concentrations of maltodextrin translate to 15-20% solution concentrations which can also provide up to three times the calories as compared to simple-sugar solutions. Also, because they don’t taste as sweet as simple sugars, athletes can tolerate higher concentrations of complex carbohydrates in their drink formulas.

The scientific models I present here do not directly address the ongoing debates focusing on how much fluid and electrolytes an endurance athlete should consume. The opposing camps base their recommendations on different sets of assumptions. One side argues that what is lost in sweat must be replaced. Others argue that because the maximum rates of absorption for water, sugar, and electrolytes are less than the rates at which they are used or lost, it is impossible to try to replace them at these higher rates, since it may unnecessarily trigger compensatory mechanisms. Both sides of the issue offer replacement guidelines.

Application is also made more difficult by the fact that as some research suggests not only conditioning but heat acclimation decrease perspiration rate and sodium loss in sweat.

We must also consider the fact that because the sodium-sugar transporter is a protein, there may be a genetic component to an individual’s ability to hydrate efficiently.

The list of considerations goes on.

I need to state a cautionary note. Although all the talk here has been about water, salt, and sugar, there is absolutely nothing wrong with consuming regular food during endurance events for either or both electrolyte and carbohydrate sources. Another corollary: don’t attempt to fix that which isn’t broken. In other words, if you are having no great problems with hydration, don’t change something just because you read this article. Many of the food items athletes might normally consume during endurance events are compatible with the hydration triangle. It doesn’t matter whether the food is solid or in liquid form. It’s personal preference that matters.

Consider, for example, a can of V8 juice. One can supplies 29% and 19% of the daily values of sodium and potassium respectively, not to mention almost 12 ounces of water and 14 grams of carbohydrates. Plus, it can be purchased cold, which increases absorption! Do you want carbohydrates with salt? Try a small bag of pretzels washed down with your favorite beverage. Normally, I avoid soft drinks, which are at the high end of the tolerable absorptive limit for sugar solutions. But hours into an event, I’ve been known to consume them, perhaps for the sugar, the salt, the caffeine, the fluid, the taste? Otherwise, I generally prefer maltodextrin-based formulations in my drink bottle in combination with electrolyte supplements all washed down with cold water from my hydration pack. This does not mean that I won’t have a regular sit-down meal with riding buddies during an event, something that I personally find extremely satisfying.

In summary, the hydration triangle introduced here represents, I believe, a good translation of basic hydration science. It notes the three items endurance athletes must ingest during an endurance event to ensure maximal hydration: water, salt, and glucose source. The availability of glucose in the small intestine—in addition to water and sodium chloride—ensures a fourfold increase in hydration above that if glucose were not available. This is due to the utilization of the Na-coupled glucose transporter (SGLT1). We need to be reminded that there are many ways to satisfy the hydration triangle based on personal preference and need. This obviously entails some knowledge of the nutritional content of available food during endurance events and one’s responses to these foods in view of our own physiology and the physical challenges we confront and accept. Because science provides information in ideal settings, it becomes the responsibility of each endurance athlete to develop a suitable, tailor-made personal hydration strategy based on individual needs.

The ideas presented here go beyond the typically stated reasons for utilizing carbohydrates as fuel and electrolytes for maintaining ionic balance and muscle and nervous function, however important. The science tells us that efficient hydration during endurance events calls for a sugar-saline solution in the small intestine in part to activate the sodium-glucose transporter first discovered in the mid-1960s. I believe both the science and its application are captured by what I’ve termed here the hydration triangle.