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Military Health System

Stay Hydrated for Optimal Performance

Image of A soldier takes a drink from his canteen. Army Pfc. Milton A. Torres takes a drink while on guard duty during a Combat Support Tactical Exercise at Fort McCoy, Wisconsin. Hydration is important for optimal health, particularly in high-heat, dry climates and during outdoors activities. (U.S. Army photo by Army Sgt. 1st Class Gary A. Witte)

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You may have heard that dehydration can occur before you feel thirsty. That’s true – but thirst is only one way your body lets you know you’re low on H2O.

Whether you’re conducting operations, working out, or simply spending a lot of time outdoors in high-heat, extreme cold, high-altitude, dry, or humid climates, keeping hydrated is paramount to optimal performance.

“Water is essential for survival,” said Jonathan Scott, a registered dietitian and assistant professor of military and emergency medicine at the Uniformed Services University of the Health Sciences (USU) in Bethesda, Maryland.

“It plays many critical roles in the body, including regulating our body temperature, moistening tissues in the eyes, the nose, the mouth,” said Scott. “It protects our body organs and tissues. It carries nutrients and oxygen to the cells in our body, lubricates joints, helps keep the kidneys and liver functioning properly by flushing out waste products.”

Water comprises about 60% of our body weight. As a result, it also “helps to dissolve vitamins and minerals and nutrients to make them more accessible to your body,” said Scott.

Losing as little as 2% of your body weight through sweating, urinating, and breathing can affect your mental and physical performance. Once you become dehydrated, you may start to feel lightheaded, dizzy, disoriented, fatigued or irritable.

“Feeling thirsty is an indication that there is a fluid imbalance within the body,” added Scott.

That means the amount of fluid you take in versus that which you lose are not in proportion, upsetting your body’s fluid balance. And while you can also take in too much water, dehydration occurs when you lose more fluid than you take in, he said.

However, thirst doesn’t let you know how dehydrated you may be. That, Scott said, depends on a variety of factors, including how long you’ve been feeling thirsty, environmental factors, etc.

“Dehydration is a common issue among our military population,” said Navy Lt. Karla Eslinger, an environmental health officer at Naval Hospital Twentynine Palms aboard Marine Corps Air Ground Combat Center in California.

Urine Hydration chart
Although certain foods can alter the color of your urine, typically, when you’re properly hydrated, your urine should be light colored and clear. (Graphic by Human Performance Resources by CHAMP)

A good way to check your hydration level is through your urine.

Although Scott warned there are foods — such as beets and certain dietary supplement ingredients — that can alter the color of your urine, typically, your urine should be straw-colored (light) and clear when you’re well hydrated, said Eslinger.

“Darker and more concentrated urine suggests dehydration,” she said.

This is important, because “long-term consequences of dehydration can include urinary tract infections, kidney stones, even kidney failure, and on the extreme end of the spectrum, seizures, and ultimately death,” said Scott.

Having a battle buddy is important, he added.

“It's usually somebody else who notices something about you before you notice it yourself,” he said. “A buddy might point out you're dragging behind or seem clumsier, ask if you’re okay, and remind you to drink something.”

How much is good enough?

To be fully hydrated, drinking small amounts of fluids constantly throughout the day is best.

Eslinger highlighted that most people need several hours to drink enough fluids to replace what they have lost through sweat.

“The sooner you get started, the less strain you place on your body from dehydration, which is a primary contributor to heat exhaustion,” she said. “Drinking at shorter intervals is more effective than drinking large amounts infrequently.”

And while the ideal amount of fluids varies by individual due to body weight, activity, environment, and other factors, Scott suggests that a good rule of thumb is aiming “to consume half your body weight in pounds in fluid ounces daily” as a starting point.

According to his suggestion, an individual weighing 150 lbs. should consume about 75 oz. of liquids daily from food and beverages — that’s equal to almost 9.5 cups. But that’s under normal circumstances and activities. If you exert yourself more than usual, such as exercising or working in the heat, you should aim for more.

The Human Performance Resources by CHAMP (HPRC) at USU’s Consortium for Health and Military Performance (CHAMP), recommends aiming for 16-32 oz. of fluid every hour, but no more than 48 oz. per hour, during exercise or high-exertion activities. Likewise, hydrating with water during activity under one hour in duration is sufficient. However, for activity lasting longer than one hour, research supports consuming sports drinks that contain electrolytes and carbohydrates.

In addition, many fruits and vegetables are rich in water and minerals, including melons, berries, peaches, oranges, cucumbers, celery, and lettuce. Milk is also an effective rehydrating option, too, Scott said.

“Dairy milk, for those that tolerate dairy, is one of our best hydrating beverages,” he said. “It naturally contains both of those electrolytes (sodium and potassium), and it’s predominantly made up of water.”

What’s even better, chocolate milk is not only rehydrating, he said, but it also helps to replenish some of the carbohydrates that your body burns during prolonged exercise of 60 minutes or longer.

During those types of activities, “your body uses carbohydrates stored in the liver and muscle in the form of glycogen as its predominant fuel source,” said Scott.

“Chocolate milk has carbohydrates, which helps replenish some of those glycogen stores; protein, which we know is important for the repair process; calcium and vitamin D, which we know are crucial for bone health,” he said. “So, it really is a complete deal.”

What to Do During Deployed Operations?

In a deployed environment, options may be limited during operations. In that case, Scott said, “consuming foods and beverages that replace fluids and electrolytes (sodium and potassium) will typically work out better than just plain water alone, but it's important to incorporate those fluids throughout the entire day.”

To make it easier, Eslinger recommends hydrating before work, during work, and after work — especially for people working regularly in high-heat climates, like the desert.

In general, Scott recommends taking rest breaks from high-exertion activities, keeping track of work-to-rest ratios, and using hydration tables based on different heat categories.

Years of military operations in extreme climates has yielded lessons learned for optimal hydration, such as monitoring the Wet Bulb Globe Temperature. Similar to the heat index, this tool is useful in measuring the heat stress on the body and can help “reduce the number of heat stress injuries for participants and leadership,” concluded Eslinger.

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Last Updated: August 10, 2021
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