Effect Of Caffeine On Symptoms And Biomarkers Of Gastrointestinal Injury After Physical Work In The Heat

Several industries require substantial prolonged physical exertion in hot environmental conditions, this increases the risk of heat-related illnesses. Endotoxemia, a hallmark of exertional heat stroke, is defined by increased levels of circulating lipopolysaccharides, often caused by gastrointestinal (GI) injury and subsequent higher permeability. Caffeine, commonly used by workers to boost concentration and mood, may lead to GI distress as a side effect. Additionally, previous research also indicates that caffeine use prior to exercise in the heat may increase core temperature (Tcore). PURPOSE: To investigate the effect of ingesting a moderate dose of caffeine prior to physical activity in a hot environment on physiological strain GI injury, and endotoxemia. METHODS: Using a double-blind, randomized, crossover design, nine participants (6 females) were recruited for this study. A 120-minute protocol was completed in a heat chamber (35°, 53% relative humidity). The protocol consisted of two, 1-hour cycles of 20 minutes of walking on a treadmill at 40% VO2peak, 5 minutes of low-intensity box-lifting activity, another 20 minutes of walking at 40% VO2peak, and 10-minute seated rest. GI symptoms, heart rate (HR), and Tcore were recorded throughout. Serum concentrations of intestinal fatty acid binding protein (I-FABP) and lipopolysaccharide-binding protein (LBP) were determined from pre- and post-trial blood samples. RESULTS: Peak Tcore was 0.2 °C higher in the caffeine condition (38.2 ± 0.4 °C) compared to placebo (38.0 ± 0.3 °C) (p = 0.02). A condition effect was also observed (p = 0.001) on HR, as peak HR was higher in the caffeine condition (152 ± 19 bpm) compared to placebo (140 ± 17 bpm) (p = 0.01). There was a higher incidence of overall GI distress symptomology in caffeine condition (p = 0.03), however, caffeine did not have a significant effect on circulating markers of intestinal damage (I-FABP) or endotoxemia (LBP) (p > 0.05). A time effect was observed for I-FABP, where concentrations increased from pre- to post-work in the heat. CONCLUSION: These findings suggest that workers who perform work in hot environmental conditions may benefit from avoiding caffeine ingestion to reduce the risk of hyperthermia and potential exertional heat illness. Supported by: University of New Mexico’s Graduate and Professional Student Association