When it comes to hydration, you can’t always trust your gut. Here’s what science tells us about optimal performance during prolonged exercise in the summer sun.
When athletes talk about hydration, they aren’t just talking about water. Generally, a fluid supplement includes added carbohydrates and electrolytes.
Anyone exercising for longer than one hour, but especially endurance athletes competing in variable weather conditions (like in the hot summer sun), will benefit from the right balance of water, carbohydrates, and electrolytes.
That info probably didn’t blow your mind. Researchers and athletes alike have known this for a while. The keywords “endurance athlete hydration” pull in about 25,000 results on Google Scholar.
Events always have some kind of sports drink available at aid stations. But even athletes who understand the importance of good hydration struggle to get the right balance.
Studies show that over 50% of professional, collegiate, high school and youth athletes arrive at workouts under-hydrated. And when subjects are allowed to replace fluids as needed, they only replace about two-thirds of what they lost during exercise. (McDermott, et al.)
Getting your hydration routine dialed before, during, and after hard exercise will help reduce fluid loss, which will allow you to maintain top performance, lower your submaximal exercise heart rate, maintain plasma volume, and reduce heat-related complications.
You’ll feel better over the duration of the event and you’ll even feel better the next day. (Duvillard, et al.)
On the flip side, endurance exercise without proper hydration can cause overheating, exhaustion, cramps, dizziness, headache, vomiting, nausea, and chills.
Ingesting water without replacing sodium--especially while exercising in extreme conditions--can have some major implications, too, like hyponatremia. (McDermott, et al.)
Not only is proper hydration imperative to top performance (and a comfortable experience), it’s also tricky to get right. Too little and you’re dehydrated. Too much and you’re overhydrated.
Humans maintain total body water and overall hydration within a narrow range of 1% hyperhydration (over-hydrated) to 3% hypohydration (dehydrated). Keeping your hydration status between +1% and -1% of bodyweight gained or lost via fluids creates optimal hydration.
And while many athletes think they can count on their own body to let them know when it’s time to take a sip, it’s worth mentioning that our bodies are notoriously unreliable at indicating hydration needs during exercise.
External factors, such as psychological stress and calorie intake can blunt thirst sensation. (Goulet, et al.)
When you exercise while properly hydrated, your body directs fluids to your brain, heart, lungs, sweat glands, and working muscles and sweats to maintain thermoregulation.
But when you go out for a 2+ hour ride or run in the hot summer sun while dehydrated, your body has to make a few compromises, like decreasing your sweat rate to ensure vital organs maintain fluid balance.
That decrease in sweat has some major consequences. Namely, your body temperature will rise by about 0.15-0.20 degrees C for every 1% of body weight lost via sweat during exercise.
All that extra heat impairs skin blood flow and can even induce cardiovascular stress. For example, heart rate increases 3 to 5 beats per minute for every 1% decrease in body mass.
Dehydration will ultimately lead to a considerable decrease in performance (and considerable discomfort). (McDermott, et al.)
A loss of only 2% body weight can cause nausea, diarrhea, vomiting, and gastrointestinal problems. It can also decrease time to exhaustion and increase likelihood of heat illness and cardiac stress.
For a 150lb athlete, 2% is only 3lbs or about 1.4 liters of fluid lost. For a 200lb athlete, 2% is 4lbs. Losses of 5% or more may decrease the capacity for work by about a third. (McDermott, et al.) (Jeukendrup, et al.)
Endurance athletes can’t rely on water alone for exercise that is either high intensity or of a long duration, especially in the heat. When you sweat, you lose an important electrolyte, sodium.
When athletes lose too much sodium, it causes intracellular swelling, leading to all kinds of physiological dysfunction.
A sodium concentration of less than 135 mmol during or within 24 hours of activity is considered a potentially fatal condition, called hyponatremia.
Hyponatremia is mostly limited to endurance sports, like marathons and triathlons, and serious complications are uncommon. That being said, the symptoms of low sodium aren’t exactly a picnic (think GI distress and dizziness) and mild hyponatremia occurs in 10%-20% of marathon finishers.
Female endurance athletes should pay particular attention to their sodium levels, as women who compete in events lasting four hours or more appear more susceptible to hyponatremia than men. (Kipps, et al.) (González-Alonso, et al.) (McDermott, et al.)
