I think you sometimes wonder what to put in your child's water bottle.
When it comes to hydration, it's important to choose your drinks based on the duration and intensity of your activity, the heat, and the amount you sweat.
For daily school life and about an hour of physical education, water or tea is often sufficient.
On the other hand, for activities where you sweat a lot, such as sports days, long club activities, and games, choose sports drinks.

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① Regular school life / During a one-hour physical education class

I'll aim for days when the high temperature is around 86 degrees Fahrenheit (30 degrees Celsius) and physical education classes are about an hour long.
For commuting to and from school, classes, breaks, and about an hour of physical education, water or tea in a water bottle is sufficient.

Reason

For workouts under an hour, hydration is the main goal.
It's important to replenish the fluids you lose through sweat, and you can stay well-hydrated with water or tea.
🔸Since sports drinks also contain sugar, there's no need to drink them daily in place of water for regular hydration.

Water intake recommendations

🔸The recommended fluid intake for exercise in hot environments varies by age.

Age	Water content guidelines
Ages 9-12	300-750 mL/hour
13-18 years old	1.0-1.5 L/hour

However, this is only a guideline. The required amount will vary depending on temperature, humidity, exercise intensity, sweat volume, physique, and physical condition.

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② For long-duration exercise, club activities, or matches

This is a guideline for situations involving more than two hours of intense exercise, such as sports days, extended club activities, matches, and outdoor activities on hot days.
When you sweat a lot, it's important to replenish not only fluids but also the salt lost through sweat and the energy used during exercise.

Contents of the water bottle

🔸Let's choose a sports drink.
🔸Commercially available sports drinks such as Pocari Sweat and Aquarius are fine.
For prolonged exercise, beverages containing electrolytes and carbohydrates are said to be beneficial for maintaining hydration and electrolyte balance, as well as sustaining endurance during exercise.

Reason

For prolonged exercise, it's necessary to replenish not only fluids but also energy and electrolytes.
🔸When exercising for a long time, you lose not only water but also electrolytes such as sodium through sweat.
Additionally, since the sugar in your body will also be used up, replenishing energy is important.

What is needed for long-duration exercise

Energy replenishment
During prolonged exercise, the body's sugar levels decrease. The carbohydrates, or sugars, contained in sports drinks serve as an energy source during exercise.
Electrolyte Replenishment
Sweat contains electrolytes such as sodium. When you sweat a lot, you need to replenish not only fluids but also sodium and other electrolytes lost through sweat.
Sometimes, water alone is not enough.
Drinking large amounts of plain water during prolonged exercise can lead to a risk of "hyponatremia," a condition where blood sodium levels drop too low. Therefore, sports drinks are useful for prolonged, intense exercise or when you sweat a lot.

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How to Choose a Sports Drink

Commercial sports drinks are fine.
You can use commercially available sports drinks such as Pocari Sweat and Aquarius.

Ingredients	Role
Carbohydrates/Sugars	Energy sources during exercise
Sodium	Replenishing electrolytes lost through sweat
Water	Dehydration prevention and body temperature regulation

🔸 Generally, as a beverage during exercise,Carbohydrates 6-8g per 100mL, sodium approximately 46-69mgis considered a guideline.

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About Potassium

🔸 General sports drinks in Japan tend to have a lower potassium concentration.
However, this is not necessarily a problem.
Potassium doesn't have to be replenished solely with sports drinks; you can also get it from meals before and after exercise.
🔸Furthermore, it is considered less necessary to be overly concerned with the strict electrolyte composition for adult athletes in the regular school life and club activities of elementary, middle, and high school students.
Regarding potassium, we will consider intake from a normal diet.

Foods high in potassium

Types of food	例
Fruit	Bananas, oranges, melons, strawberries
Vegetables and Potatoes	Potatoes, sweet potatoes, spinach, broccoli
Other	Yogurt, milk, and beans

🔸Don't rely solely on sports drinks to meet all your nutritional needs; be sure to include meals before and after exercise as well.

