Although the human body may look solid, one-half to two-thirds of it is actually water.1 Of all the nutrients you must obtain in your diet, water requires the most consistent replacement. If water intake is restricted for several days and losses of greater than 7–10 percent of weight occur, the body struggles to maintain adequate blood pressure and blood delivery to vital organs like the brain, liver, and kidneys. Eventually, organ failure and death ensue if severe fluid deficits aren’t remedied, and these deficits can set in rapidly in hot and humid environments.
While obviously not as dire as severe dehydration, moderate deficits of body water can still negatively affect you during exercise. In order to meet the swelling energy demands of exercise, your body must deliver more oxygen to your working muscles. This is precisely why your heart rate soars during exercise; every additional squeeze of this four-chambered muscular organ ejects additional blood and increases flow to your working muscles. Water is a critical component in this process because approximately half of your blood volume is water. In fact, one of the earliest adaptations to endurance training the body makes is to increase blood volume, which it mainly accomplishes by retaining extra water.2 Specifically, an uptick in thirst and a decline in urine output cause your body to store this extra aqua, and these adaptations serve to increase maximal cardiac output (the amount of blood the heart pumps per minute) and aerobic fitness.
During heavy exercise, heat production is a threat to your well-being and performance. Your body must rid itself of this heat, lest your brain be cooked. The human machine utilizes several avenues to cool itself, but sweating becomes an increasingly important method in sweltering conditions. The relevance of this is that heavy sweat losses can translate to less blood volume, and in order to maintain cardiac output, your body adjusts by making your heart pump faster. However, if fluid and blood volume losses are large enough, then your body won’t be able to completely compensate by ramping up heart rate. In sum, the following chain of events will ensue:
1.Fluid losses decrease your blood volume.
2.Reductions in blood volume compromise blood flow to your muscles.
3.Compromised blood flow means you can’t deliver as much oxygen to your muscles.
4.A reduction in oxygen delivery slows your production of ATP.
5.A reduced rate of ATP production reduces your work output.
Although there are additional reasons you fatigue in the heat, one reason is the inability to deliver the proper amount of blood and oxygen needed to maintain a particular exercise intensity.
Beyond its effects on blood volume and cardiac output, dehydration can also adversely affect body temperature regulation.3 You only have so much blood to go around, with your skeletal muscles, heart, gut, brain, liver, kidneys, and skin all competing for a piece of the proverbial pie. During exercise, your skeletal muscle—sort of like a Kardashian to the paparazzi—gets much of the attention. Not to be forgotten, however, is your skin; a main mechanism by which your body cools itself is by sending warm blood from your core to your periphery. The extra blood flowing through your skin enhances sweating, and the evaporation of sweat decreases skin temperature and cools your blood before it returns to your core. When you’re substantially dehydrated, blood flow to your skin and sweating become compromised, causing a more rapid rise in body temperature.
What exactly does all of this have to do with gut function, you ask? It turns out that this loss of body water and blood volume can hamper stomach emptying and provoke gut symptoms by reducing gut blood flow.4 This can be a vicious cycle; as you become more and more dehydrated, it becomes progressively more difficult to reverse this dehydration because the fluid you drink sits idly in your stomach. Table 7.1 shows an example of how blood flow to the skin and gut can change as you go from rest to exercise; in addition, it displays how dehydration can impair blood flow to the skin and gut in the heat. As you can see, total cardiac output surges fivefold from about 5 liters per minute at rest to about 25 liters per minute during intense exercise in the heat. Simultaneously, blood flow to the skin surges, while blood flow to the gut might decline by 40 percent. As shown in the far right panel, when you experience substantial dehydration (4–5 percent body mass loss or more) from heavy sweating, your cardiac output declines and you start to fatigue. Likewise, blood flow to your gut drops even further, leading to delays in gastric emptying and more severe gastrointestinal symptoms.
table 7.1.DEHYDRATION’S IMPACT ON THE GUT
Moderate-to-severe dehydration can impair blood flow to the gut, especially in hot and humid environments where your skin is competing for blood flow as well.
Because fluid losses can compromise blood flow to muscle and skin as well as impair heat regulation, an indisputable rule in the field of sports nutrition is that dehydration, if severe enough, harms performance. Decades of research—combined with aggressive marketing campaigns from sports beverage producers—has solidified our collective belief that athletes need to be extra vigilant about hydrating during exercise.
