Exercise: Hydration, Heat and Humidity

Your Dietitian Recommends: :Ways to combat Hydration, Heat and Humidity

by Janet L. Simpson M.S., R.D.  Sports Nutritionist

Hydration, Heat and Humidity

When summer days are upon us, the extra daylight gives us extra training time…and extra heat and humidity!

Some of us are training for our first triathlon; others have already completed an Ironman and continue training because it is simply “in our blood.”

The heat and humidity offers special challenges whether we are training or competing in an event; it may negatively impact your ability and if you are not acclimatized to the heat-can become dangerous. In moderate summer heat of 80 degrees, or even 70 degrees in high humidity, our bodies cannot cope. Signs of this are heat cramps, heat exhaustion and even heat stroke! Heat injuries are caused by three factors: loss of body fluids, loss of electrolytes and increase core temperature.

Ellen Coleman, RD, MA, MPH, CSSD, in her book Eating for Endurance, explains:

Heat cramps, or voluntary muscle spasms, occur during prolonged exercise when there has been profuse and repeated sweating. Heat cramps are probably caused by an imbalance of the body’s fluids and electrolyte concentrations.

Treatment: Rest, drink fluids containing electrolytes such as sports drinks, and add salt to foods.

Heat exhaustion is probably caused by a reduced blood volume due to excessive sweating. Blood pools in the extremities and the person may faint or feel dizzy. The symptoms of heat exhaustion also include nausea and fatigue. Although sweating may be reduced, the rectal temperature is not elevated to dangerous levels-less than 104 degrees.

Treatment: Rest in a cool place and drink fluids containing electrolytes. Medical attention may be required.

Heat stroke is a medical emergency requiring immediate action. The body’s temperature-regulating processes simply stop functioning. Sweating usually stops and the skin becomes dry and hot. The rectal temperature is excessively high-over 105.8 degrees F. Other symptoms include: disorientation, vomiting, headache, and loss of consciousness. If untreated, death occurs due to circulatory collapse and central nervous system damage.

Treatment: Aggressive steps must be taken to immediately lower the elevated body temperature. Until medical help arrives, the person can be covered with ice-packs, immersed in cold water, or rubbed with alcohol.

Be aware of the symptoms of impending heat illness. These include weakness, chills, goose pimples on the chest and upper arms, nausea, headaches, faintness, disorientation, muscle cramping, and cessation of sweating. Continuing to exercise when experiencing any of these symptoms can lead to a heat injury.”


The risk of heat related illness may also be increased by alcohol, caffeine, and some prescription or over-the-counter drugs. The issue of heat + humidity = sweat rolls off your skin rather than cooling you through evaporation. Heat, instead, builds up and your core temperature increases. A five-six percent loss of body weight increases the risk of a heat-related illness considerably.

The heat index is as important to consider as the temperature itself.

Remember! The heat index is an index that combines air temperature and relative humidity to determine an apparent temperature; how hot it actually feels. …(wikipedia)

At an apparent temperature of:
90-104 F Heat cramps or heat exhaustion possible
105-130 F Heat cramps or heat exhaustion likely, heat stroke possible
130 F Heat stroke likely.

To calculate it yourself, the formula is:

Or use this chart:

So…what to do???

The American College of Sports Medicine (ACSM) recommends that athletes drink two cups of fluid two hours before the event. This gives the fluid time to be absorbed and excrete excess water. Your urine should be a light yellow or clear color.

For running and biking, if you lose two percent or less of your body weight, it is considered safe to rehydrate after the event. For over two percent, fluids and electrolytes need to be replaced during the event! The amount depends on how much you have lost and your gastric emptying rate. About six ounces of fluid every 15 minutes typically works well for most athletes.

It is difficult to determine the amount of sodium lost in sweat. Ellen Coleman recommends:

“As a general guide, I estimate that endurance athletes lose about 1 gram of sodium per quart (2 lbs) of sweat. If the athlete loses 2 lbs of sweat per hour, then consuming one quart of a sports drink that contains a higher amount of sodium should adequately replace both sweat sodium and fluid losses.”

You may also want to choose a sports drink that contains a small amount of potassium. (I like Gatorade Endurance, but that is ONLY a personal preference.)

An athlete would also do well to determine their sweat rate in an exercise intensity and approximate heat and humidity that will simulate competition.

What about swimming?

Is dehydration an issue? Absolutely!

Bonnie Modugno, MS,RD reports:

“I worked with a swimmer who I suspected was dehydrating during her workouts. She couldn’t fathom sweating in the water so we did the simple experiment. She weighed herself before; jumped in the pool, and then after the workout another weigh in. She lost 2-3 pounds each workout. Her coach started to leave water bottles at the end of the pool for her to hydrate with. Her energy, stamina and times all improved.”

For the IM swim, ACSM recommends that the athlete drink 1 oz of fluid 4 hours before the swim. This will give adequate time to excrete any excess fluid. If you do not want to wake up 4 hours earlier, then two hours will work…it is easier to eliminate the excess fluid during the swim

Thanks again, Ellen!

Val Richardson, my swim coach adds:

“We do actually sweat in the water but the pores are clogged with water and harder to release. More than 50% of our heat is penetrating off the top of our heads. Take the cap off, especially if it is silicone. Get out of your wetsuit. The body needs to breathe.”

Thanks Val!

The opposite and just as serious as being dehydrated is hyponatremia.

Hyponatremia is defined as: “The normal concentration of sodium in the blood plasma is 136-145 mM. Hyponatremia occurs when sodium falls below 130 mM. Plasma sodium levels of 125 mM or less are dangerous and can result in seizures and coma.” (Health A-Z)

The symptoms of hyponatremia begin with confusion, and then progress to a headache, unconsciousness, coma and possibly death. It occurs from drinking excessive amounts fluid with low sodium levels. ACSM states: “inclusion of sodium (0.5-0.7 grams per liter of fluid) in rehydration solution ingested during exercise lasting longer than one hour is recommended since it may be advantageous in promoting fluid retention, and possibly preventing hyponatremia in certain individuals who drink excessive quantities of fluid.”

Hope this is helpful as you get out this summer to swim, bike and run.

Questions? I welcome you to contact me.

See you in the water, with our bikes and on the run!

Eat well and play hard,

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