Here we are at the edge of summer, and I am naturally more inclined to drink than to eat. Not in the Hemingway tradition of strong drink, but more to stay hydrated and avoid being blown away by the hot desert wind here in Arizona. In Arizona, we don’t just rehydrate, we fight desiccation.
It seems everyone knows the same rule about hydration: “Drink eight glasses of water a day.” I guess that depends on what you’re calling a glass, but most folks think that is about 64 fluid ounces. The American Academy of Science actually recommends about 88 to 120 fluid ounces a day. Out of that water allowance, you exhale some moist air, sweat, make urine and keep the blood in liquid form.
The economy of hydration is simple, and the body self-adjusts until you stress the system. Add heat and exercise and you can produce more than a quart of sweat an hour just to maintain your body temperature. The body has a hard time absorbing more than 12 to 24 ounces per hour from the gut, so when you don’t drink enough, there is more going out than in. This is not a viable long-term strategy.
But your body, unlike your checking account, will tolerate short-term debt. Most people can handle a 3% to 4% weight loss from exercise and sweating, without their performance deteriorating, or feeling too bad.
Since the ability to sweat is the principal way to rid the body of excess heat, water loss is intrinsically linked to core body temperature. For every 1% of body weight loss, the core body temperature goes up 0.4 degree Fahrenheit. It is common to have runners at the end of long, hot races to have temperatures of 101 or 102 degrees F. They have no infection – their body is simply storing heat that couldn’t be dissipated during the race.
Once weight loss over 3% to 4% from exercise and sweating occurs, things go downhill quickly. A 5% to 8% weight loss causes weakness and dizziness. Get to 10% and people start dying.
There are deaths every summer from people working or exercising too much in the heat. These deaths are completely avoidable.
The body can adjust to exertion in high temperatures; it just can’t adjust instantly. For example, the U.S. military is the expert in getting people adapted to safely work in high temperatures. It takes 14 days to fully acclimatize. One to two hours of exercise in the heat per day, while well-hydrated, gets you there – this is called heat stress. The body measurably changes, with the core body temperature dropping, increased blood flow to the skin (your radiator), and an expanded blood volume.
Once acclimatized, people are very resistant to heat-induced illness. But stay out of the heat for five days, and you lose your heat tolerance. What happens if you don’t have 14 days to work up to labor in the heat? You learn firsthand about heat illness, which generally has three stages of severity.
Heat cramps are the least of the heat-related illness. Rest, hydration and massage will provide some relief.
Heat exhaustion is a total body illness, where patients become sweaty, weak, nauseated and may have a headache (perhaps you have heard the expression “a hot mess”). For heat exhaustion, you need to get out of the heat immediately and hydrate. You may not tolerate being in the heat for a few days.
Push the exertion in the heat far enough and you have heat stroke. This is a medical emergency. Your heat-dissipating machinery has failed. Symptoms are hot, dry skin, a high fever of 104-105 degrees F, confusion and extreme weakness. Your high-core body temperature is cooking your internal organs – the brain and heart in particular. It is necessary to cool you off as rapidly as possible, while waiting for the paramedics.
Many heat-related deaths are preventable. Heat tolerance is something that takes both hard work and patience. It is not achieved by strength of will or intense desire. It is not obtained in a day or a hurry. Stay well-hydrated, and keep your exertion to an hour or less to safely build tolerance this summer.
Donald Bucklin, MD (Dr. B) is a Regional Medical Director for U.S. HealthWorks and has been practicing clinical occupational medicine for more than 25 years. Dr. B. works in our Scottsdale, Arizona clinic.
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