Heatwaves may be doing more than causing discomfort and increasing the risk of heatstroke. New data suggest that rising temperatures could be silently altering key health markers, with signs of dehydration and mineral imbalance becoming significantly more common during the summer months.

An analysis by Plum of around 22,167 health checkup records between October 2025 and May 2026 found that abnormal readings for several dehydration- and blood-related biomarkers increased sharply during the heatwave period from March to May compared with the winter months of November to February.

The study, which examined one record per patient, analyzed eight biomarkers and found that all the observed changes were statistically significant, highlighting the physiological impact of prolonged exposure to extreme heat.

Abnormal health readings

One of the clearest trends was a steady increase in abnormal readings as temperatures climbed. The average proportion of patients with abnormal results across the eight biomarkers rose from 9.4% in October 2025 to 10.6% in February 2026. However, with the onset of heatwaves, the figure jumped to 13.6% in March, peaked at 15.6% in April and remained elevated at 15.3% in May.

The findings point to dehydration and electrolyte loss as major contributors to these changes.

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Low magnesium, sodium imbalance 

Low magnesium, sodium imbalance and signs of dehydration became markedly more common during the summer heatwave period

Among the biomarkers analyzed, low magnesium levels recorded the sharpest increase, surging 219% during the summer months compared with winter levels. Magnesium is lost through sweat and is not replenished through plain water alone, making individuals more vulnerable to deficiencies during periods of extreme heat.

Low sodium levels rose by 59%, suggesting that many people may be consuming large quantities of water without adequately replacing lost electrolytes. Concentrated urine, a key indicator of dehydration, increased by 25%, while acidic urine, which may reflect stress on kidney function, rose by 28%.

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The study also found a 17% increase in low iron levels during the hotter months, indicating that prolonged sweating and fluid loss could have broader effects on blood health.

Low sodium to iron deficiency

From low sodium to iron deficiency, extreme heat is leaving measurable changes in key health indicators

A closer look at seasonal differences revealed that 32.3% of patients had concentrated urine during April and May, compared with 26% during winter. Cases of acidic urine rose to 23.7% from 18.6%, while low sodium abnormalities climbed to 10.5% from 6.6%.

Other blood-related abnormalities also became more common. Low haemoglobin levels increased by 18%, while low hematocrit readings rose by 13%. High calcium abnormalities nearly doubled, though they remained relatively uncommon overall.

According to the analysis, the changes can largely be explained by the body's response to excessive heat. Increased sweating leads to losses of water and minerals, while the kidneys respond by producing more concentrated and acidic urine. Drinking excessive amounts of plain water without replacing electrolytes may further dilute sodium levels in the blood.

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What happens to the body during heatwaves?

The findings underscore the importance of staying hydrated with electrolyte-rich fluids, maintaining a balanced diet and taking additional precautions during peak heatwave months. As extreme weather events become more frequent, the data suggest that heatwaves may be leaving a measurable — and often unnoticed — imprint on human health.

The analysis highlights that the health effects of extreme temperatures extend beyond heatstroke and exhaustion. With climate change expected to make heatwaves more frequent and intense, experts say understanding these subtle physiological changes will become increasingly important in safeguarding public health.

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Published on: Jun 16, 2026 1:53 PM IST