LAST UPDATED: January 21, 2020

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.
  • 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. The slump you feel in the mid-afternoon may be from the consumption of a large meal or it may be the effect of inadequate hydration throughout the morning.

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.

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.

Don’t 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.
  • 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.

Resources for Public Affairs and Communication Staffs
(request access - one time only)
Communication Resources
Join the discussion
Facebook Twitter YouTube Cookpad
Working together for a healthy community
U.S. Army Army Medicine Human Performance Resource Center ArmyFit Army Community Resource Guides