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Staying Hydrated in Hot Weather

Staying Hydrated in Hot Weather

Why is fluid important during hot weather:

Staying hydrated during hot weather is important to preserve body function and support exercise performance. Heat and humidity can negatively impact your performance and increase risk for heat exhaustion and heat stroke. When you exercise in hot and/or humid weather your body must cope with both the heat of the environment and the heat produced by your muscles (over two-thirds of the energy produced by your muscles is lost as heat). To help regulate body temperature, blood moves closer to the surface of the skin and sweat rate increases to remove heat through evaporative cooling. Without adequate rehydration this ongoing lose of fluid can cause you to become progressively more dehydrated.

What happens if you don’t drink enough:

If you enter into exercise dehydrated or become dehydrated during activity, you may experience a reduction in athletic performance. As a little as a 2% reduction in body mass from lost fluid can negatively affect exercise outcomes by increasing perceived effort, reducing mental sharpness, and decreasing muscle power and endurance. Dehydration can also increase your risk of heat exhaustion or heat stroke. If you lose more than about 5-6% of body weight, the risk of heat-related illness increases significantly.

When to drink:

The first step to preventing dehydration is to make sure that you are adequately hydrated before workouts and races. Your body’s thirst mechanism is a good, but imperfect indicator of dehydration. Drinking to thirst can result in body water deficits that exceed 2–3% body mass when sweat rates are high and exercise is performed in warm-hot environments. Drinking until your urine is lightly colored provides a better measurement to determine if you are well-hydrated.

If you lose less than two percent of your body weight during a run, then you can simply rehydrate after you finish, but for longer runs or in more extreme conditions you should replace fluids during exercise. It is a good idea to have a hydration plan to ensure you consume enough fluids (see below to determine sweat rate). Ensure that your rehydration plan starts shortly after the completion of exercise as remaining dehydrated after running can slow down recovery (blood and other body fluids help to remove waste products and bring nutrients to tissues for repair).

Making a hydration plan:

The amount of fluid that you should drink before, during, and after your run depends on several factors. You need to consider your pre-exercise hydration status, your sweat rate during exercise, and how quickly fluid empties from your stomach. It is important to note that there is no advantage to consuming more fluid than that lost through sweat loss (this may place you at risk for hyponatremia).

Example plan:

Before: 2-4 hours pre-activity, consume ~5-10mL/kg body weight of fluid (water is best).

During: Consume sips/gulps of fluid every 15 min to 20 min. Ideally the total consumed should reflect a pre-established sweat rate (see below).

After: Consume enough fluid to replace sweat losses by 1.25-1.5 times. Know that 1.0kg of lost body weight = 1.0L of fluid lost.

What fluids to drink:

If you are exercising in a hot and/or humid environment, cold beverages or iced/slushy beverages pre-exercise may help to lower your initial core temperature. Cold fluids can potentially enhance endurance performance when ingested before, but not during exercise (ingesting cold fluids during exercise might be a reduction in sweating and therefore skin surface evaporation, due to the activation of thermoreceptors probably located in the abdominal area).

For shorter activities, water is appropriate; however, for longer activities it may be beneficial to consume fluids that also contain carbohydrate and/or electrolytes. Following exercise, fluids that help to rehydrate you the best are those that contain nutrients (i.e. protein, fat, and vitamins and minerals). This may be because when you drink it signals the kidneys to get rid of the extra water by producing more urine; however, when beverages contain nutrients and electrolytes like sodium and potassium the stomach empties more slowly with a less dramatic effect on the kidneys. Beverage choice to help with rehydration could include oral rehydration solutions, milk/milk alternative, and orange juice.

Calculating sweat rate

The most practical way to monitor sweat losses from exercise is to measure changes in body mass. Individual sweat rates and fluid losses can vary widely.  For example, body size, gender, exercise intensity, environmental conditions, the amount of clothing worn, level of aerobic fitness, and acclimatization status all influence sweat rates. The best way to gain information about your sweat rate for a given event or training session is to monitor losses over several sessions under similar conditions. Plan to continue this over training as you might have a decrease in sweat rate with heat acclimatization or increased fitness.


Sweat Rate Calculation Sample Calculation
Deficit = (pre-weight – post weight) x 1000mL

Pre-weight: _________kg

Post weight: _________kg

Deficit: ___________kg x 1000mL


Pre-weight = 52kg

Post weight = 50kg

Deficit = (52kg – 50kg) x 1000mL

= 2000mL


% Body Mass Lost = (Deficit/10) / pre-weight


% Body Mass Lost = (2000/10)/52kg

= 3.85% decreased body weight


Fluid Intake = (pre-fluids – leftover fluids)

Pre-Fluids: ________ mL

Leftover Fluids: ________ mL


Pre-fluids = 1000mL

Leftover fluids = 500mL

Fluid Intake = 1000mL – 500mL

= 500mL


Sweat Loss = deficit + fluid intake


Sweat Loss = 2000mL + 500mL

= 2500mL (2.5L)


Time = # of minutes of exercise/ 60 minutes (per hour)


Time = 90 minutes

= 90 minutes/60 minutes (per hour)

= 1.5 hours


Sweat Rate = Sweat loss / time Sweat Rate = 2.5L / 1.5 hours

= 1.67 L/hour


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