Beware of Hot Days! Increased Risk of Heatstroke Due to Salt Deficiency—Is "Reducing Salt" a No-Go? A Doctor Provides a Thorough Explanation

Table of Contents

1. Introduction: The Reality of Global Warming and Frequent Heatstroke

2. Chapter 1: What is Heatstroke? - Pathophysiology and Clinical Picture

3. Chapter 2: The Science of Sodium and Fluid Balance

4. Chapter 3: The Pitfalls of the "Salt Reduction Boom"

5. Chapter 4: Reading "Heat x Hyponatremia" with the Latest Evidence

6. Chapter 5: How Much Should You Take? - Specific Intake and Beverage Selection

7. Chapter 6: Practical Guide for the Elderly, Children, and Those with Chronic Illnesses

8. Chapter 7: Situation-Based Measures: Home, Workplace, School, and Outdoor

9. Chapter 8: Comparison of Overseas Cases and International Guidelines

10. Chapter 9: Regional Disaster Prevention and Future Strategies from the SDGs Perspective

11. Appendix: First Aid Flowchart & Self-Check List

Introduction: The Reality of Global Warming and Frequent Heatstroke

In the summer of 2025, Japan recorded the highest number of extremely hot days over 35°C in history. The number of days with a WBGT (Wet Bulb Globe Temperature) index over 31°C was 48 in Tokyo and 74 in New Delhi. Climate change has also led to an increase in "nighttime heatstroke," with statistics reporting that about 20% of emergency transports occur at night.Japan Medical Association

Chapter 1: What is Heatstroke? - Pathophysiology and Clinical Picture

Heatstroke is a complex condition of . Mild heat cramps are mainly caused by sodium deficiency lost through sweat, and seizures will recur unless 0.5–1 g of salt is consumed even after rehydration. As it progresses, it can involve central nervous system symptoms, kidney damage, and DIC, with a mortality rate reaching 30%.CDC

1-1 Classification

Chapter 2: The Science of Sodium and Fluid Balance

Sodium is the main cation that controls the osmotic pressure of extracellular fluid, and adults lose 0.5–1.0 g (equivalent to 1.3–2.5 g of salt) per day even at rest. Walking for an hour at 30°C and 60% humidity results in about 800 mL of sweat, with sodium loss rapidly increasing to 600–960 mg. If left unchecked, the volume of extracellular fluid decreases, leading to hypotension and cerebral edema.Otonanswer

Chapter 3: The Pitfalls of the "Salt Reduction Boom"

The Ministry of Health, Labour and Welfare's "Dietary Reference Intakes for Japanese 2025" set a target salt intake of less than 7.5 g for adult men and 6.5 g for women from the perspective of preventing lifestyle-related diseases. However, during the summer, sweat loss is added, so mechanically adhering to the target value poses a risk of deficiency. Dr. Hideki Wada warns that "10–15 g is physiological in summer."Ministry of Health, Labour and WelfareKaigo Post Seven

Chapter 4: Reading "Heat x Hyponatremia" with the Latest Evidence

4-1 Outdoor Temperature and Onset Risk

A 2025 study analyzing 21,924 cases in Stockholm, Sweden, found that severe hyponatremia sharply increased above 20°C, with the number of onset days quadrupling above 22°C compared to normal temperatures.PubMed

4-2 Military Data Showing the Dangers of Excessive Water Intake

According to the U.S. military's annual report, the incidence of hyponatremia in 2024 was 10.4 per 100,000 people. It was recorded that 73% of cases consumed ">1 L/hour" of only water.Healthy Army

4-3 Increase in Hospital Transport Cases

A 2024 paper in JAMA Network Open reported that cases with Na <125 mmol/L in emergency departments increased 2.3 times during heatwaves, with a notable increase among elderly women.JAMA Network

Chapter 5: How Much Should You Take? - Specific Intake and Beverage Selection

WHO-recommended ORS (Na 75 mmol/L = about 4.4 g/L of salt) most efficiently corrects sweat loss. At home, adding lemon juice to "3 g of salt + 40 g of sugar + 1 L of water" speeds absorption and improves taste.WHO Apps

Chapter 6: Practical Guide for the Elderly, Children, and Those with Chronic Illnesses

• Elderly: Decreased sensitivity to thirst. Aim for 1 L/day and an additional 2 g of salt at a room temperature of 28°C. Patients with heart failure or kidney failure should consult a doctor.PubMed

• Children: Rapid dehydration due to a large body surface area ratio. Outdoor activities should be under WBGT 28°C, with ORS 100 mL + half a salt candy every 20 minutes.Japan Medical Association

• Diabetes/Hypertension: Use oral rehydration powder with zero sugar and 0.3% sodium. Those taking antihypertensive drugs should be cautious of blood pressure drops and decide on salt supplementation with a doctor.

Chapter 7: Situation-Based Measures: Home, Workplace, School, and Outdoor

• Home: Prevent "sleep dehydration" with miso soup and barley tea first thing in the morning and 100 mL of oral rehydration jelly before bed.
  
  

• Workplace: The revised Industrial Safety and Health Regulations, effective June 2025, mandate water and salt breaks every 10–20 minutes when WBGT exceeds 28°C. Keep salt tablets on hand and maintain health records.Ministry of Health, Labour and Welfare

• School: Physical education classes are generally canceled at WBGT 31°C. When classes resume, create an environment where students can voluntarily request water and salt supplementation through a "hydration call system."
  
  

• Outdoor: During mountain climbing, sweat volume is unpredictable due to altitude changes and temperature differences. Routinely change sweat-soaked shirts and take 1 g of salt every hour.

Chapter 8: Comparison of Overseas Cases and International Guidelines