By: Daniela R Liscio
Physical exercise can elicit high sweat rates and substantial water and electrolyte losses that compromise exercise performance. In general, we consume food and fluid during exercise to provide energy, delay fatigue, prevent or delay dehydration, replace electrolytes and minimize gastrointestinal distress. But how much fluid to rehydrate with during exercise?
Significant differences in sweat rates between sports, individuals themselves (affected by genetic predisposition, heat acclimatization and training status, at a minimum), exercise duration, climates, and other factors, affect this determination. Individualized rehydration plans need to take account of all of this. Athlete tables can be used to help approximate weight loss depending on your size and activity (and I work with clients to make those estimations). You can also just weigh yourself immediately before and after your workout to determine how much weight you are losing during a typical workout at the gym (you will always need to do this more than once but that is particularly true if working out outdoors as climate, including humidity and wind, terrain and precise metabolic requirements for your particular exercise will all affect your fluid loss workout to workout). The goal is to avoidexcessive weight loss, which refers to a loss of > 2% of body weight. For each pound of water weight lost, you’ll want to drink about 20 ounces of fluid. You won’t necessarily be able to drink that much during your activity, but drinking at least some of that will help minimize that excessive weight loss.
Fluid composition is also important; this is where the issue of “electrolytes” comes in. Electrolytes are substances in fluids that conduct electrical currents (there’s the science) and include substances that are involved in fluid balance in our bodies (there’s the reason we care). These electrolytes include the positively charged ones (sodium, potassium, calcium and magnesium) and the negatively charged (chloride, bicarbonate, phosphate and protein). Why do I waste time listing them? I answer that below. Generally, sodium and potassium are the ones you will see highlighted in sports drinks and will help replace sweat electrolyte losses. Sodium also helps to stimulate thirst – a good thing as it will encourage you to drink. In addition, carbohydrates help sustain energy during high-intensity exercise of approximately 60 – 90 minutes or longer, as well as less intense exercise over longer periods. The general guidance from The Institute of Medicine for persons performing prolonged physical activity in hot weather is that fluid replacement beverages contain “approximately 20-30 meq·L−1 sodium (chloride as the anion), approximately 2-5 meq·L−1 potassium and approximately 5-10% carbohydrate (although these amounts may vary further depending on weather conditions).” Translated, this amounts to 460 – 690 mg/L of sodium, 78 – 195 mg/L of potassium, and carbohydrates of approximately 30-60 grams, per hour of activity.
Keep in mind, however, that unless you are working out for multiple hours, you are unlikely to deplete your stores of electrolytes during a single training session. My general rule of thumb is that unless I am working out for more than two hours, I stick to water. Experimentation here is key and you will need to evaluate all of the factors I listed above as they pertain to you – how hard and in what activity are you working, in what climate do you typically work out in, are you a heavy sweater, are you a salty sweater, does your sport involve a lot of clothing, etc.? Also, take into account your energy stores heading into a workout – are you already dehydrated and energy depleted? You may then need more than water to keep you going.
Generally speaking, and particularly for those trying to drop weight, don’t ruin the benefits of your workout by sucking back excess sports drinks that are simply not necessary for optimal performance. Recall the Gatorade-police got on me about this (you can read about that here) … but all Gatorade cares about is selling more Gatorade. On that note, when you do use sports drinks, experiment with different types and brands. I have always found all versions of Gatorade resulted in stomach distress for my longer training sessions and, after a lot of experimentation, I settled on watered down “Gu Brew” (including in any of my races sponsored by Gatorade, which just meant I had to carry more stuff with me but that was a better alternative than puking over my handlebars). Choose what works best for you.
We will get into food choices during workouts another time since, if you are able to consume food during a workout, including food substitutes like gels and the like, this will very much affect the types of fluids needed. Generally speaking, the only fluid you will need with food is water – this is extremely important as an excess concentration of carbohydrate in the gut will do you a disservice, and we will get into why that is another time. I spent time actually listing the electrolytes involved in fluid balance above as a reminder that a diversified and nutrient dense diet will ensure trace minerals are present and in good supply in all those cells and body parts for when you are digging deep during a workout.