This summer’s record-breaking heat waves have seriously inhibited many people’s ability to engage in outdoor adventures comfortably and safely. Fortunately, for the dedicated adventurer, there are well-educated researchers such as Dr. Melissa Roti who have made a career out of studying and disseminating knowledge about sports nutrition. Here are some of her tips to avoid dehydration and worse while exercising in hot temperatures:
In the summer heat a primary concern for any active individual is staying cool and hydrated. Hydration is so important because evaporation of sweat, which consists of water and electrolytes, is our primary means of cooling ourselves. Exercise increases our body temperature, the extent to which depends upon exercise intensity and duration, fitness level, clothing and environmental conditions. When we perform muscular work, heat is produced. More intense and/or longer durations of exercise produce more heat. When the body temperature becomes too hot hyperthermia can take effect and lead to heat illness (cramps, exhaustion and heat stroke).
There are several mechanisms our body uses to thermoregulate. Our cardiovascular system works to bring hot blood from our core to the skin by dilating our surface blood vessels. This heat can then be transferred to sweat glands and evaporation of sweat can draw heat away from the skin. One key point is the term “evaporation”. If the sweat just drips off our body it is not cooling us, we are simply losing more fluid and electrolytes. With elevated humidity, evaporation of sweat is impeded, contributing to more stored body heat over time. We can also cool our bodies by convection, where airflow over our skin helps cool the blood at the surface. Technical clothing serves a very real purpose by allowing air movement near the skin. This permits evaporation of sweat and convective cooling to help prevent hyperthermia.
With all this sweat loss to cool our bodies, we are at risk of dehydration. As we lose more fluids the body cannot thermoregulate efficiently, which greatly increases our risk of hyperthermia and heat illness. Exercise performance is negatively affected due to muscle fatigue, loss of coordination and loss of concentration.
Therefore, staying hydrated is key to staying safe and maintaining performance. Thirst is a signal that you are already becoming dehydrated, so it is ideal that you drink before you feel thirsty. Unfortunately, we cannot store extra fluids before an adventure so it is good to be well hydrated beforehand and plan on drinking regularly during exercise and rehydrate afterwards. The American College of Sports Medicine (ACSM) recommends:
- 16-20 fluid ounces 4 hours before exercise
- 8-12 fluid ounces 10-15 minutes before exercise
- 3-8 fluid ounces every 15 minutes during exercise
- Rehydration equivalent to body weight lost after exercise
Water is fine at rest or for short bouts of exercise, but for any exercise lasting an hour or longer a glucose-electrolyte beverage is recommended.
You can monitor your own hydration status with two simple techniques: body weight and urine color. Record your body weight before and after exercising in the heat and you will see it decrease due to water lost as sweat. If you lose one pound of body weight this should be replaced with 24 fluid ounces. Dehydration effects can be seen with as little as one percent dehydration (loss of one percent body weight). Ideally, try not to become more than two percent dehydrated as health risks dramatically increase. Recording body weight can help determine your personal sweat rate, which can vary dramatically between individuals, so you can determine your own fluid needs.
You can also monitor your urine color (and volume). If you produce a large amount of pale urine (like lemonade) you are well hydrated. If you produce a smaller amount of dark urine (like apple juice) you are dehydrated.
You can also minimize hyperthermia by dressing appropriately, avoiding direct sun radiation, avoiding high heat and humidity, and lowering exercise intensity and duration. But as long as you stay properly hydrated, you can maintain better performance and thermoregulation.
For additional information, see the ACSM position stand on Exercise and Fluid Replacement (2007).