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The Danger of Dehydration

Safety Corner logo by Frank Hubbard

October 2015

The heat this summer will not quit. We are still riding in the 90s and a day with low humidity is cherished. This brings up the subject of dehydration. We are all aware of the fact that we need to drink enough water but all of us tend to underestimate our need for hydration. At the end of a recent century, I was asked to see a rider who had severe quadriceps muscle cramps. When I told him that his problem was dehydration, he told me that he could not accept this diagnosis as he had drunk almost 4 bottles of water on the ride. Fortunately, he did accept the treatment and was able to drink fluids and eat salty foods avoiding a trip to the emergency room.

The ride was mostly shaded but there were some stretches with full sun. Temperatures had reached 94 deg and humidity was near 90. As I had also experienced some early muscle cramping, I decided to total the amount of fluid that I had consumed before and after the ride and check my remaining weight loss when I got home. My weight was down 6 lbs. I had consumed 8 liters at that point and eliminated 1.5 liters at the ride stops. Loss of fluid by sweating was about 1.5 liters per hour and on the ride I had drunk only about 3.5 liters. Yes I was dehydrated like the rider that I had seen.

As we exercise and heat up, our circulation shifts to shunt more blood to the skin and exercising muscles. Vaporization of sweat is an efficient method of eliminating body heat as heat is released as liquid converts to gas. In humid environments, however, sweat isn’t vaporized but beads up and drips off leading to reduced capacity for release of body heat. Sweat losses must be replaced to prevent dehydration. 2% body fluid loss does not lead to deterioration of athletic performance but with further losses in blood volume the circulation must compensate. Blood flow to exercising muscles and to the intestines falls in an attempt to maintain blood flow to the skin. This likely is the cause of intestinal and muscle cramps. Muscle contraction slows with 3% dehydration. Further circulatory compromise can lead to loss of the ability to eliminate body heat and lead to the later stages of heat illness: faintness, confusion, seizures, vomiting and hyperthermia.

Ride hydration is obviously important and we all need to anticipate when riding long distances in hot weather. On a long ride, we need water, calories and salt. Water must be palatable if we are going to consume 1 liter an hour. Diluted fruit juice is probably the simplest. If you prefer your drinks cold, ice your water bottles 1/2 full and add fruit juice on the day of the ride. On hot days, drink an extra bottle of fluid one hour before the ride start. When drinking a lot of water anticipate your salt needs.

Remember that the most common cause of death at marathons is not myocardial infarction but dilutional hyponatremia. Plan your salt intake: pretzels, potato chips, nachos, peanut butter, crackers, sliced turkey, cheese, hummus etc. or add a pinch of salt to your water bottle. Start replacing salt as you start drinking early in the ride.

Remember, when you feel thirsty, you are probably 2% dehydrated. If you have to stop to use the portapotty, you are adequately hydrated. If you are experiencing muscle cramping and urine flow has ceased, you are at least moderately dehydrated and should spend more time at the next rest stop.

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