Dehydration and Over Hydration (Hyponatremia) for the Cyclist

More information about dehydration and how to avoid it can be found in Nutrition for Cyclists: Eating and Drinking Before, During and After the Ride which can be purchased on  For information about the book and how it relates to what I’ve posted to Tuned In To Cycling, please check out this post.

There’s a lot of water around us; roughly 70% of the earth’s surface is covered in water. There’s also a lot of water in us; roughly 60% of the human body is water. The balance between the water inside and outside our bodies has to be kept within a fairly narrow range or bad things can happen. Either too much or too little water inside the body can, in extreme cases, be lethal.

When you ride your bike your body loses water in the form of sweat. This is a good thing because the evaporation of sweat from the skin is the main way your body sheds heat while you’re on the bike. Without that cooling the increase in body core temperature from the heat generated by your working muscles would kill you fairly rapidly.

Sweating is good but the fluid loss that comes from sweating is not so good. When you lose water through sweat you become dehydrated. At extreme levels dehydration can lead to heat stroke which can be life threatening. However, even relatively mild levels dehydration can have negative effects on cycling performance.

How dehydration affects your body

For the endurance cyclist the main effect of dehydration is to decrease the volume of blood in the system. This has two major consequences and both of them are bad.

First, a decrease in blood volume reduces the body’s ability to shed heat and thus leads to an increase in core temperature. This works in two ways. The main way the body sheds heat during exercise is through sweating. When blood volume is decreased through dehydration sweating decreases because the water in sweat is derived from blood plasma. You can’t sweat it out if it’s not there in the first place.

In addition to sweating, the body sheds excess heat by radiation and conduction if the air temperature is lower than the body temperature. When body temperature rises the blood vessels near the surface of the skin expand (vasodilation). This brings more of the blood into close contact with the surface of the body so that heat carried by the blood can be lost through conduction and radiation. A decrease in blood volume decreases the amount of blood that can be brought into contact with the body’s surface thereby partially offsetting the benefits of vasodilation.

The second negative effect of the decrease in blood volume caused by dehydration is that the blood becomes thicker or more viscous. The heart has to work harder to pump the thicker fluid through the body. Blood flow becomes more sluggish. During diastole (the resting phase when the heart fills with blood) the heart may not completely fill with blood so that the volume of blood pumped with each heartbeat declines. Blood flow throughout the body declines and blood flow to the working muscles is critical for the cyclist because it brings fuel to the muscle in the form of glucose, and carries away waste materials and heat.

How dehydration affects performance

Dehydration and increase in body temperature are separate factors that have independent effects on athletic performance. They also interact with each other to decrease performance. It’s helpful to keep in mind how both of these factors are affecting performance both individually and in combination.

A loss of as little as 2% body weight (3 lbs. for a 150 lb. person) can negatively affect athletic performance. This negative effect increases as the amount of time spent performing the exercise increases. A study carried out with runners showed that a roughly 2% loss in body weight due to dehydration produced approximately a 3% loss in performance over 1500 meters and a 5% loss in performance over 5K or 10K meters. A loss of 5% body weight through dehydration (7.5 lbs. for a 150 lb. person) has been shown to produce approximately a 30% loss in performance.

VO2 max is a measure of the maximum amount of oxygen a person can use during exercise and is widely used as a general measure of aerobic fitness. Studies carried out in cool laboratory environments have shown a 5% decrease in VO2 max with a 3% decline in body weight through dehydration. The negative effect of dehydration on VO2 max is increased in the warm or hot environments the cyclist usually experiences. Decreases in VO2 max are most probably caused by the decrease in blood volume produced by dehydration that was discussed earlier. Note also that increased body temperature can reduce VO2 max even when individuals are fully hydrated. In other words, dehydration and increased body temperature act alone and in combination to decrease VO2 max.

Whether or not it is accompanied by a decrease in VO2 max, dehydration produces a decline in endurance as measured by the time it takes to reach exhaustion. A loss of 5% body weight through dehydration can decrease endurance even for low intensity exercise (e.g., low intensity walking). The loss in endurance increases markedly as either the intensity of exercise increases or the level of dehydration increases.

Endurance is also affected by core temperature; as core temperature increases, endurance decreases. Because dehydration has a large effect on the body’s ability to shed heat, core temperature rises more quickly and endurance decreases more quickly as dehydration increases. In addition, dehydration produces a lower tolerance for increased core temperatures. Exhaustion occurs at lower core temperatures for dehydrated individuals as opposed to hydrated individuals.

As if all that isn’t enough, there is evidence that suggests that dehydration in combination with increased core temperature my cause glucose to be burned more quickly and less efficiently in working muscles. Glucose is the fuel that powers muscle activity, it’s almost always in short supply for the endurance cyclist, and the evidence suggests it’s used less efficiently when dehydration is accompanied by increased core temperature (which it almost always is). You’re trying hard to keep going and avoid the bonk by paying attention to what you eat while you’re riding, you are always short on fuel, and dehydration is causing you to burn the limited fuel you have available less efficiently. Not good.

Stay hydrated.

Over hydration or hyponatremia

Is it possible to drink too much water? Yes, the condition is called hyponatremia and in rare cases it can be fatal. When the body is over saturated with water the sodium in the body becomes diluted. When this happens individual cells throughout the body swell with the result that a variety of bodily functions may be disrupted.

At present we don’t know as much about athletically induced hyponatremia as we would like. The condition was first described in 1981 and much of the data that exists about hyponatremia is drawn from samples of convenience taken at popular athletic events such as marathons and reports from the military documenting the consequences of water consumption during training. The problem with samples of convenience is that important variables are left uncontrolled that need to be controlled in order to draw sound and justified conclusions from the data.