While it’s undeniable that moderate-to-severe dehydration usually impairs exercise performance, recently there’s been a growing debate as to whether milder forms of dehydration have the same effects. On top of that, there are concerns that drinking loads of fluid during exercise has its own downsides. Consequently, in the subsequent sections of this chapter, I address the evolution of hydration guidelines over time, as well as the effects of different hydration strategies on gut function and symptomology.
RECOMMENDATIONS FOR FLUID REPLACEMENT DURING EXERCISE
In 1996, the American College of Sports Medicine released a paper detailing their position on hydration and exercise.5 Along with describing the health and performance consequences of dehydration, the authors recommended that athletes consume enough fluid so that “water losses due to sweating during exercise be replaced at a rate equal to the sweat rate.” While the authors acknowledged that, in the real world, athletes rarely achieve those levels of consumption (in large part due to gut intolerance), they nevertheless suggested that athletes “consume the maximal amount of fluids during exercise that can be tolerated without gastrointestinal discomfort up to a rate equal to that lost from sweating.” Other sports medicine organizations mirrored these guidelines; for instance, the National Athletic Trainers’ Association published a paper in 2000 stating that “athletes should aim to drink quantities equal to sweat and urine losses.”6
These recommendations may be realistic for an average sweater (one who loses between 15 and 30 ounces of sweat per hour), but how well would they apply to an athlete who sweats like Ted Striker from the movie Airplane!? (If you don’t get the reference, just google the GIF.) Is it practical for prolific sweaters to replace upward of 50–70 ounces of fluid per hour? (That would be equivalent to downing four to six cans of soda each hour!) The obvious answer is no, especially because the heaviest sweaters are typically exercising the hardest, making their guts less able to tolerate food and fluid intake.
While these sports medicine organizations admitted that athletes rarely met these hydration objectives of their own volition, they probably understated the potential risks of gut distress when athletes consume large volumes of fluid during exercise. After about a decade, the American College of Sports Medicine updated their position on exercise and hydration, in part as recognition that their recommendation to match drinking to sweat rates was unrealistic for many athletes. The updated paper no longer explicitly stated that athletes should consume fluid at a rate equal to sweat losses. Instead, the authors remarked that the general goal of drinking was to prevent greater than a 2 percent body mass loss, though they acknowledged that several factors (exercise duration, opportunities to drink, etc.) will ultimately influence exercise hydration plans.7
Around the same time, other organizations and researchers began questioning the wisdom of avoiding anything greater than a 2 percent body mass loss during prolonged exercise. Take for example an article from the International Marathon Medical Directors Association, in which the authors argued that body mass loss isn’t a completely accurate marker of fluid loss during prolonged exercise and that the wide spectrum of weather conditions, body sizes, and running speeds that characterize races (and that determine sweat losses) makes formulating a universal hydration recommendation impractical.8 Furthermore, the research used as the basis for developing the 2 percent threshold often wasn’t reflective of real-life conditions. Some studies, for example, had people begin exercise in a fluid deficit (which is clearly different from developing dehydration gradually during exercise), while others implemented problematic methods to induce dehydration before exercise (e.g., exhausting exercise or heat exposure).
In support of these criticisms, a relatively recent scientific review found that if a person starts exercise in a hydrated state, drinking fluid to prevent a 2 percent body mass loss doesn’t necessarily lead to superior performance on realistic, racelike tests (i.e., finishing a set distance as fast as possible).9 In this analysis from researcher Eric Goulet, it was shown that dehydration levels of about 2 percent weren’t associated with reduced performance and that drinking to the dictates of thirst likely improved performance in comparison to drinking more or less. Yet, this analysis was based entirely on cycling studies with exercise durations lasting around one to two hours, so whether these findings translate to other sports and longer exercise durations is unclear.
To add to the lack of clarity, a second analysis from Goulet published about a year later found that body weight losses of 2 percent or more did in fact reduce exercise capacity on what are referred to as time-to-exhaustion or fixed-power tests.10 An example of this type of test is asking a person to cycle or run for as long as they possibly can at a fixed intensity (e.g., 75 percent of VO2max). While this type of test doesn’t exactly replicate most endurance races or sporting events, it does imitate some types of training as well as activities that are common in military and occupational settings.
