Inappropriate timing of salt intake increases the risk of heat-related illness: An observational study

The importance of salt intake in preventing heat-related illness (HRI) is well established, however, the specific method of ingestion has not been sufficiently studied. This study, therefore, aimed to investigate the optimal timing of salt intake to prevent HRI during hot outdoor work. We recruited 28 healthy male firefighters working at a fire department in Japan. They were provided a questionnaire to complete before and after receiving training in the summer season. We assessed their salt intake as before, during, and after training or none. In addition, they completed a brief self-administered diet history questionnaire to evaluate their daily salt and alcohol intake. HRI was determined through subjective and objective symptoms listed in the questionnaire, and environmental data were obtained from a national database. Subsequently, factors related to HRI were determined using a logistic regression model. The mean age of the participants was 31.0 ± 7.7 years. The study was performed within 250 working days, and we detected 28 HRI symptoms (11.2%). The median alcohol intake was 25.6 g/day when calculated according to the actual work system. Logistic mixed effect model analysis revealed that salt intake before training (OR: 5.893, 95% CI: 1.407-24.675), and salt intake before and during training (OR: 22.889, 95% CI: 4.276-122.516) were positively associated with HRI symptoms. The results indicate that inappropriate timing of salt intake increases the risks of HRI. Thus, a timely intake of salt in adequate amounts may be important in preventing these risks.