As summer temperatures ramp up I thought it would be a good idea to discuss heat stress and how our body can be negatively affected by long hot days on the field or trail. With proper knowledge and some dietary modifications it’s possible to prevent heat stress from occurring. Here’s some background information to help put it in perspective.
Water, water everywhere
Our bodies are mostly water (50-70%, depending on fat vs. muscle composition). Water serves many important functions: nutrient digestion and transportation; removal of body waste (urine); amniotic (joint) fluid, saliva, and bile; maintenance of blood volume; and body temperature regulation. With regard to temperature regulation, dissipation of heat via sweat evaporation is the most effective way of keeping our bodies cool during prolonged exercise in hot or humid environments.
Sweating the details
When our core body temperature rises, our cardiovascular systems shunts hot blood from our core to our skin. Sweat then transfers this heat outside of our body where it is dissipated via evaporation. In addition to heat and water, sweat contains a number of important electrolytes, namely sodium, chloride, and small amount of potassium. If we exercise in hot environments and/or for more than 60 minutes, it very important to replace the water and minerals lost via sweat.
Failure to replace lost water and/or electrolytes may result in one of three forms of heat stress: 1) heat exhaustion, 2) heat stroke, and 3) hyponatremia. Heat exhaustion is defined as a 3% or greater loss of body weight in fluids. Less fluid volume = thicker blood = less oxygen transport to our muscles = premature fatigue (exhaustion). Heat exhaustion is characterized by excessive sweating (our body’s way of trying to keep us cool). A 1-3% loss of bodyweight negatively affects performance, and if heat exhaustion goes unchecked it may progress to a life-threatening case of heat stroke.
Heat stroke occurs after a significant amount of fluid (>3% body weight) volume is lost and represents an overload on our temperature regulation system. Our body simply does not have enough fluid (blood) to shunt to our skin, and therefore we stop sweating — a very, very bad sign. If physical activity is not stopped and/or body fluids are not replaced, core temperature may rise to deadly levels (>105 degrees Farenheit).
Hyponatremia refers to low sodium levels. Sodium is needed to maintain fluid balance in our body and also plays a critical role in muscle contraction. Hyponatremia occurs when we replace water lost via sweat, but forget about the electrolytes (mostly sodium and potassium) lost via sweat. Chugging gallons of water in the heat is better than developing heat exhaustion or heat stroke, but it presents its own problems. You lose water and sodium via sweat. If you replace just the water then you are further diluting the sodium concentration in your body. This can result in severe, debilitating muscle cramps and compromised athletic performance.
Typically we get plenty of salt (Na+Cl–) in our diet to maintain sodium and chloride stores during moderate intensity and duration exercise. However, as exercise duration increases past 1-2hrs in hot and/or humid climates, electrolyte replacement (particularly sodium and to a lesser extent potassium) can benefit performance. To avoid hyponatremia, drink a sports beverage that contains water AND electrolytes: 100-150mg sodium and 30-40mg potassium per 8oz of fluid is ideal. While you’re at it, you should also consume a simple form of carbohydrate to maintain energy levels. Suggested intake is 30-60 grams of carbohydrate per hour, ideally in a 6-8% solution — your body cannot absorb greater quantities of carbohydrate, especially during intense exercise. A 6-8% solution is very important because this matches the carbohydrate concentration (osmoality) of your blood.
Food for thought
Sports beverages like Gatorade have been specifically developed to help you prevent or cope with heat stress. It’s possible to make your own sports drink from diluted fruit juice (a source of potassium) and table salt. Avoid juices with intact fiber. How much water, juice, and salt would be needed to replicate the ideal electrolyte concentration and osmoality? In addition to sport beverages, what other modifications could you make to your diet to prevent heat stress?