Experts Explain! The 6 Major Misconceptions About Hydration You Shouldn't Believe — A Thorough Examination from the "2 Liters a Day Myth" to the "Coffee Causes Dehydration" Theory

Table of Contents

1. Introduction: Why Examine the "Hydration Myth" Now?

2. Misconception ① "Drinking 2 L a Day is Enough"

3. Misconception ② "Coffee and Tea Dehydrate You"

4. Misconception ③ "It's Too Late When You're Thirsty"

5. Misconception ④ "Clear Urine is Perfect, Dark Urine is Bad"

6. Misconception ⑤ "Sports Drinks are Better than Water"

7. Misconception ⑥ "Cold Water is Bad for Your Stomach"

8. Comparison of Global and Japanese Official Guidelines

9. Practical Advice by Age, Activity Level, and Climate

10. Common Q&A (Boundary Between Dehydration and Water Intoxication, etc.)

11. Conclusion: Design Your "My Hydration Plan"

12. 📚List of Reference Articles (External Links, Chronological Order)

13. ▶︎Link to This Article

1. Introduction: Why Examine the "Hydration Myth" Now?

In Japan, the number of people transported for heatstroke surges from July to September each year, and emergency visits by foreign tourists also increase. Meanwhile, many "hydration tips" spreading through social media and word of mouth lack scientific evidence, leading to cases of hyponatremia (water intoxication) due to excessive water intake or dehydration. It is urgent to update global standard knowledge and create a guide that allows both residents in Japan and visitors to act safely.Ministry of Health, Labour and WelfareMinistry of Health, Labour and Welfare

2. Misconception ① "Drinking 2 L a Day is Enough"

Key Points of Misconception
The saying "Adults should drink '8 cups = about 2 L'" originated from a misinterpretation of a 1945 recommendation by the U.S. Food & Nutrition Board. Current international recommendations clearly state that "the required amount varies greatly depending on body size, activity level, and climate."Harvard HealthIris

Latest Evidence

• The WHO's domestic water indicator is "5.3 L/person/day" (total including climate and labor intensity), but this includes cooking and cleaning. The guideline for individual pure drinking water intake is 30–35 mL per kg of body weight.

• Harvard School of Public Health suggests "4–6 cups (about 1–1.5 L) of 'plain water' plus meals and other beverages" for healthy adults.Harvard Health

Practical Advice

• Use body weight × 0.03 L as the base amount, and add 250–500 mL for exercise over 30 minutes or in hot and humid conditions.

• Drinking more than 500 mL at once can overwhelm the kidneys, leading to increased urinary excretion.

3. Misconception ② "Coffee and Tea Dehydrate You"

Source of Misconception
The diuretic effect of caffeine led to the oversimplification that "coffee = dehydration."

Scientific Facts
Meta-analyses by Mayo Clinic and the European Coffee Science Information Centre conclude that "normal caffeine intake (less than 400 mg/day) results in a positive total fluid balance" and is effective as a hydration source.Mayo Cliniccoffeeandhealth.orgVerywell Health

Points of Caution

• For those unaccustomed to caffeine or consuming high doses at once, diuresis may become pronounced.

• High-energy additives like sugar and cream pose separate health risks.

4. Misconception ③ "It's Too Late When You're Thirsty"

Background
Education has repeatedly taught that "thirst = 1% body weight loss = already mild dehydration."

Latest Guidelines
The Ministry of Health, Labour and Welfare and the Japanese Association for Acute Medicine state that while "the sense of thirst is dull or cannot be conveyed in the elderly and children, prompting regular spontaneous fluid intake," in healthy adults, thirst is a sufficiently functioning physiological switch and is not "too late."Ministry of Health, Labour and WelfareMinistry of Health, Labour and Welfare

Practical Advice

• For those primarily doing desk work, aim for 200 mL every hour.

• For outdoor workers, 100–200 mL every 15–20 minutes, with 0.1–0.2% salt.

5. Misconception ④ "Clear Urine is Perfect, Dark Urine is Bad"

While urine color is indeed a simple indicator of dehydration, it's important to note that vitamin supplements, food coloring, and medications can change the color. Additionally, consistently clear urine may indicate excessive water intake or electrolyte dilution. WHO and Harvard consider "pale straw color" ideal, with extreme transparency or dark brown as warning signals.Harvard HealthIris

6. Misconception ⑤ "Sports Drinks are Better than Water"

Isotonic solutions containing 6–8% carbohydrates and 40–80 mg/100 mL sodium are useful during prolonged exercise or heavy sweating, but in daily life, the excessive sugar content poses issues of energy surplus and dental caries risk. WHO recommends that "for light to moderate exercise under an hour, water is sufficient."Iris

7. Misconception ⑥ "Cold Water is Bad for Your Stomach"

While liquids cooler than body temperature temporarily alter gastric blood flow, they pose no issue for healthy individuals. In fact, under extreme heat, cold water helps efficiently lower core body temperature, contributing to heatstroke prevention. The Japanese Association for Acute Medicine guidelines also recommend beverages around 15°C.Ministry of Health, Labour and Welfare

8. Comparison of Global and Japanese Official Guidelines

9. Practical Advice by Age, Activity Level,