What's Your Fluid Replacement Drink Number?

By Rick Morris

You have a lot or choices when picking a fluid replacement drink for your training runs and races. You have a choice of plain water or any of a multitude of commercial fluid replacement drinks. Some of the drinks contain electrolytes while others do not. Some include a variety of sugars for carbohydrate replenishment and some don't. The drinks that contain carbohydrate replacements may have anywhere from 1% to over 10% carbohydrate content.

Those always popular carbohydrate drinks will improve your endurance and running performance because they help replace the carbs that you are burning through like a pile of dried out hay. But, that steady supply of carbohydrates can come at a price. That price is related to the gastrointestinal upsets or even the dreaded runners trots that sometimes go hand in hand with carbohydrate replacement drinks? Do you always need a fluid replacement drink containing carbohydrates? What is the best percentage of carbohydrate in your sports drink? What is your fluid replacement number? The answer to those questions begins with a basic understanding of how your body absorbs those fluids.

Some runners are under the mistaken belief that the fluid enters their body as soon as they drink it. The truth is that the fluid is considered to be "outside" your body until it is absorbed into your blood from your small intestine. Obviously, you want those fluids and carbohydrates to absorb as quickly as possible to help you prevent dehydration and to replenish the carbs you have used up.

There are two components that go into your body's ability to absorb fluids; the speed at which the fluid is emptied from your stomach and the pace that it is absorbed from your small intestine into your bloodstream. In a perfect world, those two components are working in perfect symmetry. Your stomach is emptying quickly and at the same rate that the fluids and nutrients are being absorbed through your small intestine. Unfortunately, running is rarely a perfect world. If your stomach empties faster than the fluid can be absorbed you end up with gastrointestinal problems and most likely a good case of runners trots. On the other hand if your stomach is emptying too slowly you will not be able to deliver the fluid and nutrients fast enough to keep up with your bodies demands during running.

There are a number of things that affect your body's ability to absorb fluids, including:

• The Amount of Fluid in Your Stomach - A stomach that is full of fluid will empty faster than a more empty stomach. As your gastric fluid level drops so does your rate of gastric emptying.
• The Percentage of Carbohydrates in Your Drink of Choice - Studies have shown that increasing the percentage of carbohydrate content will slightly delay gastric emptying. Plain water, with its 0% carbohydrate content has the fastest gastric emptying rate. While increasing the carbohydrate content does slightly delay gastric emptying, the delivery of energy is higher due to the sugar content.
• The Type of Sugar in Your Drink - All sugars are not alike. Your fluid replacement drink might contain glucose, fructose, sucrose or glucose polymers. Each of those can have a different effect on how your body absorbs fluids. The additional of any of these sugars can slightly delay your gastric emptying but glucose has been shown to counteract that decrease by increasing the rate of absorption from your small intestine while  fructose can make is worse by delaying it. For this reason you should avoid drinks containing only fructose as the energy component.
• The Intensity of Your Running or Workout - I just love it when most of the scientific studies agree on something; it rarely happens. The studies do agree that running at lower intensity levels (less than 70% of VO2 max or slightly slower than marathon pace) do not affect your rate of gastric emptying. Running at higher intensity levels ( more than 70% of VO2 max) may slightly slow your emptying rate.
• Your Stress Level - If you are stressed out or have high levels of anxiety, the rate at which your stomach passes on its fluids could be delayed. This can be important if you suffer from race anxiety symptoms.
• Your Current Level of Hydration - If you are dehydrated your rate of gastric emptying is slowed.

So, what does all this have to do with your fluid replacement drink number? Keeping things simple,  it means you should adjust the carbohydrate percentage according to the length and intensity of your workout or race as well as the ambient temperature. There isn't one specific number that is ideal for all situations. Below is a chart outlining my recommendations for your fluid replacement drink number.

How about that upset stomach that seems to go along with carbohydrate containing sports drinks? Is that a myth or do the carbs in sports drinks really cause gastrointestinal upsets. It turns out that the main culprit may not be the carb content but more likely the type of carbohydrate and the volume of your stomach. An investigation by the Gatorade Sports Science institute recruited the help of 36 runners who drank a variety of concentrations from 0% to 8% carbohydrate. They found that there was no statistically significant difference in gastrointestinal complaints at any concentration between 0% and 6%. They did find a small increase in the incidence of complaints at the 8% level.

Is seems that the most likely causes of runners trots and other gastrointestinal complaints in distance runners is the volume of fluid in the stomach and the use of a drink with mostly fructose, which delays the absorption of fluids in your small intestine. The best thing you can do to avoid runners trots is to stay away from drinks containing large amounts of fructose.

While it may seem like taking frequent small drinks to keep the fluid content of your stomach low would be a good strategy, that may not be the case. Keep in mind that a full stomach empties faster than an empty one. So, if you need to maximize the delivery of fluids a more full stomach is the way to go. It's a kind of Catch 22. A full stomach empties faster but it may also cause gastric problems. So, what do you do? For most runners, I would suggest a compromise. Start with a stomach that is about half full of fluid and then drink when needed to keep your fluids flowing. You might also try doing your training runs with a fuller stomach so your body adapts to running in that condition.

References:

Gastric Emptying During Exercise, Sports Science Exchange, SSE#46 Volume 6 (1993) Number 6

Effects of CHO electrolyte solution on prolonged exercise, Perceptives in Exercise Science and Sports Medicine, Benchmark Press, Carmel, 1991 pp35-89

The influeance of volume on gastric emptying and fluid balance during prolonged exercise, Med. Sci. Sports Ex. 23 314-319

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