During a heat wave the body does not lose only water; it loses electrolytes, strains the cardiovascular system and accelerates oxidative stress. Here is a practical, evidence-based guide to nutritional preparation.
Heat waves are becoming more frequent, longer and more intense. According to World Health Organization data, between 1998 and 2017 more than 166,000 people died from heat waves around the world, and experts predict that number will increase dramatically with climate change. [1] What many people do not realize is that the risk of heat-related illness is not simply a matter of ambient temperature; it depends greatly on the body’s nutritional readiness: electrolyte levels, antioxidant status and the body’s capacity to regulate temperature.
This article walks through the key nutritional mechanisms that determine how our body copes with extreme heat and what we can concretely do, from food to supplements, to improve that capacity.
Why Is a Heat Wave Dangerous: What Happens on the inside
When ambient temperature rises above body temperature, our body relies almost exclusively on sweating as its cooling mechanism. The evaporation of sweat from the skin surface removes heat, but simultaneously triggers a cascade of losses: fluids, electrolytes and energy resources. Sweat is not pure water. It contains sodium (average 35 mmol/L, range 10–70 mmol/L), potassium (5 mmol/L), chloride, magnesium and calcium. [2] During intense sweating an adult can lose 1–2 litres of fluid per hour, and up to 4.8–6 grams of sodium per work shift in hot conditions. [3]
At the same time, heat stress increases the production of free radicals: reactive oxygen species (ROS) that attack cell membranes, proteins and DNA, causing oxidative stress. A review published in the International Journal of Hyperthermia documented that high temperature directly stimulates mitochondria to increase ROS production, analogous to the effects seen during intense physical exertion. [4] The combination of dehydration, electrolyte disturbances and oxidative stress forms the basis of why heat waves can be fatal, particularly for the elderly, children and people with chronic conditions.
| 35 mmol/L | 1–2 L/h | 4,8–6 g | 18% |
| average sodium concentration in sweat (Shirreffs 2000) | possible fluid loss per hour during intense sweating | sodium loss per shift in hot working conditions (JOMT) | increase in mortality per 1°C rise above temperature threshold (meta-analysis of 62 studies) |
Hydration: More Than “Drink Plenty of Water”
The Paradox of Drinking Too Much Water
The most common advice during a heat wave, “drink as much water as possible”, is not only inadequate but can be dangerous. A clinical study published in the New England Journal of Medicine followed 488 runners in the 2002 Boston Marathon and found that 13% finished with hyponatraemia (serum sodium ≤135 mmol/L), exclusively as a consequence of drinking large amounts of plain water without electrolytes. In 0.6% critical hyponatraemia developed with sodium levels ≤120 mmol/L, a condition that can cause seizures, cerebral oedema and death. [5] The same danger threatens anyone who, during a heat wave, consumes large quantities of plain water without replacing electrolytes lost through sweat.
The mechanism is clear: sweat contains sodium, and when we replace it with plain water we dilute the remaining sodium in the blood. Heat-related hyponatraemia is particularly common in the elderly, whose capacity to excrete free water is reduced, as documented by the study published in the New England Journal of Medicine. [6] The solution is not to reduce fluid intake, but to replenish electrolytes together with water.
How Much Fluid and When?
The American College of Sports Medicine (ACSM) recommends proactive hydration: 5–10 mL/kg of body weight of fluid in the 2–4 hours prior to heat exposure, followed by enough fluid to replace sweat losses. A practical marker of adequate hydration is urine colour — pale yellow to clear indicates good status, dark yellow or amber signals dehydration requiring immediate replacement. [7] For an average adult in the heat without intense physical activity, this typically means 2.5–3.5 litres of fluid per day from all sources (food and drink combined), increasing with physical activity or exceptionally high temperatures.
“The body does not know the difference between an athlete and a worker sweating in the sun. Both lose sodium, potassium and magnesium. Plain water cannot replace those losses.”
Electrolytes: Which, How Much and From Which Sources?
