There is a robust debate in the global sports science world regarding this question. Due to this it is common to read articles that tout one way and suggest the other way is less desirable to sports performance and maybe even dangerous to health. Given there are two conflicting sets of guidelines (both based on research) what is an athlete to do?
It may help to understand a few points. Both views agree that during exercise: 1. drinking nothing or 2. over drinking (causing weight gain) can lead to problems and should be avoided. The central question at play: “Is it necessary to replace some, most or all fluids lost in sweat?” Most organizations in the US including the American College of Sports Medicine suggest replacing enough to prevent a 2% drop in body weight during exercise. https://journals.lww.com/acsm-msse/Fulltext/2007/02000/Exercise_and_Fluid_Replacement.22.aspx
Losses of 3% or more are not only performance killers but can be dangerous in causing a decrease in blood volume making the heart work harder. It also can lead to heat illness like heat stroke where the body is unable to manage temperature. The other point of view states that a 2% body weight loss is normal and not a detriment to performance and health. This group also stresses that pushing too much fluids during exercise can lead to a dilution of the blood where sodium levels drop to dangerously low levels (called hyponatremia) which is dangerous, and in some cases fatal.
The bottom line: there is no one right way that applies to all athletes and all circumstances.
When is it best to drink to thirst: Training sessions that are less intense and more skill based and last less than 90 minutes. Exercise in cooler conditions. Best for athletes who find drinking during exercise natural and easy to do. Best for athletes who are adequately trained, and heat acclimated. Also, for athletes with a history of exercise induced hyponatremia.
When is it best to drink according to a plan: Intense and long (greater than 90 minutes) training or racing sessions especially in high heat and humidity. Best for athletes who tend to forget to drink or do not pay close attention to thirst signals. Best for heavy sweaters with significant sweat losses. Also, athletes doing two or more heavy training sessions in one day. Athletes not heat acclimated or with a history of heat illness.
Other factors to keep in mind that will affect your ability to hydrate well:
· For exercise under 1 hour, water is usually adequate. Over an hour add sodium & carbs (unless purposely training low, in which case you may be able to omit carbs – talk more with your sports dietitian about this).
· Electrolytes (Sodium 180-225 mg/8oz fluid) will enhance fluid absorption and retention.
· Carbohydrate (10-15 grams/8oz fluid) will enhance fluid absorption.
· Cool to cold fluids are easier and more desirable to drink in hot conditions.
· Flavor matters. If you dislike the taste of a beverage you simply won’t drink enough.
· Training the gut to tolerate fluids during exercise can help. Athletes who don’t do this often develop an upset stomach making adequate hydration harder to achieve.
· As we age our thirst mechanism kicks in later so senior athletes are at increased risk of both dehydration and hyponatremia. Junior athletes’ thirst mechanisms are immature.
Beyond general guidelines athletes can dig deeper to know what is best for their unique physiology. There are many variables you can measure and monitor to find out what hydration strategy works best for you. While these tests are not perfect and do not compare to an exercise physiology lab, they still can give you some useful information. Keep in mind that the answers will change due to changes in temperature, humidity, altitude, acclimatization, fitness level, age and phase of the menstrual cycle for women.
1. Do a sweat rate test: https://www.gssiweb.org/toolbox/fluidLoss/calculator Drink to prevent a 2% or more body weight loss and not so much that you gain weight. Work with your sports dietitian to assess the most appropriate fluid target for you and your exercise (ultra-endurance athletes and cold weather athletes may have atypical fluid recommendations and body weight loss recommendations).
2. Monitor the color of your urine. Urine as dark as an amber beer is a sign of inadequate fluid intake. Use a clear, disposable cup to assess the color. Keep in mind that other factors can affect urine color. https://www.medicalnewstoday.com/articles/324469
3. Use a sweat patch to determine the amount of sodium lost in your sweat.
4. Do a urine specific gravity test to assess hydration level upon waking.
5. Keep a diary of your fluid intake and your perceived exertion to look for correlations. Keep in mind that many factors affect how energetic you feel (several nutrition variables, sleep quality, fitness level, weather and even psychological factors).
6. Work with a sports dietitian to help make sense of these sources of information.