Athlete's Kitchen: Hot Weather Hydration Tips

by Nancy Clark, MS, RD, CSSD

Steaming hot summers bring up nutrition questions for runners who are training and competing in the heat:

  • How can I tell if I’ve had enough to drink?
  • Should I be consuming extra electrolytes?
  • Is it possible to drink too much?”

With summers getting hotter and longer, here are some practical hot weather sports nutrition tips.

To start, let’s look at the physiology of keeping the body cool. Normal body temperature is 98.6°F (37°C). When you exercise, your body temperature increases. At 104°, you are in the danger zone. If you were to really overheat and get to 107.6°F (42° C), your cells would get damaged –similar to how raw egg white coagulates as it starts to cook. You don’t want that to happen!

•To dissipate the heat generated by working muscles, blood flow to the skin increases and your sweat glands get activated. As sweat evaporates from the skin, it provides a cooling effect.

• Humid heat (New England) is physiologically more stressful than dry heat (Arizona). Hence, runners who will be traveling to an event want to acclimatize to the environment in which they will be competing.

•With repeated training in the heat for more than an hour a day, the body acclimatizes over the course of 7 to 14 days. You’ll notice greater exercise capacity. In one study, endurance increased from 48 to 80 minutes.

• The more you train in the heat, the more you sweat. While this helps keep you cooler, the additional fluid loss can easily lead to progressive dehydration if you do not fully replace sweat losses on a daily basis.

•Sweat losses of 2 to 3 pounds per hour are common among runners who exercise vigorously in the heat; some lose more than that. You don’t need to replace every drop of sweat, but you do want to minimize losses, so you end up losing less than 2% of your body weight (3 pounds for a 150-pound athlete).

• “Drinking to thirst” generally works for day to day living and fitness exercisers, but not always for runners. Studies suggest drinking to thirst often results in body water deficits of 2% to 3% among athletes who sweat heavily in the heat. That level of dehydration impairs athletic performance. Hence, ironman Triathletes, marathoners, and other endurance athletes should have a drinking plan that balances losses with intake.

• To learn how much sweat you lose during runs, weigh yourself nude before and after a hard workout, accounting for any fluid consumed during the session. If you have lost, let’s say 2 pounds per hour (32 ounces, 1 quart), target drinking 6 to 8 ounces every 15 minutes the next time you run at that intensity and under those weather conditions. Practice drinking that volume of fluid, to train your gut to handle it comfortably.

• Monitor progressive under-hydration by taking daily weights first thing in the morning. A downward weight trend can be a warning sign of inadequate fluid replacement, particularly if the morning urine is dark and concentrated. (Yes, it could also reflect fat-loss.)

• You can tell if you have adequately rehydrated by monitoring the color and volume of your urine—as well as how often you need to urinate. For example, if you sweat heavily during your morning workout  and then don’t pee for five hours afterwards, you are underhydrated. Urine that is dark and concentrated is another warning sign.

• On a daily basis, your goal is void a significant volume of urine that looks like lemonade, not beer, every 2 to 4 hours. Google urine color chart for a visual resource.

• When you sweat, you lose not only water but also electrically charged minerals (electrolytes), more commonly known as sodium, calcium, magnesium, and potassium. Sodium (a part of salt) is the main electrolyte of concern.

• Because you sweat off proportionately more water than sodium, the concentration of sodium in blood actually increases during exercise. In standard (i.e., not extreme) exercise situations, replacement with electrolyte supplements is needless; food eaten at meals/snacks offers ample electrolytes.

• The primary purpose of sodium in a sports drink is to enhance fluid absorption and retention, as well as enhance absorption of carbohydrate. The amount is inadequate to replace sodium lost in sweat. For example, a slice of bread offers about 125-200 mg sodium; 8-oz. Gatorade offers only 110 mg.. Gatorade Endurance formula, 200 mg.

• If you will be exercising for hours on end in the heat (i.e., ultra-run, marathon, all-day bike ride, or tennis tournament), you can lose a significant amount of sodium.  Assuming you will be consuming food during the extended exercise session, you can replenish lost sodium with peanut butter & jelly sandwiches (500 mg sodium), thin pretzels (490 mg/1-oz) and cheese sticks (200 mg/stick).

• Caution: Do not over-consume plain water and/or sports drink during extended exercise unless you are taking in other sources of sodium. Excess water dilutes the reduced amount of sodium in the blood and can lead to hyponatremia (low blood sodium), a life-threatening condition that can result in death. This can happen, for example, with slow (4-hour) marathoners who diligently drink at every water station, regardless of thirst.

• After exercise, if you need sodium, you will crave salt and should honor those salt cravings with crackers and cheese, pickles, pizza, potato chips, V-8 Juice—or more simply, sprinkle salt on your recovery meal.

• Most healthy, sweaty runners can set aside public health guidelines to “limit your salt intake.” Replacing sodium losses is important to rebalance your body.

• When you know you will be sweating for more than an hour or two in the heat, plan to boost your pre-exercise salt intake. By consuming 300 to 500 mg sodium before you exercise, the sodium will already be in your body, working to retain water and retard dehydration. During runs longer than two or three hours, plan to target 500 to 700 mg sodium per hour (and more if you experience muscle cramps).

• Chocolate milk is preferable to sports drink to enhance rehydration. It offers more sodium (150 mg vs 110/ 8 oz) —as well as more carbohydrate (to replenish glycogen stores) and protein (to repair muscles). Drink wisely!


Sports Nutritionist Nancy Clark, MS, RD counsels sports-active people in the Boston-area (617-795-1875). Her Sports Nutrition Guidebook offer additional hydration info. Visit www.NancyClarkRD.com for more information

References (open access)

Consensus recommendations on training and competing in the heat. British Journal of Sports Medicine 2015, 49(18): 1164-1173  

Practical Hydration Solutions for Sports. Nutrients 2019; 11(7):1550


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