Sodium: The Primary Electrolyte of Sweat
Sodium is the electrolyte lost in the greatest quantity through sweat and its adequate replacement is the most important priority. Research published in the Journal of Occupational Medicine and Toxicology showed that outdoor workers in hot conditions lose an average of 4.8 to 6 grams of sodium per shift, far more than the average diet automatically replenishes. [3] A review on fluid and electrolyte replacement during heat stress, published in the American Journal of Clinical Nutrition, concludes that adding sodium to hydration beverages significantly improves fluid retention and prevents the development of hyponatraemia. [8] Practical sources: broth, pickles, salted nuts, electrolyte beverages containing sodium; not just plain water or coconut water, which is rich in potassium but relatively low in sodium.
Potassium: Sodium’s Partner in Fluid Regulation
Potassium is the second most important electrolyte lost through sweat, with a concentration in sweat of approximately 5 mmol/L on average. A potassium deficit manifests as muscle cramps, fatigue and cardiac rhythm disturbances; symptoms that in the heat are easily confused with dehydration. Foods rich in potassium include bananas (422 mg each), avocado (975 mg per fruit), pumpkin, potato with skin, legumes and dark leafy vegetables. [9] Electrolyte beverages such as coconut water can contribute (around 470 mg of potassium per cup), but due to their low sodium content they are ideal as a supplement, not a replacement for sodium-richer electrolyte drinks. [10]
Magnesium: The Often-Overlooked Player
Magnesium is lost through sweat in smaller absolute amounts (0.8 mmol/L on average), but due to chronically suboptimal intake in the European population it is often already on the borderline of deficiency. Magnesium deficiency increases heat sensitivity and worsens muscle cramps. Research from the Journal of Research in Medical Sciences documented how magnesium supplementation reduces signs of heat and physiological stress. [11] Natural sources: nuts (especially almonds and cashews), pumpkin seeds, dark chocolate, dark leafy vegetables and legumes. During heat wave periods, consider a supplement of magnesium citrate or glycinate.
Antioxidants: The Defensive Shield Against Heat-Induced Oxidative Stress
Heat stress increases the production of free radicals (reactive oxygen species, ROS) that damage cell membranes, proteins and DNA. A review published in the International Journal of Hyperthermia describes how high temperature directly stimulates ROS production in mitochondria, which is the mechanistic basis for antioxidant protection during a heat wave. [4]
Vitamin C
Vitamin C (ascorbic acid) is one of the most important water-soluble antioxidants in the body. A meta-analysis published in Frontiers in Immunology in 2025 reviewed 17 randomised controlled trials and concluded that combined supplementation with vitamins C and E statistically significantly reduces plasma oxidative stress markers. [12] Vitamin C also participates in collagen synthesis, which is essential for the integrity of blood vessels under strain from heat-induced vasodilatation. A practical approach in the heat: increase vitamin C intake from food (bell peppers, strawberries, citrus fruits, broccoli) and/or consider a supplement of 500–1,000 mg per day.
Vitamin E
Vitamin E (alpha-tocopherol) is a fat-soluble antioxidant that protects cell membranes from lipid peroxidation induced by free radicals. The membranes of blood vessels and red blood cells, which are under increased strain during a heat wave, are particularly vulnerable to this mechanism of damage. The same meta-analysis from Frontiers in Immunology confirmed that the combination of vitamins C and E provides better antioxidant protection than either vitamin alone. [12] Foods rich in vitamin E: almonds, sunflower seeds, olive oil, avocado and dark leafy vegetables.
