What's Cool In Road Cycling

Toolbox: Hydration and Hyponatremia

The mantra of slamming back the fluids during exercise in the heat has been pounded into the public consciousness over the past decade by scientists, health officials, and industry. But can drinking too much be a bad thing, and just how much can and should you be drinking during hot summer rides?

Rising Water
Walk into any gym, a children soccer league, or a shopping mall, and chances are that you will see lots of people with water bottles slung to their hips or otherwise close at hand. Certainly within Canada and the United States, the message of the importance of adequate hydration has become entrenched into the general consciousness; however, this has come about both as a result of health educational initiatives but also marketing by the sports drink and beverage companies, making it potentially difficult to separate evidence-based knowledge from myths.

While it is generally accepted that a lowered hydration status is detrimental to exercise capacity and heat tolerance, the definition of the word “adequate” remains unclear. Part of the problem is the difficulty in generating a health message that is simple to communicate to enhance acceptance and adoption, but a lively scientific debate continues on many apparently simple but fundamental questions concerning hydration. The simplest one really comes down to this: how much do you need to drink during exercise, and is there danger from drinking too much?

Sweating Physiology
Heat stress and heat illnesses are major considerations for both exercising individuals and the general population, and adequate countermeasures are critical in reducing the risk. Short of removing oneself from the heat, the most practical intervention is to ensure that adequate hydration is maintained during exercise and in hot environments. Sweat rates exceeding 1 are typical during moderate exercise, with a high of 3.7 having been recorded by the American runner Alberto Salazar during preparation for the 1984 Olympic Marathon.

An initial response to hypohydration is less total blood volume in your body. During exercise then, blood becomes shunted away from the gut, skin and other visceral organs to maintain blood flow to your muscles and overall blood pressure. Hypohydration also progressively impairs sweating ability, with increasing levels of hypohydration both increasing the core temperature threshold for the initiation of sweating and also decreasing the sensitivity of the sweating response, resulting in a lower sweating response overall and potentially less defence against continued heat storage. Therefore, while core temperature may be able to stabilize at a level slightly higher than baseline when euhydration (normal fluid balance) is maintained, the typical observation is a continued increase in core temperature when individuals exercise at the same intensity in a hypohydrated state.

State of the Art: 1996
Coupled with studies demonstrating that even 1% hypohydration, measured by body weight loss, can impair aerobic performance and exercise tolerance time in the heat, the most straightforward approach would appear to be drinking the same amount of fluid as what you might anticipate losing through sweating. Indeed, this is the primary health message advanced by the American College of Sports Medicine (ACSM), the largest sports medicine society in the world, in its Position Stand published in 1996. Namely, this document advocated that “During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated.”

For a clear example of this strategy in action, we can look at Floyd Landis’s epic solo performance on the (in)famous Stage 17 of the 2006 Tour, where he rode for much of the stage with one bottle constantly in his hand, either chugging fluids or pouring it over his head to keep cool on an infernally hot stage.

Since the 1996 publication, however, scientists have been reassessing their recommendations and asking just how much drinking needs to be done during exercise in the heat. The brass tacks, of course, comes down to whether mild dehydration actually impairs physical performance as much as we have thought.

Things may not be as dire as lab studies may suggest. Anecdotally, many world-class marathoners drink only very small amounts of water and well below either their anticipated sweat rate. For example, it was estimated that Mizuki Noguchi, the winner of the women’s marathon at the 2004 Athens Olympics, held with a starting temperature of 35єC, only spent 30 s drinking during the entire race. Therefore, with such an apparent imbalance between fluid consumption and anticipated sweat loss, it is arguable whether moderate dehydration significantly impairs elite performance even in hot environments. Indeed, some current research is testing whether mild dehydration may actually benefit performance by decreasing body weight and therefore raising power-to-weight ratio.

Dr. Tim Noakes at the University of Cape Town in South Africa, author of the seminal book “Lore of Running,” has led the questioning of this common wisdom. Noakes is especially concerned that the prevailing emphasis on high volumes of rehydration during exercise may lead to the onset of hyponatremia, marked by a reduction in blood sodium concentrations (as opposed to heavy sweating typically increasing blood sodium concentrations). This dilution can produce dangerous swelling of the central nervous system and symptoms such as confusion, disorientation, progressively worsening headache, nausea, vomiting, aphasia, impaired coordination, muscle cramps, and muscle weakness. Extreme complications of severe hyponatremia include cerebral and pulmonary edema that can result in seizure, coma, and cardiorespiratory arrest.

Unfortunately, these symptoms can be very similar to those from severe dehydration, making the correct diagnosis and medical response critical amongst first aid and other emergency personnel. This issue of “water intoxication” has caused recent deaths during the 2007 London Marathon and also during a radio promotional stunt where contestants competed to drink large amounts of water while not using the bathroom. Such clinical problems can arise due to an abnormally high rate of water consumption, especially in individuals who have very high sodium concentrations in their sweat. Another consideration for medical support at endurance events may be the slower participants, who may be exercising at a lower intensity and subsequently lower sweating rates, excacerbated by greater rates of drinking and total time spent out on the course.

ACSM Position Stand 2007
Arguments such as these, along with continuing evolution of research into hydration and exercise, has led to a significant revision of the fluid replacement guidelines on this specific issue of rehydration goals for volume by ACSM in its new 2007 Position Stand on Exercise and Fluid Replacement.

One highlighted change in the revised document is recognition of the risks of hyponatremia from fluid consumption exceeding the sweating rate. Specifically, the “goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance” while deliberately steering clear of a specific volume of recommended rehydration. Rather, the document provides information on various factors that may modify both sweating and electrolyte excretion rates, suggested methods of monitoring fluid loss, and finally individual and situational characteristics that may affect the ability to rehydrate during exercise.

Overall, the Position Stand suggests a range of 0.4-0.8 L/h as a very general guideline, which individual athletes should modify based on their own situations. For example, large individuals who are heavy sweaters exercising in the heat may lean towards the higher end of this range, while small individuals exercising at low intensities in a cool environment may aim towards the lower rates. Importantly, people exercising at relatively low intensities for long duration (e.g., back of the packers in marathons) should not be encouraged to constantly drink large volumes of water at every aid station.

The first message I hope you will get out of this article is that science is constantly evolving, so it’s important to stay on top of the latest research and rely on current evidence-based statements rather than traditional dogma.

In terms of actual drinking guidelines, it appears that a mild bit of dehydration is inevitable, due to both our delayed thirst response and max capacity of fluid absorption of about 1 L/h. It remains to be seen whether this may actually be an ergogenic benefit. However, the best thing for most of us is to continue with a rough starting guideline of about a water bottle every hour during exercise in the heat, and to modify it based on the particular event we’re riding.

Convertino, V. et al. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sports Exerc 28(1):i-vii, 1996.

Sawka, M.N. et al. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sports Exerc 39(2):377-90, 2007.

About Stephen:
Stephen Cheung is a Canada Research Chair in Kinesiology at Brock University, with a research specialization in the effects of thermal stress on human physiology and performance. He can be reached for comments at [email protected].

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