This is an excerpt from The Athlete’s Gut: The Inside Science of Digestion, Nutrition, and Stomach Distress by Dr. Patrick Wilson. The Athlete’s Gut is available for pre-order and starts shipping in April.
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:
- Fluid losses decrease your blood volume.
- Reductions in blood volume compromise blood flow to your muscles.
- Compromised blood flow means you can’t deliver as much oxygen to your muscles.
- A reduction in oxygen delivery slows your production of ATP.
- 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.
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.
- M. N. Sawka, S. N. Cheuvront, and R. Carter, “Human Water Needs,” Nutrition Reviews 63, no. S1 (2005): S30–S39.
- V. A. Convertino, “Blood Volume: Its Adaptation to Endurance Training,” Medicine and Science in Sports and Exercise 23, no. 12 (1991): 1338–1348.
- M. N. Sawka et al., “Thermoregulatory and Blood Responses During Exercise at Graded Hypohydration Levels,” Journal of Applied Physiology 59, no. 5 (1985): 1394–1401.
- P. D. Neufer, A. J. Young, and M. N. Sawka, “Gastric Emptying During Exercise: Effects of Heat Stress and Hypohydration,” European Journal of Applied Physiology and Occupational Physiology 58, no. 4 (1989): 433–439.