LAST UPDATED: April 29, 2021

Performance suffers from even small amounts of dehydration

Dehydration happens quickly with physical activity, especially in extreme climates. Mid-afternoon slumps could be from the consumption of a large meal or the effect of inadequately hydrating throughout the morning.

As measured by change in body weight:

  • 1% dehydration has been shown to have a slightly negative influence on mental function—slowed working memory, increased tension/anxiety and fatigue, and increased errors on visual vigilance. Dehydration can also affect physical work capabilities.  And can reduce the body’s evaporative cooling abilities (sweating).
  • 2% dehydration has a more severe impact on mental function, mood, and fatigue.

You can easily become dehydrated regardless of fitness level, body composition, or age. It happens quickly with physical activity, especially in extreme climates. Dehydration often begins as a minor headache and fatigue. As dehydration progresses, you might feel dizzy standing up too quickly; experience random muscle spasms and cramps, get a bad headache or migraine, and/or lose your ability to focus and concentrate.

Water consumption table (Body armor: add 5°, MOPP 4: add 10°)
Heat Category WBGT Index, °F Water Intake (quart/hour)
Easy Work Moderate Work Hard Work
Walking on a hard surface at less than 2 mph with less than a 30-pound load; weapon maintenance; marksmanship training; drill and ceremony Walking in sand at 2.5mph with no load; calisthenics; patrolling; individual movement techniques (i.e., high/low crawl) Walking in sand at 2.5mph with a load; field assaults
1 78° - 81.9° ½ ¾ ¾
2 82° - 84.9° ½ ¾ 1
3 85° - 87.9° ¾ ¾ 1
4 88° - 89.9° ¾ ¾ 1
5 > 90° 1 1 1
Rest - sitting or standing in the shade if possible
These fluid replacement volumes will sustain performance and hydration for at least 4 HOURS of work in the specified heat category. Fluid needs can vary based on individual differences and exposure to full sun or full shade.
CAUTION: Hourly fluid intake should not exceed 1.5 quarts. Daily fluid intake should not exceed 12 quarts.

So how much fluid does the average, healthy adult need? The U.S. National Academies of Sciences, Engineering, and Medicine determined that an adequate daily fluid intake is about:  

• 15.5 cups (3.7 liters) of fluids a day for men
• 11.5 cups (2.7 liters) of fluids a day for women 

These recommendations cover fluids from water, other beverages, and food. About 20% of daily fluid intake usually comes from food and the rest from beverages. Daily fluid intake recommendations vary by age, sex, physical activity, pregnancy, and breastfeeding status.

General hydration tips

Drink regularly and frequently. Drink at least 8–10 cups of water a day at regular intervals. In extreme climates, you will need even more water to prevent dehydration. To avoid dehydration, you might have to make yourself drink when you are not thirsty.

Water is your best choice

Cool water is the best performance fluid replacer for any physical activity that lasts less than 60–90 minutes. Water is always better than soda, energy drinks, coffee, beer, or full-strength fruit juice, and equal to sports drinks for replacing the fluid you lose. Cool water is absorbed into your bloodstream quickly and has none of the drawbacks that other fluids can have.

Do not wait until you are thirsty to drink

  • By the time you feel thirsty you are already dehydrated.
  • Drink beyond your feeling of thirst. If you stop drinking when your thirst is satisfied, you have replaced only about two-thirds of the water you have lost.
  • Sip frequently rather than gulp all at once; drinking small amounts of fluids at a time is more effective than large amounts only occasionally.

Monitor fluid loss

  • Monitor urine color; when you are hydrated, urine is clear or pale yellow. It is dark yellow or brown when you are dehydrated. The recommendation is to monitor the color of the first morning void.  Monitoring other voids throughout the day may be less accurate indicator of hydration.
  • Weigh yourself before and after activity to see how much water you have lost. Drink 2–3 cups for every pound you lose during physical activity.

Weight loss can be used to measure water loss

Weight lost over several hours of physical activity is body water lost in the form of sweat. In a 150-lb person, a 1.5 lb. weight loss would be a loss of 1% of body weight and about 3 cups of sweat.

What should you drink?

Go for Green® Coding Algorithm: Beverage Table

Beverages are coded based on added sugar content, artificial ingredients, saturated fat content, and healthful nutrients such as vitamins and minerals. 

Green coded

  • Water (plain or carbonated)
  • Naturally flavored water, including fruit/vegetable/herb-infused (no artificial sweeteners)
  • Herbal tea
  • Unsweetened or very lightly sweetened iced or hot tea (≤ 11 grams of sugar per 16 oz.)
  • Unsweetened or very lightly sweetened iced or hot coffee (≤ 11 grams of sugar per 16 oz.)
  • 100% vegetable juice
  • Milk, unsweetened (skim, 1%)
  • Milk alternatives: soy, almond, rice, unsweetened or plain with added calcium and vitamin D

Yellow coded

  • Sports drinks
  • 100% fruit juice
  • Lightly sweetened iced or hot tea (12–18 grams of sugar per 16 oz.)
  • Coffee with small amounts of sugar, cream, or milk (12–18 grams of sugar per 16 oz.)
  • Artificially sweetened beverages (diet or light sodas, tea, juices, and many flavored waters)
  • Milk, unsweetened (2%)
  • Flavored milk (skim, 1%, 2%) (vanilla, chocolate, etc.
  • Flavored milk alternatives: coconut, soy, almond, and rice
  • Hot chocolate made with water or milk (skim, 1%, 2%)

Red Coded 

  • Energy drinks
  • Sweet tea (≥ 19 grams of sugar per 16 oz.)
  • Coffee with large amounts of whole milk or cream and sugars or syrups (≥ 19 grams of sugar per 16 oz.)
  • Sweetened beverages of any kind (sodas, fruit punches, and juice drinks) (≥ 19 grams of sugar per 16 oz.)
  • Milk, plain or flavored (whole)
  • Hot chocolate made with whole milk, cream, or half-and-half
For more information on Go for Green ® External Link


1.    "Report Sets Dietary Intake Levels for Water, Salt, and Potassium to Maintain Health and Reduce Chronic Disease Risk," U.S. National Academies of Sciences, Engineering, and Medicine. Last modified February 11, 2004. https://www.nationalacademies.org/news/2004/02/report-sets-dietary-intake-levels-for-water-salt-and-potassium-to-maintain-health-and-reduce-chronic-disease-riskExternal Link

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