Hydration science is inherently difficult to do rigorously (you can’t blind someone to whether they’re drinking fluid, right?), but if I were to distill these findings into a basic recommendation, it’s this: Drinking fluid based on the dictates of your thirst is a reasonable hydration strategy for competitions that last one to two hours. This conclusion was largely reaffirmed by a more recent analysis from Goulet.11 That said, situations exist where it may be better to use a more aggressive hydration approach; examples include competitions held in extremely hot/humid conditions, when exercising at a constant intensity for as long as possible, and during more protracted exercise (more than two hours).
Given that we know dehydration can reduce blood volume, it seems somewhat counterintuitive that drinking to thirst would optimize performance in many situations even when it can cause 2–4 percent body mass losses in a relatively short amount of time (one to two hours). If drinking more can prevent such significant losses in body mass and blood volume, then why wouldn’t it also lead to better performance in these situations? While there are a few answers to this question, the most important one comes back to gut tolerance. In brief, drinking well above thirst makes it more likely you’ll be plagued by excessive fullness, stomach discomfort, and nausea. Indeed, a 1991 experiment showed that drinking about 1,600 milliliters (about 54 ounces) of fluid per hour during two hours of cycling doubled stomach fullness ratings as compared to intakes that were about half as much.12 Another study conducted a few years later found that attempting to fully replace sweat losses during one hour of intense cycling led to more gut distress and caused cyclists to cover less distance than when they ingested no fluid.13
These findings were extended to runners in a study that demonstrated that drinking a carbohydrate beverage at a rate of about 30 ounces per hour during a two-hour run was associated with fullness ratings that were nearly twice as severe than from drinking 13 ounces per hour.14 In another study, 10 runners completed two half-marathons in hot conditions while ingesting water to the dictates of thirst or based on prescribed rates that aimed to prevent 2 percent body mass losses.15 Fluid consumption was dramatically different between trials (roughly 20 versus 70 ounces), and the prescribed strategy led to gut discomfort ratings that were twice as high by the end of the half-marathon.
While the gut discomfort that accompanies aggressive fluid intakes doesn’t automatically hurt performance, it clearly can in some situations. Take for example an experiment that examined how drinking different volumes of a carbohydrate beverage influenced running performance.16 Nine men completed a 10-mile run on three occasions while drinking (1) no fluid, (2) as much fluid as they pleased, or (3) a prescribed volume to minimize dehydration. The amount drank in the prescribed trial was about three to four times as much as when subjects drank whatever they pleased (10 versus 35 ounces), and although prescribed drinking led to less dehydration (0.6 percent versus 1.4 percent body mass loss), it caused the runners to finish the race about one minute slower! The most plausible explanation for the less-than-stellar performance during the prescribed trial was gut intolerance, as discomfort ratings by the end of the race were twice that of the ratings from when they drank as much as they pleased. The gut discomfort was likely the result of fluid remaining in the participants’ stomachs since the max rate of emptying is not much more than 30–35 ounces per hour and is further reduced at intensities above 70 percent of VO2max.17 Interestingly, performance with prescribed drinking was essentially the same as when the runners consumed no fluid, substantiating the idea of a Goldilocks zone for fluid intake.
Hydration recommendations during exercise will likely continue to remain ambiguous because of the markedly different environments, modalities, and durations of exercise people engage in. With that said, an optimal hydration zone likely exists for each athlete and each situation (see Figure 7.1). If you drink too little during prolonged exercise, dehydration can ensue and endurance may suffer. Additionally, consuming too little fluid can aggravate gut woes by impairing gut blood flow. On the opposite end of the spectrum, drinking too much can also cause stomach discomfort and performance may suffer. Ultimately, the goal is to consume an optimal volume of fluid that minimizes the risks of dehydration and gut discomfort at the same time.
figure 7.1OPTIMAL FLUID INTAKE
Both over- and underconsuming fluid during exercise carry certain risks, including gut distress, and each athlete may need to use trial and error to find their own Goldilocks zone.
Like other nutrition strategies, trialing different drinking plans during training should help you dial in your own optimal hydration zone for competition, whether it be drinking to thirst or to a preplanned schedule. In addition, the following recommendations may be of use when making decisions about hydration:
For competitions that last between one and two hours, drinking based on thirst may be the easiest and most effective strategy for optimizing gut comfort and performance in the vast majority of cases.
Heat and humidity increase sweating and the risk of dehydration, and therefore drinking to thirst may be an insufficient strategy for a majority of one-to-two-hour competitions held in hot and humid environments.