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Key Points That Apply Across the Board

Before exercise

🔸Be sure to drink plenty of water before starting your activities.
🔸On days when it’s hot from the morning on or when there’s P.E. or club activities, make sure to drink some water before heading to school.

While Exercising

🔸Make sure to drink water frequently, before you get thirsty.
🔸It’s better to drink small amounts during breaks or between exercise sessions than to wait until you’re thirsty and then drink a lot all at once.
🔸During prolonged exercise, you can also aim to drink 180–360 mL every 15–30 minutes. However, the amount needed varies depending on your body type, how much you sweat, the temperature, and the intensity of your exercise.

After Exercise

🔸Even after exercising, you need to replenish the fluids you’ve lost through sweating.
🔸If your weight has dropped significantly from what it was before exercise, this could be a sign of dehydration, so you should be careful.
🔸In sports nutrition, there is a general guideline that if you lose 1 kg of body weight, you should replenish approximately 1 to 1.5 liters of fluid over time.
🔸Instead of drinking a large amount all at once, drink small amounts gradually between the time you get home and your next meal.

Meal

🔸A balanced diet is also important.
🔸By eating a diet that includes fruits, vegetables, root vegetables, and dairy products, you can replenish not only fluids but also the nutrients your body needs.

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Drinks to Avoid

🔸Caffeinated beverages and energy drinks are not suitable for elementary, middle, and high school students to use for hydration during physical activity.
Examples of beverages to avoid:
• Coke
• Energy drinks
• Soft drinks high in caffeine
🔸Energy drinks, in particular, are not intended for hydration. Do not put them in your water bottle for school or club activities.

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Note: This serves a different purpose than an oral rehydration solution.

🔸Although both sports drinks and oral rehydration solutions contain water and electrolytes, they serve different purposes.
🔸Sports drinks are beverages used for rehydration during exercise or when sweating.
🔸On the other hand, oral rehydration solutions are beverages used when there is a risk of dehydration due to diarrhea, vomiting, fever, or other causes.
🔸Unlike regular sports drinks, this product is formulated to replenish fluids and electrolytes during dehydration.
🔸Therefore, it is not appropriate to use oral rehydration solutions for everyday hydration or as a beverage during exercise.

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Summary

Scene	Guidelines for Filling a Water Bottle
Everyday School Life	Water or tea
About an hour of physical education	Water or tea
Sports day	Sports drink
Long club activities	Sports drink
Match	Sports drink
Outdoor activities that cause a lot of sweating	Sports drink
When you're concerned about dehydration due to diarrhea, vomiting, or fever	Consider oral rehydration solution

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To Parents/Guardians

Hydration is important not only in terms of "what to drink," but also "when to drink" and "how much to drink."
🔸 Be sure to drink plenty of water and tea throughout the school day.
For long workouts or days with heavy sweating, choose a sports drink to replenish fluids, electrolytes, and energy lost through sweat.
🔸Also, sports drinks contain sugar.
It's important to use it when needed, rather than drinking it daily instead of water.
🔸 Before and after exercise, aim for a balanced diet that includes fruits, vegetables, and dairy products, and hydrate and nourish yourself with more than just drinks.

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References

Pérez-Castillo ÍM, et al. Compositional Aspects of Beverages Designed to Promote Hydration Before, During and After Exercise. Nutrients. 2023.

American Academy of Pediatrics. How to Keep Young Athletes & Active Kids Safe in Hot Weather. HealthyChildren.org. 2024.

Watson A, Mjaanes JM. Soccer Injuries in Children and Adolescents. Pediatrics. 2019;144(5):e20192759.

Capra ME, et al. Nutrition for Children and Adolescents Who Practice Sport: A Narrative Review. 2024.

Putukian M, Leclere LE, Herring SA, et al. The Adolescent Athlete and the Team Physician: A Consensus Statement. 2025 Update. Medicine & Science in Sports & Exercise. 2026.

McDermott BP, Anderson SA, Armstrong LE, et al. National Athletic Trainers’ Association Position Statement: Fluid Replacement for the Physically Active. Journal of Athletic Training. 2017;52(9):877-895.