Learn how each works and when it’s best to use one versus the other.

Most runners have heard over and over again that drinking fluids as the summer months approach is of the utmost importance; neglecting to drink when it’s hot outside is committing one of the cardinal sins of sports nutrition.

Well, that’s somewhat true, but it isn’t quite that simple. Instead of telling you to drink more fluids when the mercury rises, over the next few pages we’ll take a look at when you should be drinking water versus when you should be drinking sports drinks or an electrolyte beverage. This is also an important topic to explore in regard to hydrating during marathons or fueling for marathon-specific long runs, but that’s a different topic for another article. Here, we’ll stick to a discussion of hydration and drinking protocols during training.

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Finally, I’ll help you calculate exactly how much fluid you need to consume on any given training run, and how to apply that information to your training.

Fluid Absorption Rate

First, it’s important you understand how sugar and electrolytes impact your fluid absorption rates. The speed at which water, electrolytes, and sugars can be absorbed into the blood stream is one of the main determinants of what type of beverage you should choose when trying to stay hydrated.

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The absorption of fluids into the body is largely dependent upon two factors: 1. the rate at which it is absorbed through the walls of the small intestine; and 2. the speed at which it is emptied from the stomach. Both of these factors are controlled by the composition of a liquid in terms of its carbohydrate (sugar), and electrolyte concentrations.

As a general rule, the higher the carbohydrate content of your beverage, the slower the absorption rate will be. Consequently, trying to maintain proper hydration and balanced electrolyte levels during a run with sugary sports drinks is difficult. On the other hand, plain water passes through the body too quickly without providing the necessary sugar to spark the insulin response and ignite the recovery process.

Puts you in a bit of a pickle, right?

Your choice for hydration will depend on whether your primary aim is rehydration (keeping the body cool and maintaining fluid balance) or the replenishment of energy (restocking sugar and electrolyte stores).

What To Drink Before/During Running

Most sports drinks on the market are what sports scientists call isotonic, which means they contain a carbohydrate solution that is at 6-8% concentration. These drinks are in the middle of the spectrum in terms of absorption rate, with water being the most readily absorbed (hypotonic) and something like fruit juice, being greater than 8% sugar concentration (hypertonic) and therefore the least absorbable. Because the sugar concentration of most sports drinks is higher than that of most body fluids, they are not readily absorbed into the blood stream and are thus not optimal for the purpose of hydration.

Before and during a run, rehydration should be your main priority. When training in warm conditions, rehydration will allow you to maintain fluid balance and stay cool. Accordingly, your best choice before and during your run would be water, a heavily diluted sports beverage, or water with electrolyte tablets.

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By drinking water alone, diluting your sports drink, or using electrolyte substitutes, you provide your body with the best combination of electrolyte replacement and immediate absorption. Likewise, electrolytes, especially sodium and potassium, reduce urine output, speed the rate at which fluids empty the stomach, promote absorption from the small intestine, and encourage fluid retention.

Furthermore, not only do you want to shy away from consuming unnecessary amounts of simple sugar when you can avoid it, research shows that when a runner consumes high-glycemic (Gl) foods such as high-sugar sports drinks or energy bars an hour before a run, he or she may become fatigued more quickly.

What To Drink After Running

After you are finished working out, water or a diluted sports drink is not the best choice for your recovery needs. Water and diluted sports drinks do not contain enough of the sugars and electrolytes that your body needs in order to bring itself back into balance.

In addition, because water or highly diluted drinks are rapidly absorbed into the blood stream, consuming high quantities results in a rise in plasma volume (in non technical terms, this means your body is oversaturated with water). This rapid absorption leads to a further imbalance of electrolytes and frequent bathroom stops, which will only increase fluid loss and decrease your desire to drink.

Your best choice after a workout is a drink that contains a good amount of sugars, electrolytes and possibly some protein. Scientific literature has consistently shown that drinking a beverage that contains a 4 to 1 ratio of carbohydrates to protein is optimal for recovery. Therefore, at the very least, you should be drinking a sports drink after you exercise to help ignite the recovery process. Consuming one with protein will also help speed things along.

Calculating Your Sweat Loss

Every runner is different in regard to rehydration and replenishing electrolytes after training. Some runners are “salty sweaters” and some people sweat very little and there’s a fine line between not drinking enough and drinking too much. Consuming the right amount — and right kind — of fluids during and after running will help you avoid an upset stomach from drinking too much, becoming a victim of hyponatremia, or not drinking enough and becoming dehydrated.

Unfortunately, most generalized advice doesn’t cut it when it comes to how much you need to rehydrate: some experts say drink to thirst, which may not keep up with your own body’s sweat loss rate if you’re a heavy sweater, while others say to consume 8-10 ounces of fluid per hour, which doesn’t factor in temperature, humidity, or environmental factors.

Rehydrating properly during and after a run can sound sound daunting and confusing, but calculating your exact fluid loss in any given temperature and humidity is actually quite easy if you use a sweat loss calculator and create a reference chart. All you need to input is your weight before and after each run, any fluid taken or lost (when going to the restroom), and the distance/time you ran. The calculator will do the hard work for you.

Use this calculator a few times in different temperatures and record your results. You’ll now have an easy reference chart to see exactly how much fluid you need replenish on any given run and in any temperature. This will help you avoid dehydration, over-hydrating, or getting a sloshing stomach while on the run this summer.