Hydration guidelines for events lasting longer than two hours are murkier because most experimental studies testing different hydration strategies haven’t used exercise tests lasting that long; although using thirst is a perfectly acceptable option for some athletes, others may require more regimented approaches to prevent levels of dehydration that impair performance.18
Athletes choosing preplanned hydration strategies should obtain estimates of their sweat rates using body weight changes and fluid intakes (see Table 7.2).
Environmental conditions and exercise intensity both impact sweating, so athletes will need to estimate their individual sweat rates for the situations they expect to compete in; this obviously makes preplanned hydration strategies more burdensome on the athlete.
If using a preplanned strategy, replacing 50–75 percent of sweat losses is a reasonable goal when sweat rates are expected to be between 17 and 34 ounces (500–1,000 milliliters) per hour.
When sweat rates are expected to exceed 34 ounces (1,000 milliliters) per hour, replacing roughly 25–50 percent of sweat losses is a reasonable target.
The higher the sweat rate, the lower the percentage of replacement; for example, an elite marathoner who sweats 80 ounces of fluid per hour is much more likely to tolerate replacing 30 percent of losses (24 ounces) than 50 percent of losses (40 ounces).
There’s some evidence that repeatedly drinking large volumes of fluid over multiple exercise training sessions can reduce gut discomfort over time (discussed in Chapter 9), which reenforces the point that it’s important to practice preplanned hydration strategies over the long term.
FLUID LOADING
What we’ve learned so far is that drinking fluid well above and beyond your perception of thirst may create gut discomfort that, in some cases, negatively impacts performance. This is despite the fact that drinking above thirst can prevent a loss of blood volume and improve thermoregulatory responses to exercise in some situations. What if there were a way to stay hydrated without having to down loads of fluid? A way of sort of bypassing your gut during exercise? At first thought this may seem like an improbable idea, but there are a couple of strategies that might bear some fruit.
One approach would be to hook someone to an IV so they can receive a constant stream of fluid directly into the vascular system, but that’s obviously impractical/unethical outside the confines of a lab. An alternative strategy—known simply as fluid loading—may allow you to avoid the problems of dehydration without having to choke down bottles of water and sports drink during exercise. Fluid loading is similar to the more well-known strategy of carbohydrate loading, and just as carbohydrate loading increases reserves of high-octane fuel in your body, fluid loading expands your stores of water. In basic terms, the more fluid there is in you before exercise, the less you may need to drink during exercise, or at least that’s how the theory goes.
From a practical perspective, ingesting loads of water by itself isn’t a super effective method of increasing water stores. Plain aqua is low in electrolytes like sodium, so when it’s absorbed and incorporated into your vascular space, your blood concentrations of sodium decline. The body reacts to this dilution of blood sodium by producing more urine, causing you to pee out, over the next few hours, much of the water you drank.
One way that scientists have tried to enhance the retention of ingested water is by adding glycerol to beverages. Glycerol is a three-carbon molecule that, through osmotic forces, allows your body to hold on to more of the water you drink. Notably, a review of studies, again conducted by hydration researcher Eric Goulet, found that including glycerol in pre-exercise beverages improved performance by an average of 2.6 percent on exercise tests lasting about one to two hours.19 Of note, most studies used dosages of about 0.5 grams per pound of body weight mixed with 1.5–2.0 liters of water. In addition, a glycerol beverage should be consumed over a period of about one hour, with the goal of finishing the entire beverage 30–60 minutes before exercise commences.
Until recently, glycerol was prohibited by the World Anti-Doping Agency (WADA) because it was thought to be a way for cheating athletes to mask the physiological responses to blood doping.20 However, effective January 1, 2018, WADA removed glycerol from their prohibited substances list because of additional research showing that it minimally affects parameters of the Athlete Biological Passport, which is an electronic record that’s used to monitor biomarkers of doping in athletes. Even though it’s now legal, glycerol is known to cause gastrointestinal side effects like nausea and bloating if it’s taken in concentrated dosages over a short period of time, which is why it should be mixed and consumed with fluid over an hour or so.21 Certain individuals should also avoid taking glycerol for health reasons, including (but not limited to) pregnant women and those with cardiovascular diseases, diabetes, kidney disease, migraines, and liver disorders. Finally, it’s important that fluid isn’t overconsumed during exercise when using glycerol ingestion as a pre-exercise fluid-loading strategy, as the combination of the two can cause hyponatremia.
