WRITTEN BY: AMANDA STEGMANN RD, MS
In the mid-’90s, Sprite had a powerful and simple slogan: “obey your thirst.” Ultimately the purpose was to obey your thirst by drinking sprite, but a dehydrated athlete reaching for a lemon-lime flavored, sugar-filled carbonated concoction just doesn’t seem to be as realistic or effective as reaching for plain, never fail, ice water.
Credit is due, however, that there is truth and power in that slogan. “Obey your thirst”. As it turns out, our thirst cue is a strong indicator of our body’s hydration status and could ultimately have an effect on athletic performance.
Our body is composed of up to 65% water, although after this hot summer, maybe more like 57%. Anyone else sweat just by sitting still this August? Water is used by all organs of the body, serving as a transportation system for nutrients, and metabolic reactions. The ubiquitous rule we’ve all heard about how much to drink is 8 glasses a day. It’s difficult to apply one universal amount for all human bodies. I doubt Dwayne “The Rock” Johnson and Danny Devito have the same fluid requirements, so a more appropriate universal rule: 35ml/Kg is just fine. That would mean a 70kg adult would need about 2.4 Liters of water a day to be properly hydrated.
Dehydration, by definition, is a deficit in total body water with an accompanying disruption of metabolic process. Dehydration can occur in extreme temperatures, from lack of consumption of fluids, or through sweat during prolonged exercise. Exercising while dehydrated includes increased cardiovascular strain, increased heat strain, altered central nervous system function, altered metabolic function. (1)
During heat stress, like the summer of 2018, dehydration can compromise one’s athletic performance. A meta-analysis conducted by Cheuvront et al found that dehydration of 2-7% of body weight consistently decreased endurance performance. Temperatures played a role in performance as well.
Dehydration of at least 2% or greater of body weight reduced exercise performance by about 40% on a hot dry climate. That’s a large margin of performance loss that could be largely recovered if proper hydration protocols are followed. Cheuvront et al found only a 20% reduction in performance output when a 2% body weight dehydration occurred in a cool climate as it allowed increases central blood volume and lowered core body temperature (1).
Ultimately, in all temperature conditions, dehydration by 1-2% doesn’t appear to alter performance when under 90 min duration. This means the sweat from a typical Saturday Pilates class or after work jog won’t have a negative impact on your speed or number of sit-ups, but dehydration over 2% in hot environments is when you might see a decline in performance (2).
Now we know dehydration in amounts greater than 2% of body weight, especially in extreme temperatures, can have a negative effect on performance. But how does one remedy this? Does the timing and amount of water matter?
One of the largest meta-analysis of its kind found that “drinking according to the command of thirst was associated with an increase in power output compared with a rate of drinking below…and above. The probability that drinking to the dictate of thirst confers a real and meaningful practical advantage on performance under field conditions compared with drinking below and above thirst sensation is of the order of 98% and 62%, respectively” (3).
Well, it may sound obvious but: obey your thirst. Drink according to thirst cues. According to Goulet et al, repeated replenishment and satisfaction of thirst throughout exercise should preserve extracellular fluid homeostasis and maximize endurance performance (3).
If intuition is still a little too vague to give accurate advice, and concrete instructions serve you better, what exactly does drinking according to thirst cues look like? Even though Goulet’s research supports it, simply telling a client to obey their thirst may undermine your credibility and professionalism as a nutrition coach; even if that recommendation is supported with sound science!
Since 8 glasses a day doesn’t fit all, here is a more uniquely tailored method to proper hydration so your nutrition client feels individually catered to according to their needs.
The first step, pre-hydration. If sufficient fluid intake in the amount of 35ml/kg has been consumed within 12 hours of the last exercise session, ideally a person should be close to appropriately hydrated. In anticipation of a prolonged exercise regimen of greater than 90 minutes, especially in hot conditions, 5-7ml/kg at least 4 hours prior to the event will suffice (4). For a 70kg individual, 350-490ml will do. For those of you not following the metric system, that would be about 1.5-2 cups of water.
This will allow time for urine output to return to normal. If more fluid is still necessary, 2 hrs before the event add another 3-5ml/Kg of fluid or another cup of water for a 70kg individual. While water is essential, it’s not the only thing lost during excessive perspiration. Electrolytes like sodium and potassium are lost as well and play a vital role in maintaining core temperature and muscle contractions. Consuming beverages with about 50mEq/L of sodium will help to retain the consumed fluids or about 1100mg for those who are a few years beyond high school chemistry atomic weight conversions (4).
For those competing in endurance activities in excessive heat, adding 2mEq/L of potassium, or 80mg, as well as 5-10% carbohydrate solution will aid to prevent dehydration and a potential decline in performance (4).
Once the activity is over, the best way to rehydrate is to replenish 100-150% of the weight lost during activity with water which universally looks like about 1-1.5L of fluid per each kg lost. If that’s too much math, you can always just obey your thirst.
- Cheuvront SN, Carter R, Sawka MN. Fluid Balance and Endurance Exercise Performance. Current Sports Medicine Reports. 2003;2(4):202-208
- Goulet ED. Dehydration and endurance performance in competitive athletes. Nutrition Reviews. 2012;70.
- Goulet EDB. Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis. British Journal of Sports Medicine. 2011;45(14):1149-1156
- Shephard R. American College of Sports Medicine Position Stand: Exercise and Fluid Replacement. Yearbook of Sports Medicine. 2007;2007:254-255