Skip to content

Calculate Your Sweat Rate

How Much Should You Drink During Triathlon & Marathon Training?

One of the most common questions I hear from athletes is, “how much do I need to hydrate during my race and in training.” The answer is a lot more simple than you might assume and can be calculated by knowing your sweat rate and understanding what an acceptable level of fluid-loss is during workouts regardless of your preferences for how you hydrate during training! 

Recent Research in Hydration Strategies

According to recent research on rehydration strategies during endurance exercise from Dr. Lawrence E. Armstrong, an expert in hydration assessment and Professor at University of Connecticut, “There is overall consensus in the literature that dehydration of 2 to 4% represents the range in which endurance exercise performance declines. A body water deficit of 1–3% was less likely to impair endurance exercise performance significantly than dehydration of 4–7%.” 

Interestingly this same research challenged perceptions between 5 drinking preferences of most endurance athletes of: 

1. Only drinking when thirsty 

2. Consuming fluid whenever and in whatever volume they prefer that day

3. Planned drinking based off sweat rate

4. Drinking nothing

5. Drinking as much as possible. 

In summary the findings were to consume a volume of fluid that avoids dehydration greater than 2–4% of body mass (weight loss in pounds) and avoids overhydration regardless of drinking preference (sorry drinking nothing crowd you probably need to make some life changes).. 

Here’s a summarized version of the 9 recommendations that are appropriate for most endurance and ultraendurance activities surrounding hydration: 

  1. Measure body weight before and after exercise a change of body mass (in pounds) during exercise is a reasonable, but not perfect, measure of water gain or loss.

  2. Do not gain weight from fluids during endurance exercise.

  3. Consume fluid at a rate less than or equal to 24 oz/hr to reduce the risk of EHN (Exertional Hyponatremia) . 2001 guidelines of the International Marathon Medical Directors Association and the American College of Sports Medicine recommend a  13.5 oz-27 oz/hr rate of fluid intake during endurance exercise.

  4. Be alert for cues that discourage drinking. When stomach fullness, bloating, or vomiting are experienced, decrease fluid intake.

  5. Know modest levels of dehydration up to 2–3% of body weight are tolerated well, with little risk, symptoms, or a decline in exercise performance. 

  6. According to The International Marathon Medical Directors Association a weight loss that exceeds 4% of body mass justifies a medical consultation. 

  7. After endurance exercise, white salt deposits on a shirt, jersey, or shorts indicate both a high sweat rate and a high sweat sodium concentration. Training weekly in excessive heat or a day-long Ironman or marathon event in a hot environment may lead to whole-body salt deficiency due to large sweat sodium losses, inadequate sodium intake, or both. If salt depletion is suspected due to an increased appetite for salt, it is smart to consider adding specific dietary food items to ensure that daily sodium intake replaces exercise-induced sodium loss.

  8. Athletes should be aware that sodium intake, while not discouraged, may provide little or no defense against EHN during prolonged exercise and the effects are unpredictable. 

  9. Experiment with rehydration options based on your drinking preferences during training sessions, before using them in competition or in hot environments….

-Lionel Sanders, Pro Triathlete

Danger of Dehydration from Pro Lionel Sanders

Here’s a recap of the danger of dehydration and under-hydrating in excess of the 2-4% body weight loss range described by Lionel….

“I had a traumatizing experience at Ironman Texas in 2015. On the bike in Texas I consumed less than one Gatorade bottle per hour. The bike took me over four hours, and I also probably sweat a bit in the hour long swim. By the time I got off the bike my vision was beginning to get blurry, and my muscles were not functioning very well. I felt like a concrete block. These symptoms only got worse during the marathon, to the point where I was swerving all over the path. “

Coming off the bike, Lionel’s estimated deficit was 5.66L of fluid. In terms of percentage of body weight, 5.66L is equal to 12.45 pounds, and before the race he weighed about 165lbs, so he had lost a minimum of 7.5% of my body weight, leading to a dangerous outcome…

How To Stay Under The Danger Zone Of 2-4% Fluid Loss

Step 1: 

  • Weigh yourself before your workout with minimal clothing (try to use the bathroom before taking your weight). 
  • After your 45 min – 2 hr workout, dry off, take your weight again in minimal clothing BEFORE using the bathroom . 

Step 2: 

  • Use our handy dandy calculator to find your results.
  • Aim to stay under 2-4% fluid loss during training and racing. Practice your hydration strategy in training and re-test when weather conditions change.

My favorite sports drink for training is what is available out on course, for Ironman events that is Gatorade Endurance. I’ve found that helps not only meet my fluid goals but also my helps with my sodium (2x sodium over normal Gatorade) and my carb goals during training and racing during sessions under 2 hours. In the future, I’ll be posting an article about creating a race-nutrition plan for triathlons and marathons. 

I’ve had luck and less GI distress when drinking Gatorade Endurance during sessions longer than an hour (or sessions that require less than 2.5% body weight loss) and drinking in intervals of every 10-15 minutes according to my sweat rate and goal of losing less than 2.5% which is where I personally find my performance declines. During races, 

For athletes who are in high-volume training, I’ve found it’s beneficial to follow the same hydration strategies for shorter sessions as well because of cumulative effects of dehydration over their weekly workouts.

During weekly training, it’s good to track your fluid intake and make sure you’re not reaching a dangerous deficit if you’re a long-distance athlete. It’s an important part of your training and racing to stay hydrated. While reaching 100% re-hydration during workouts and is not the goal, try to limit fluid loss well below Lionel Sander’s story to avoid dangerous outcomes. On hot days, have a “weigh in” at the beginning and after long workouts. “Weight loss of 6% or more has been a good indicator of serious problems.” -Specifics, from the American College of Sports Medicine as verified by Biochemist Dr. Bill Vaughn, PhD

Good Luck Out There!

-Coach Dan, Training To Race

Training To Race participates in the Amazon Services LLC Associates Program, which is an advertising program designed to provide a means for TTR coaching to earn fees by linking to Amazon.com. The compensation received will never influence the content, topics or posts made on this site. 

Related News