Hyponatremia is usually a concern during hot, intense, and long events. It can be avoided by adding an electrolyte tablet to water bottles and sports drinks. A basic electrolyte tablet will include sodium at a bare minimum, but should also include potassium, calcium, iron, magnesium, and chloride.
Unfortunately, there isn’t a simple answer to how much fluid per hour you should consume. There is a huge variation in sweat rate, or the amount of sweat lost, and sodium loss between athletes.
Multiple studies show that sweat rate can fluctuate between 0.5 to 4.0 L/h! Sodium loss can range between 0.2 and 7.3 g/h.
What’s more, factors like duration and intensity of exercise, as well as external factors like ambient temperature and humidity can also impact hydration needs. That being said, you can use the two strategies below to get an idea of your specific needs. (Gonzalez RR, et al.) (Baker, et al.) (Patterson, et al.) (Montain, et al.) (McDurmott, et al.)
The easiest way to assess hydration is by calculating body mass change. Start by gathering a baseline. Weigh yourself on three consecutive days while well-hydrated.
Weigh yourself again before the training session or race and then weigh yourself after. Most (if not all) of the fluctuation in weight will be fluid loss. Ideally, any fluctuation will be between +1% and -1% body weight. (McDurmott)
If you find your fluid intake is inadequate, you can get a rough estimate of how many liters per hour you need to consume using the sweat-rate calculation below.Sweat-Rate Equation
Sweat loss (L) = Body mass before exercise (kg) – Body mass after exercise (kg) + (Volume of fluid consumed during exercise [L]) – (Urine volume, if any [L])
Sweat rate (L/h) = Sweat loss (L) / Exercise duration (h)
a If body mass is assessed in kilograms and fluid consumed in liters, the sweat rate = L/h.
(McDurmott, et al.)
Like the change in body mass calculation, a urine color assessment is inexpensive, easy, and noninvasive. For best results, compare the color of morning measurements to this chart.
Unfortunately, you can’t just assess color in the toilet bowl if you want accurate info. For one, there isn’t a standard for amount of water per toilet bowl. For best results, assess urine in a clear container.
Unlike total fluid levels, sodium levels have to be measured with a blood test. To make matters more complicated, the symptoms of overhydration and dehydration overlap, with symptoms including headache, dizziness, physical exhaustion, muscular twitching, and nausea. (McDurmott, et al.)
If your body mass change calculation often comes back overhydrated, you may give this issue special consideration, particularly when exercising hard in hot temperatures and during day-long or multi-day events.
Serious complications from low sodium levels are possible under extreme conditions but are unlikely in heat-acclimatized athletes or those participating in shorter events. (Hew-Butler, et al.)
The general rule of thumb is that there is no need to supplement fluid intake for exercise lasting an hour or less. This is just an estimate, and how much consideration you give to your hydration plan depends on the duration, intensity, and environmental factors.
For example, a jogger headed out for a light, 30-minute jog around the block doesn’t need to be concerned about fluid intake beyond their usual routine. But a slightly dehydrated athlete competing in a seven-hour race in the backcountry on a 95 degree F day could find themselves in a bad situation.
Plan ahead to ensure you have enough water and sodium to perform and feel your best.
Most athletes don’t need to consume extra fluids before training. However, athletes competing or training in a location with limited access to fluids can benefit from slight hyperhydration.
That being said, if you’re well-hydrated, there is no reason to chug a bunch of water right before an event. It will probably just make you nauseous. Instead, pay attention to your urine color for a few days before the race and adjust fluid intake as needed.
As stated above, there is no single amount of fluid per hour that works for everyone.
During exercise, try to drink enough fluids to limit body mass loss to 2% or less and avoid gaining any mass.
After you calculate body mass change for a variety of conditions, you’ll get better at predicting how much fluid you need under what circumstances.
Before grabbing a beer, try to replace fluids with a recovery drink that includes protein. Consider replenishing sodium, too. Up to 150% of fluids lost need to be replaced to return to baseline.
About The Author:
Matt Mosman (MS, CISSN, CSCS) is a research scientist, endurance athlete, and the founder and Chief Endurance Officer at EndurElite. Matt holds his B.S. in Exercise Science from Creighton University and his M.S. in Exercise Physiology from the University of California. Matt and his family reside in Spearfish South Dakota, where they enjoy running, mountain biking, camping, and all the outdoor adventures Spearfish has to offer.