Beyond glycerol, there’s another fluid-loading approach that might be worth a try: taking in more sodium. As mentioned earlier, a drop in blood sodium is one of the stimuli that sparks urine production when you consume a large volume of plain water, and by ingesting a good bit of sodium with water, reductions in blood sodium are blunted and urine output is curtailed. While only a handful of studies have examined whether this strategy impacts exercise performance, there’s reason to feel somewhat positive about it. In a study led by exercise physiologist Stacy Sims, pre-exercise ingestion of roughly 25 ounces of water containing a large amount of sodium (about 3,700 milligrams per liter) allowed runners to exercise longer at 70 percent of VO2max in a hot environment as compared to ingesting low-sodium water (230 milligrams per liter).22 A second study by Sims found a similar result when female cyclists consumed either high-sodium or low-sodium beverages before cycling to exhaustion in 90°F conditions.23 One caveat to both of these studies is that the athletes weren’t allowed to drink fluid during exercise, which may have exaggerated the performance benefits.
To summarize, pre-competition fluid loading may enhance body water storage and decrease the need for fluid ingestion during exercise, at least in sweltering conditions. This may allow you to consume a smaller, more gut-friendly volume of fluid during exercise without becoming dehydrated. However, the interactions between fluid loading and fluid ingestion during exercise haven’t been well studied, so the idea that you need to drink less during exercise when you’ve fluid-loaded is based on theoretical grounds, and any athlete trying this approach should be careful not to overdrink during exercise (because of the risk of hyponatremia). Both glycerol and sodium-rich beverages are viable options for pre-exercise fluid loading, but the wider availability of sodium-rich beverages makes them the more practical choice for most athletes. If you ultimately decide to consume a concentrated sodium drink to fluid load, you should start consuming the beverage about two hours before competition, taking a 3-to-4-ounce serving every 10 minutes for about one hour. You can make your own beverage, but be aware that table salt (sodium chloride) is only 40 percent sodium by weight. Alternatively, you could purchase a pre-formulated version from companies like Osmo or The Right Stuff.
FLUID TEMPERATURE
Ultimately, anything that retards emptying from your stomach could contribute to fullness, bloating, and nausea, so it’s important for you to consider how the foodstuffs you ingest affect this process. As it relates to fluids, one factor that impacts stomach emptying is beverage temperature. In a study that fed people orange juice at different temperatures, 30 percent of juice kept at 39°F had left the stomach 12 minutes after ingestion, contrasted against 45 percent of the juice that was kept near body temperature.24 On the other hand, people usually opt to consume more fluid while exercising in the heat if a beverage is cool or cold, and chilled fluids delay the rise in body temperature and prevent premature fatigue during exercise in hot conditions.25 Given these benefits, is reduced stomach emptying from a colder beverage really something you should be concerned about in terms of gut comfort or distress?
The short answer is that beverage temperature is unlikely to profoundly impact gut symptoms. Once they reach your stomach, chilled beverages are warmed to near body temperature within 5–10 minutes, making the effects on gastric emptying short-lived.26 Indeed, the same investigation that found hindered stomach emptying 12 minutes after cold juice ingestion also found no differences in emptying between the cold and warm juices at later time points. And while studies comparing the consumption of fluids kept at different temperatures have frequently neglected to collect information on gut symptoms, the fact that cool fluid ingestion has often improved exercise capacity in these studies suggests that any increase in symptoms is either negligible or not severe enough to impact performance.
VOLUME AND FREQUENCY OF DRINKING
Other than fluid temperature, the frequency and volume of fluid ingestion can also affect stomach emptying. It’s long been recognized that large volumes of fluid held in the stomach elicit the fastest rates of emptying, but as fluid leaves the stomach, emptying begins to slow.27 This means that the rate of emptying falls exponentially over time. So, if you continually drink, say, every 10–15 minutes, with the goal of keeping your stomach topped off with fluid, that should result in a superior rate of fluid delivery to your small intestine. Consequently, exercise science textbooks sometimes recommend that athletes consume fluid every 10–15 minutes during prolonged exercise in order to maintain a high rate of gastric emptying.
On a practical level, however, it’s questionable whether this modest increase in emptying and fluid delivery leads to better exercise performance because, as I explained earlier, drinking above the dictates of thirst hasn’t been consistently shown to improve performance and can actually hinder performance if it causes gut discomfort. In addition, the frequency with which an athlete drinks often depends on the constraints of the sport, and in some sports (e.g., professional soccer), drinking every 10–15 minutes might not be feasible.