Drink too little and performance may suffer. Drink too much and there are other risks.
In endurance sports (and most other sports), getting your hydration just right is important. Dehydration is common and may cause performance to suffer. Overhydration, although less common, can be a much more serious issue. Knowing how to hydrate properly will help you achieve peak performance and avoid serious risks.
Why do we sweat?
In order to make sure body temperature stays within acceptable limits and we don’t overheat, we sweat. The higher the exercise intensity (read as higher speed/pace/power output), the more heat is produced and the more we need to sweat to stay cool. In hot conditions, it is even more important because sweating may be the only way we can cool down our bodies. Other factors can affect sweat rate. Sun, high humidity, insulating clothing can all result in increased sweat rate. On the other side, shade, wind and low humidity can aid in cooling and reduce sweat rate.
How much sweat loss is OK?
Sweat loss is usually measured as a percent of our body weight. In very short efforts such as a 5K, sweat rate may be high but time spent exercising (sweating) is short so that total body weight loss will likely be low (less than 2% of body weight). As duration increases and more time is spent sweating, losses can easily reach 2-5% of body weight.
Research shows that performance can begin to degrade as sweat losses go beyond 2-3%. Guidelines from American College of Sports Medicine and International Institute of Race Medicine (Optimal Hydration) respectively suggest targeting 2% and 2-5% body weight loss. CORE aligns with these guidelines.
Why is dehydration a problem?
When we lose too much sweat we become dehydrated; this reduces blood volume, increases heart rate, and makes it harder maintain our body temperature. It also increases our perceived exertion. All of these reduce our ability to compete and may increase the risk of heat exhaustion and heat stroke.
Why is overhydration a problem?
Drinking too much (overhydration) can lead to a dangerous medical condition called hyponatremia (low serum sodium levels). Early symptoms can include swelling, headaches, vomiting. More serious symptoms include disorientation and seizure. In the worst cases, if not treated, hyponatremia can result in coma and death. The risks of hyponatremia increase significantly with overhydration. If you do not lose weight during your race, you are 7 times more likely suffer hyponatremia.
How do I prevent dehydration and overhydration?
In order to prevent dehydration, it is important to start a race hydrated and maintain proper hydration throughout the race.
Before the race, drink ~500ml during the 2 hours before the start; excess water will be eliminated through urine. You can confirm you are well hydrated by checking that your urine color is pale.
During the race (or training), there are two approaches to hydration.
Drinking to thirst is a recommendation that works fine for the slower athlete: if you are thirsty, drink; if you are not thirsty, don’t drink.
Drinking to a plan can work for everyone, especially if you are going a bit faster, if your race is longer and if you understand your sweat rate. Your planned fluid consumption should always be targeted at a rate that ensures you will not gain wait; a good gauge for this is to consume at or below your sweat rate. If you want to find out how to measure your sweat rate, visit the article Sweat rate calculation.
It is wise to use the early parts of a race when the gastrointestinal tract is working well to absorb both carbohydrate and fluid. Later in the race, even though you may be thirsty, your gut may not absorb as much. Don’t drink excessively, and use common sense. The goal should be to lose a little weight (1-2 kg, 2-4lbs or ~2-3% of body weight) at the finish line.
It is important to note that if bloating occurs and fluids seems to accumulate in the stomach there is no point ingesting more fluids. Reducing your run intensity a bit and giving the stomach some time to pass fluid on to the intestine for absorption will relieve bloating.
Baker LB, Jeukendrup AE. Optimal composition of fluid-replacement beverages. Compr Physiol. 4(2):575-620, 2014.