The study compared biomarker data from peak summer months between March and May with cooler winter months from November to February and found a sharp rise in clinically abnormal readings linked to heat stress and dehydration.
Among the clearest signals was concentrated urine, a direct marker of dehydration, which was 25 per cent more common in summer, affecting nearly one in three patients. Acidic urine, another indicator of kidney stress linked to dehydration, rose 28 per cent during the same period.
One of the most striking findings was a 59 per cent increase in low sodium cases during summer months. Researchers said the trend points not to sodium loss from sweating alone, but to excessive intake of plain water without adequate electrolyte replacement, resulting in dilution of blood sodium levels. “The fix isn’t just more water, it’s electrolytes,” the study noted.
Magnesium deficiency showed the sharpest rise among all biomarkers studied, with cases more than tripling during summer months. Magnesium is lost through sweat and cannot be replenished through water alone, making prolonged heat exposure a potential trigger for fatigue, cramps, and irregular heartbeat.
The study also found that elevated calcium levels, associated with kidney stone risk during dehydration, were nearly twice as common in summer compared with winter, though overall incidence remained low.
Blood-related biomarkers also shifted during heatwave months. Low haemoglobin levels were 18 per cent higher in summer, while low hematocrit and low iron levels rose to 13 per cent and 17 per cent, respectively. Researchers said these changes likely reflect the body’s natural cooling response, in which blood plasma expands during prolonged heat exposure, diluting red blood cell concentration.
The dataset covered approximately 22,167 health checkup records between October 2025 and May 2026, with one record retained per patient. To ensure fair comparison, summer and winter cohorts were matched by age, gender, and health checkup package type.
Interestingly, the findings emerge not from hospitalised heatstroke patients, but from otherwise healthy urban professionals undergoing routine annual checkups. The trends suggest that the physiological effects of extreme heat may already be visible in everyday health data, even outside emergency medical settings.
The findings come as India experiences increasingly frequent and prolonged heatwaves, with health experts warning that rising temperatures could create a growing burden of silent and cumulative health stress across the population.
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