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.
Recognizing the weather conditions and work requirements can help to prevent dehydration. The recommended air temperature threshold for initiating hot weather WBGT index guidelines is ~75 °F, though exertional heat illness can occur in cooler environments. As the WBGT index increases, physical work intensity should be reduced or more frequent and longer rest periods should be taken. Under extreme conditions when the WBGT index is > 90 °F, training may require extreme modifications or temporary suspension. Work schedules should be customized for the environment, task, and military situation when possible. Table 3–2 provides work-rest and fluid replacement guidelines for heat-acclimatized Service members in a training environment.
Table 3-2. Fluid replacement and work-rest guidelines for training in warm and hot environments
Easy Work (250 W)
Moderate Work (425 W)
Heavy Work (600 W)
Very Heavy Work (800 W)
WBGT Index (°F)
Water Intake (qt/hr)||
Water Intake (qt/hr)||
Water Intake (qt/hr)||
Water Intake (qt/hr)|
Very Heavy Work|
- Weapon maintenance
- Marksmanship training
- Drill and ceremony
- Patrolling with 30-pound load
- Low and high crawl
- Dig defensive position
- Patrolling with
- 45-pound load
- Four-person littercarry (180 pounds)
- Jogging 4 mph
- Two-person litter carry (150 pounds)
- Move under direct
- Obstacle course
NL = no limit to work per hour (up to 4 continuous hours).
- Applies for average-sized and heat-acclimatized Service member wearing the Operational Camouflage Patten (OCP) uniform.
- The work-rest times and 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 (± ¼ qt/hr) and exposure to full sun or full shade (± ¼ qt/hr).
- Rest means minimal physical activity (sitting or standing) accomplished in shade if possible.
- CAUTION: Hourly fluid intake should not exceed 1½ qt.
- CAUTION: Daily fluid intake should not exceed 12 qt.
- If wearing heavy protective clothing (CBRN, JSLIST), add 10 ⁰F to WBGT index for easy work and 20 ⁰F to WBGT index for moderate and heavy work.
TB MED 507, Heat Stress Control and Heat Casualty Management (12 April 2022)
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.
- 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
- 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%)
- 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