Lycopene and Carotenoids: Protection From Within and Without
Lycopene, the red pigment in tomatoes and watermelons, has demonstrated the ability to reduce oxidative stress and protect the skin from UV-induced photo-oxidation. Interestingly, watermelon consumption has a triple advantage during a heat wave: it provides lycopene, its high water content (92%) contributes to hydration, and it delivers natural sugars and electrolytes without added sweeteners. A review on carotenoids published in the journal Nutrients concluded that regular consumption of carotenoid-rich foods correlates with better antioxidant protection of tissue under conditions of oxidative stress. [13]
Diet During a Heat Wave: Concrete Strategies
Foods With a High Water Content
Food can cover up to 20–30% of daily fluid intake. Cucumber (96% water), watermelon (92%), strawberries (91%), celery (95%) and courgette (95%) are excellent examples of foods that simultaneously hydrate, deliver electrolytes and provide antioxidant protection. According to a review on hydration in the American Journal of Clinical Nutrition, consuming hydrating food has a better effect on long-term hydration status than drinking fluids alone, because food slows absorption and reduces renal losses. [8]
Light Meals and the Thermic Effect of Food
Digesting food itself generates heat; the so-called thermic effect of food (TEF). Proteins have the highest TEF (20–30%), meaning that consuming large amounts of meat in the heat internally heats the body further. During a heat wave it makes sense to favour light meals rich in water and fibre (fruit, vegetables, legumes), and to plan heavier, protein-rich meals for the cooler part of the day. Smaller, more frequent meals place less strain on the thermoregulatory system than large ones.
Alcohol and Caffeine: With Caution
Alcohol has a diuretic effect and accelerates fluid loss through the kidneys, which during a heat wave can quickly lead to dehydration. Caffeine in moderate amounts (1–2 coffees) is not a clinically significant diuretic for habitual consumers, but in combination with high temperatures and physical activity it can contribute to a negative fluid balance. It is recommended to replace each alcoholic drink consumed with an equal amount of electrolyte water.
Vulnerable Groups: Special Attention Required
Older people are disproportionately at risk during a heat wave for several physiological reasons: reduced capacity for sweating, a weaker sense of thirst (meaning they do not notice dehydration until it has already progressed), reduced renal capacity to excrete free water, and a higher prevalence of chronic conditions and medications that interfere with thermoregulation. A review published in Environmental Health Perspectives documented that the risk of heat-related mortality in people over 65 is 25% higher than in the general population, and significantly higher still in those over 85. [14]
A systematic review published in Maturitas (2011), which analysed 30 studies on heat waves and mortality in the elderly, concluded that adequate hydration, alongside cooling and avoiding heat, is the most important preventive measure. [15] For older people, an active reminder to drink fluid every 1–2 hours is recommended, because the thirst mechanism is not a reliable indicator of dehydration in that population. Children, pregnant women and people with chronic heart, kidney or diabetic conditions also require heightened vigilance and an individualised hydration plan in consultation with a physician.
The Most Common Myths About Nutrition During a Heat Wave
| MYTH: Drinking plenty of water is all you need. FACT: Drinking excessive amounts of plain water without electrolytes can cause hyponatraemia, dilution of sodium in the blood, which is a medical emergency. Electrolytes are obligatory companions to hydration during a heat wave. |
| MYTH: Ice-cold water cools the body faster. FACT: The temperature of ingested fluid has a minimal effect on core body temperature — the difference is physiologically negligible. Consistent hydration matters far more than the temperature of the drink. Ice-cold beverages consumed quickly in large quantities can trigger gastrointestinal cramps. |
| MYTH: Coconut water is the perfect electrolyte replacement for sport and heat. FACT: Coconut water is rich in potassium but low in sodium, the main electrolyte lost through sweat. As a light-day drink it can be useful, but during intense sweating or heat stress sodium sources are also needed. Add a pinch of salt or combine with sodium-rich foods. |
| MYTH: A high-protein diet keeps our energy up and helps us tolerate heat better. FACT: The opposite is true; proteins have a high thermic effect (20–30%), meaning that digesting them heats the body more than carbohydrates or fats. During a heat wave, light, water-rich meals with moderate protein are the smarter choice. |
Practical Guide: Nutritional Preparation for a Heat Wave
QUICK OVERVIEW — HEAT WAVE PRIORITIES
- Water + electrolytes: every hour of activity in the heat, replenish with foods or beverages containing sodium, not just plain water
- Urine colour: pale yellow = good, dark yellow = drink immediately, clear = good sign
- Potassium: 2–3 portions of potassium-rich fruit and vegetables per day (banana, avocado, pumpkin, legumes)
- Magnesium: nuts, seeds, dark chocolate, or a supplement of 300–400 mg/day if under stress
- Vitamin C and E: increase intake of red bell pepper, citrus fruits, almonds, olive oil
- Water-rich foods: watermelon, cucumber, strawberries, celery- deliver fluid and antioxidants
- Light meals: avoid heavy, large meals in the middle of the day; the thermic effect of food heats the body
- Alcohol: replace each unit consumed with electrolyte water
- Older people and children: active reminders to drink every 1–2 hours, without waiting for thirst
SIGNS OF HEATSTROKE — CALL EMERGENCY SERVICES IMMEDIATELY
- Body temperature above 40°C with little or no sweating
- Confusion, disorientation or loss of consciousness
- Rapid, weak pulse and difficulty breathing
- Nausea, vomiting and a severe headache
- Heatstroke, unlike heat exhaustion, is a medical emergency. Cool the person immediately and call emergency services.
