Bicycling and Dehydration – Why It Happens and What You Can Do
By: M. Ramin Modabber, MD
As the Medical Director and Chief Medical Officer of the Amgen Tour of California, dehydration is a significant concern for cyclists. Especially as we head into the warmest months of the year, it is essential for every athlete to understand what is happening to the body and how it reacts when dehydration occurs.
It all starts with the typical human physiological response to exercise. By understanding how each physiological component is involved, how the elements are related, how they are affected by hydration/dehydration, the complicated condition of dehydration can be better understood and (hopefully) prevented in many cases. During exercise, our heart must deliver fuel to our muscles. ‘Fuel’ in this sense consists of 2 basic things – oxygen and sugar. So, ANYTHING that affects the heart’s ability to deliver those two fuels to the muscles can affect performance. Water, believe it or not, plays a crucial role in each step along the way.
Two significant examples of this are body temperature and blood volume. When dehydration occurs, it causes a decrease in blood volume. Unfortunately, a reduction in blood volume results in the concentration/thickening of the blood, leading to a decline in cardiac output which (as stated earlier) affects the ability for our heart to supply fuel to our muscles. In essence, our heart has to work that much harder to pump the same amount of fuel to our muscles. This is why an increase in heart rate is a sign of dehydration.
Similarly, thicker blood does not have the same ability or ease to flow through our lungs nor can it optimally absorb oxygen as it passes through them. Since our muscles are continuing to demand oxygen as we exercise, our heart and our lungs come under increased stress with dehydration. This explains why we see respiratory rate and heart rate both rise during dehydration. Urinary output decreases with dehydration and (obviously) increase with boosting hydration. The kidneys (through urine output) are the body’s way of optimizing our hydration state – it can hang on to water in times of early dehydration and can also excrete water when in excess. In effect, people produce less urine and more concentrated urine in states of dehydration, but our urine is “copious and clear” when we are optimally hydrated.
Body temperature is also critically important in athletic performance. Muscles perform optimally under specific temperature ranges and when our body temperature increases, our muscles’ ability to function optimally declines. Sweating is a critical way for humans to regulate body temperature. Since sweat production decreases during dehydration in the body’s effort to retain fluid, body temperature rises. This phenomenon again helps to explain further the decrease in muscle performance seen with dehydration. Dehydration results in sub-optimal body temperature and a decrease in fuel delivery. Some other signs include skin changes – dry, “saggy” skin are seen and result from efforts by the body to maintain water content to vital organs and minimize water loss to ‘less vital’ areas. Our skin is itself an organ and similar to a very thin sponge; it takes on a different appearance based on its water content.
Nausea is another common symptom of dehydration, and although food/drink are common causes, dehydration (along with intense exercise) can lead to decreased blood flow to digestive organs, resulting in nausea.
Lastly, let’s consider (arguably) our most vital organ, our brain. When our blood gets thicker from dehydration, our blood pressure typically rises to pump this thicker blood through the vascular system. The brain usually does not like significant changes in blood pressure – which can explain why headaches are a telltale sign that we need more fluids. The brain does NOT like to be deprived of fuel/oxygen – so when dehydration progresses, brain function slows (confusion), or if severe, it has its way of protecting itself by “re-booting” (fainting).
Keeping all of the above in perspective is essential. During athletic activities, all athletes exhibit an increase in heart rate, respiratory rate, and muscle fatigue, so these signs are not helpful without associated findings and proper context.
Given the above factors which contribute to the body’s state of hydration/dehydration, it is hopefully now easier to understand that there is NO FORMULA for what an ideal amount of fluid consumption is. Athletes demonstrate a wide variability in baseline physiology, overall health status, medical conditions, injuries, training regimens, and other factors. Also, endurance events vary in duration/intensity of activity, temperature, humidity, access to fluids, and more, so each of these can play a role. The overall picture must be considered, and education for anyone who engages in sport, support staff, and race and sporting event personnel should help them be able to recognize signs of severe dehydration and help affected individuals obtain treatment. These more severe signs include a decrease in sweat response, mental status changes, and decreased or absent urine output.
When it comes to dehydration, here is the best advice:
- Try to simulate event conditions while training (whenever possible), so you can learn from your own body’s response to efforts under similar circumstances. If this is not possible, make an effort to understand what effects are most common or likely during whatever the unique event conditions might be – temperature, altitude, and duration.
- Pay particular attention to pre-hydration (topping off the tank before endurance activities.) Doing so is often overlooked and is probably the most critical variable that an athlete can control. One strategy commonly used by cyclists is to begin drinking a favorite sugar/electrolyte solution (the one your body is familiar with to avoid stomach and other issues) 12-24 hours BEFORE an endurance activity. Commonly, this consists of drinking small volumes (4-6 ounces) frequently (every 1-2 hours) until urine output increases and is clear. The above is not a fool-proof method but eliminates a common issue by not being dehydrated on the start line.
In summary, hydrate, hydrate, hydrate. Of note, oral rehydration involves more than just drinking water. Since we consume sugars and lose water and electrolytes through exertion, it is optimal to replace all of these when managing clinical symptoms of dehydration. There are a variety of specialty drinks which are ideal since they contain varying mixtures sugars/electrolytes to replace losses from exercise optimally. Whether it’s a cycling race or other strenuous activity you’re engaging in, being aware of the sneaky way dehydration can strike and heading it off at the pass are some of your best tools to stay healthy and active any time of year.