Hyponatremia is diagnosed based on the level of sodium in the blood and measuring serum sodium level is relatively easy. The problem is that the serum sodium level that is widely accepted as indicating hyponatremia may be accompanied by a variety of symptoms ranging from confusion or seizures, through headaches and stomach distress, to, in many cases, no symptoms at all.

Hyponatremia began to be commonly observed along with the rise of marathon running as a popular hobbyist sport. In order to prevent dehydration and heat stroke race organizers frequently stress the importance of staying hydrated during the run and they provide frequent water stations along the route. In addition, manufacturers of “sport drinks” often market their products at open running events and pay the organizers to make their drink available to runners along the route.

Under these circumstances you might expect relatively inexperienced hobbyist runners to drink too much during their run. There appears to be evidence that this is the case. Hyponatremia has been observed to be much more common among inexperienced runners who train at slower speeds and take more time to complete the marathon.

At present, it is unclear whether women are more susceptible to hyponatremia than men. Some studies suggest they are, other, better controlled studies, suggest there’s no difference. Also, there is no evidence that the sodium content of many “sports drinks” serves to prevent hyponatremia .

How can you tell if you’re drinking too much water when you ride? A rough method is to weigh yourself right before and right after your ride. If you gained weight and drank a lot of fluids during the ride, you were probably over hydrating.

In the absence of medical complications, avoiding hyponatremia is basically a matter of common sense. Anecdotal reports in the medical literature about people who experienced extreme hyponatremia include very slow runners who took very long times to finish marathons and who reported drinking at every water station along the way and a woman who prepared for her marathon by drinking 10 liters of water (!!) the night before. Use your head for something besides a place to keep your helmet while you ride and don’t drink excessive amounts of fluids and you shouldn’t have a problem. Most important, don’t become dehydrated because you’re afraid of hyponatremia.

Wiggins, Froome, Team Sky and the 2012 Tour de France

One of the saddest days of the summer: The Tour de France is over for another year.  Some in the media and in online commentaries have complained that this year’s tour was boring.  They tried to sell the idea that Team Sky was torn by internal competition between Bradley Wiggins and Chris Froome.  They claimed that Froome humiliated Wiggins by waiting for him on the final climb on the Peyragudes.  That’s not how it looked from here.

(Analyze, plan, test, evaluate, revise) iterate, train, execute.  Win.  That’s what Team Sky’s Tour de France looked like to me.  I thought it was brilliant. But more than that, I thought the entire Sky team carried out their Tour de France with integrity, dignity and class.

Bradley Wiggins showed himself to be everything you would want in a team and race leader.  He didn’t just ride for himself, he rode for his team.  When was the last time you saw the man in the yellow jersey at the front of the entire peloton going under the 1K flag on the final lap around the Champs-Elysees leading out his team’s sprinter?  Rather than ride safely in the peloton Wiggins performed the same service for Cavendish on Stage 18’s sprint finish the day before the final time trial that Wiggins needed to cement his overall victory.

When Cadel Evans (who was still in contention as one of Wiggins’ main rivals) flatted because some moron threw nails on the road on the Mur de Péguère, Wiggins tried to slow the peloton down so that Evans could catch up.

In his press comments Wiggins always praised his team.  While this is the standard response riders give to the press, Wiggins appeared to mean it, unlike some others who sound like they are reciting a memorized script.  Moreover, Wiggins appeared to be genuinely pleased on the road when his teammates did well.  While he showed triumphant emotion at the finish of the penultimate day’s time trial when he locked up the Tour victory, he never engaged in self-conscious displays of ego or self-aggrandizement. Compare Wiggins demeanor with Thomas Voeckler’s seemingly self-absorbed “Adore Me. Worship Me” freewheel to the line in his terrific Stage 16 victory, or Peter Sagan’s self-conscious what-victory-display-should-I-do-today-to-draw-attention-to-myself behavior during the first week of the Tour.

Wiggins’ behavior reflected that of his team.  After a foolish tweet by Chris Froome’s girlfriend, the media reacted like a bunch of hysterical little girls with their panties in a twist about internal division within Team Sky or about Sky sacrificing Froome for Wiggins.  Team Sky responded in a way that I wish more people and organizations would when the media creates ridiculous tempests in teapots.  They basically told the media they were being silly and then disengaged and ignored them.  The TV commentators’ indignant and self-righteous “We’re not making this up!” response was hilarious and seemed an apt demonstration of just how lame the media can be.

As for Froome, when asked about his role on the team, he appeared to answer honestly when he said he thought he had a chance to win the Tour, not taking that chance and possibly becoming the first British rider to win the Tour was a personal sacrifice, and it was a sacrifice he was going to make because he was there to ride for the team and the team was there to win the tour with Wiggins.  Of much more importance, he rode the truth of what he said.  Some interpreted his waiting for Wiggins on the Peyragudes as Froome humiliating Wiggins by showing the world that he was the stronger rider.  What I saw was a loyal rider supporting his team leader and doing exactly what he said he was there to do.  After watching him in this Tour de France, if I was putting together a professional cycling team I would take one Chris Froome over ten Frank Schlecks on the basis of personal demeanor and integrity alone.

I thought that throughout the 2012 Tour de France Bradley Wiggins, Chris Froome and Team Sky behaved with irreproachable dignity, integrity and class. They gave professional cycling exactly what it needed after years of doping allegations and controversy. Brilliant.