Different for Exercise and Daily Life? What is the "True Amount of Water" Your Body Needs

Most of our body is composed of water, and proper hydration is essential for maintaining health. However, many people likely have questions such as "How much water should I drink per day?" or "Are the indicators for determining dehydration accurate?" In reality, common measurement methods often cited, such as "the speed at which the skin on the back of the hand returns after being pinched" or "how quickly the color returns after pressing a fingernail," are not necessarily perfect.

On the other hand, while we often hear theories like "you should drink at least 2 liters a day" or "performance drops if you lose X% of your body weight," the actual basis for these claims is sometimes not well explained. In this article, we will organize these common questions and explain the basics and practical points of proper hydration for both daily life and exercise.

1. Methods for Measuring Dehydration and Their Limitations

1-1. Representative Dehydration Measurement Techniques

Commonly known methods include the following simple checks.

• Skin Turgor Test
  A method of pinching the skin on the back of the hand and releasing it to see if it returns to its original state within a few seconds. It is generally said that it should return to flat within 1 to 3 seconds, and if it takes 5 seconds or more, a tendency toward dehydration is suspected.

• Capillary Refill Test
  A method of pressing a fingernail for a few seconds and releasing it to measure the time it takes for the color under the nail (nail bed) to return to pink. If it does not return within 1 to 3 seconds, it may suggest a lack of water.

These checks can certainly serve as a guide. However, results are easily influenced by individual differences such as skin elasticity due to age, body fat percentage, and blood circulation, making them uncertain as "absolute indicators."

1-2. Why They Cannot Be Called Perfect Measurements

The simple tests mentioned above use "skin condition and blood circulation changes," but in reality, they are also heavily influenced by aging, constitution, temperature, and humidity. Furthermore, even looking at scientific literature, data that strictly links "how many seconds it takes for pinched skin to return" to "X% dehydration" is limited.
Therefore, in actual research and medical settings, more objective methods such as blood tests or urine specific gravity measurements are generally used. What is important here is to recognize that "simple checks are just one of many guidelines."

2. Guidelines for Water Intake in Daily Life

2-1. The Amount Average Adults Should Use as a Guide

Regarding the question, "So how much water should I drink per day?", you may have long heard the figure of "2 to 2.5 liters per day." Even in actual research, for resting conditions (no exercise, normal indoor environment), a guideline has been proposed to consume about 240 milliliters (8 ounces) of water per hour during the first 10 hours after waking up.

Specifically,

• Consume a total of "a little over 2 liters (about 2,400 milliliters)" over the 10 hours after waking up.

• It is not necessary to drink in divided portions every single hour; as long as you can average it out in total, such as drinking more at one time, that is fine.

This is the way of thinking. It is said that this allows you to maintain the minimum necessary water balance.

2-2. Specific Schedule Example

For example, if you wake up at 7:00 AM, you should secure your water intake over the 10 hours from then until 5:00 PM.

• At breakfast (7:00–8:00 AM): About 250–500 mL (1–2 glasses)

• Morning (9:00 AM – 12:00 PM): A total of about 500–700 mL every 1–2 hours

• Lunchtime (12:00 PM – 1:00 PM): 250–500 mL

• Afternoon (1:00 PM – 5:00 PM): A total of about 500–700 mL every 1–2 hours

While some hydration is necessary in the evening and beyond, it does not need to be as frequent as during the day. If you are concerned about drinking too much, it is a good idea to adjust your intake so you do not drink large amounts before going to bed.

3. Hydration during exercise and the Galpin Equation

3-1. What is the Galpin Equation?

For water intake during exercise, there is a simple metric called the Galpin Equation. This was derived from the research and experience of Dr. Andy Galpin, a professor of exercise physiology at California State University, Fullerton, and is calculated as follows:

For example, for a person weighing 150 lbs (approx. 68 kg), 150 ÷ 30 = 5 ounces. In other words, the guideline is to consume 5 ounces (approx. 150 mL) every 15–20 minutes.

When calculating using the metric system, it is:

3-2. How to consume before, during, and after exercise

The key to hydration during exercise is not just the total amount, but 'frequent intake.' You should refer to the following flow.

1. Before exercise
   If you have met your base intake for the day (the aforementioned '2–2.4 L in the 10 hours after waking'), there is no problem. It is safer to drink about one glass of water just before starting.

2. During exercise
   Following the Galpin Equation, it is ideal to drink little by little every 15–20 minutes. For sports where you cannot drink frequently, such as running, it is better to drink in batches at appropriate intervals, such as every 30 minutes.

3. After exercise
   Replenish the water lost by the body promptly. If you have been hydrating frequently during exercise, there is no need to gulp down water immediately after, but you should adjust flexibly according to your thirst and how much you have sweated.

4. Applications for specific situations and precautions

4-1. Special conditions such as hot environments or night shifts

• High temperature and humidity environments
  During outdoor work under the scorching sun or long-duration outdoor sports, you lose more water and minerals than you might imagine. Considering the loss of salt and minerals through sweat, it is recommended to incorporate beverages containing salt and electrolytes, not just water.

• Night shifts or shift work
  Because your activity rhythm and sleep rhythm differ from the typical daytime schedule, your body's water requirements will also shift. It is a good idea to set your '10 hours after waking' according to your own life rhythm and apply the same guidelines.

4-2. Caffeinated beverages such as coffee and tea

Beverages containing caffeine, such as coffee and black tea, are often thought to promote dehydration due to their diuretic effect, but the latest findings suggest that 'within a moderate range, they do not significantly impair the body's water balance.' However, since consuming large amounts can potentially increase urine output too much, it is safer to rely primarily on water or electrolyte-containing beverages rather than relying solely on caffeinated drinks.

When considering daily water intake, it is effective to be aware of the guideline of 'replenishing about 2 to 2.4 liters frequently during the first 10 hours after waking up,' rather than just a uniform number like 'you should drink 2 liters.' Also, during exercise, hydrating based on the Gallpin equation (body weight divided by 30 = ounces to consume every 15-20 minutes) helps prevent performance decline and is useful for managing your physical condition.

Simple dehydration tests, such as pinching your skin or checking the color of your nails, should realistically be viewed only as 'one of the indicators.' Because there are large individual differences, it is difficult to judge accurately based on these alone, but they are useful as signs that are easy to notice on a daily basis.

Ultimately, it is important to comprehensively consider thirst, physical sensations, environment, amount of exercise, and individual constitution and flexibly adjust your hydration amount. Try to find the appropriate water balance that suits you by taking water frequently and supplementing electrolytes as needed.

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