Effect Of Caffeine On Acute Kidney Injury Markers During Simulated Occupational Work In The Heat

Caffeine is a stimulant widely used by workers in labor-intensive occupations to enhance mood and concentration. Previous research shows that caffeine has a diuretic effect and may increase heat production during activity in the heat, potentially contributing to dehydration and hyperthermia. The prevalence of dehydration and hyperthermia is high in workers who are exposed to hot conditions for extended durations, putting them at risk for acute kidney injury (AKI). Despite the high prevalence of caffeine use, the possible effect on AKI is unknown. PURPOSE: To investigate the effect of caffeine on markers of kidney function and AKI risk during physical work in the heat. METHODS: In this double-blinded, crossover design, 14 habitual caffeine users (6 males) ingested 5 mg per kg of bodyweight (up to 400 mg) of caffeine or placebo before 2 hrs of physical work in the heat (35 °C, 50% RH). Participants performed a combination of treadmill walking and box-lifting to simulate occupational work. Core temperature (Tc) was monitored throughout the trial. Urine and blood samples were collected pre-, post-, and 1-hr post-trial to measure markers of AKI risk (product of urinary insulin-like growth factor-binding protein 7 and tissue inhibitor of metalloproteinase 2; IGFBP7×TIMP2, urinary neutrophil gelatinase-associated lipocalin; NGAL) and kidney function (serum cystatin C). Urine specific gravity (USG) and urine flow rate (UFR) were analyzed to further assess kidney function. RESULTS: Caffeine increased average (p=0.05) and peak Tc (38.2 ± 0.4°C vs 38.0 ± 0.3°C; p=0.02). Measures of USG, UFR, IGFBP7×TIMP-2, and NGAL showed no effect of condition, time, or interaction (p>0.05). There was a significant time×condtion interaction effect for serum cystatin C (p=0.01), where levels increased pre- to post-exercise in the caffeine condition (∆ 0.24 ± 0.43 mg/L, p=0.03), indicating a decline in glomerular filtration rate (GFR), while the placebo condition did not change (∆ -0.07 ± 0.17 mg/L, p=0.81). CONCLUSION: Caffeine ingestion prior to physical work in the heat did not increase markers for AKI risk but may transiently impair GFR, potentially through exacerbated Tc increase. While caffeine may increase kidney strain under this condition, further research is needed to explore if this strain contributes to AKI development in workers.