A heat wave is not merely a meteorological phenomenon. For the body it is a physiological challenge that demands preparation. Electrolytes, antioxidants and thoughtful food choices are not the privilege of athletes, they are tools that every person can use to reduce the burden on their body in extreme heat. As always, individual needs vary, and people with chronic conditions, the elderly and pregnant women should consult a physician about adjusting their fluid and electrolyte intake during heat waves.
Scientific references
[1] World Health Organization. Heat and health. WHO Fact Sheet. Geneva: WHO; 2018. www.who.int/news-room/fact-sheets/detail/climate-change-heat-and-health
[2] Shirreffs, S. M., and R. J. Maughan. “Whole body sweat collection in humans: an improved method with preliminary data on electrolyte content.” Journal of applied physiology 82.1 (1997): 336-341.
[3] Bates, Graham P., and Veronica S. Miller. “Sweat rate and sodium loss during work in the heat.” Journal of Occupational Medicine and Toxicology 3.1 (2008):
[4] Belhadj Slimen, Imen, et al. “Reactive oxygen species, heat stress and oxidative-induced mitochondrial damage. A review.” International journal of hyperthermia 30.7 (2014): 513-523.
[5] Almond, Christopher SD, et al. “Hyponatremia among runners in the Boston Marathon.” New England Journal of Medicine 352.15 (2005): 1550-1556.
[6] Phillips, Paddy A., et al. “Reduced thirst after water deprivation in healthy elderly men.” New England Journal of Medicine311.12 (1984): 753-759.
[7] Armstrong, Lawrence E. “Assessing hydration status: the elusive gold standard.” Journal of the American College of Nutrition26.sup5 (2007): 575S-584S.
[8] Stand, ACSM Position. “Exercise and fluid replacement.” Medicine and science in sports and exercise 39.2 (2009): 377-390.
[9] National Institutes of Health, Office of Dietary Supplements. Potassium- Health Professional Fact Sheet. ods.od.nih.gov.
[10] Kalman, Douglas S., et al. “Comparison of coconut water and a carbohydrate-electrolyte sport drink on measures of hydration and physical performance in exercise-trained men.” Journal of the International Society of Sports Nutrition 9.1 (2012): 1.
[11] Abbasi, Behnood, et al. “The effect of magnesium supplementation on primary insomnia in the elderly.” Journal of Translational Medical Research 19.3 (2012): 223-234.
[12] Moabedi, Mahdi, and Alireza Milajerdi. “The effect of co-administration of vitamin E and C supplements on plasma oxidative stress biomarkers and antioxidant capacity: a GRADE-assessed systematic review and meta-analysis of randomized controlled trials with meta-regression.” Frontiers in immunology16 (2025): 1547888.
[13] Fiedor, Joanna, and Květoslava Burda. “Potential role of carotenoids as antioxidants in human health and disease.” Nutrients 6.2 (2014): 466-488.
[14] Basu, Rupa. “High ambient temperature and mortality: a review of epidemiologic studies from 2001 to 2008.” Environmental health 8.1 (2009): 40.
[15] Kovats, R. Sari, and Shakoor Hajat. “Heat stress and public health: a critical review.” Annual review of public health 29.2008 (2008): 41-55.
