Nutritional Strategies during Exercise, which affect Thermoregulation and/or Athletic Performance in the heat: A Systematic Review

Purpose Increased core temperature (Tc) can reduce athletic performance in the heat due to the increased demands on the thermoregulatory system. This can lead to serious detriments in physical performance with increased risk of the athlete developing heat illness. Many investigations have studied one to two nutritional factors that may affect thermoregulation during exercise in the heat however, there has not been a systematic review summarising all of the nutritional factors and providing recommendations. Yet running blogs mention several factors, which could influence performance and/or thermoregulation during exercise in the heat. The aim of this review was to investigate the popular nutritional interventions, which are often included in sports drinks and marketed to improve exercise performance or thermoregulation in the heat. Methods Literature was identified from the earliest record until May 2015 from PubMed, Medline (Ovid), Web of Science and Cochrane library, using search words; caffeine, carbohydrate, hydration, electrolytes, exercise, heat and thermoregulation. Inclusion criteria for the systematic review was; English language, published in a peer-reviewed journal, an intervention study (randomised control cross over design) with placebo, healthy adults aged 18 - 65 years, continuous endurance exercise (> 1 hour), the exercise was conducted in the heat (moderate > 24 ⁰C or hot > 28 ⁰C). The studies had to have either a performance outcome measure or Tc outcome to be included. For caffeine studies, the caffeine had to be ingested within 2 hours prior to exercise only (as ingestion during exercise would not be available for use by the body) and for all other substances, they had to be ingested/manipulated during exercise only. Studies were excluded if they were; repeated studies, used WADA prohibited substances, opinion papers, intermittent exercise protocols or exercise protocols of < 60 minutes, participants wore protective clothing or carried heavy loads during the exercise protocol. For this systematic review, the literature was grouped into nutritional strategies reviewing caffeine, carbohydrate, hydration, fluid temperature and electrolytes. For each intervention study specific data was extracted, analysed for statistical significance and subsequently quality rated. Two independent reviewers examined and extracted the relevant data. Data was analysed using STATA 13, for random effects meta-analysis model for standardised mean difference (SMD) and heterogeneity (I2). Forest plots and funnel plots were used to assess bias and sample size effect. Results Due to the heterogeneity of the papers in all nutritional categories (except carbohydrate), no statistical analysis was performed. There was a significant effect of carbohydrate intake and athletic performance in both Time Trial (TT) (p = 0.024) and Time to Exhaustion (TTE) (p < 0.001) studies and there was significant effect of carbohydrate intake and Tc in both TT (p = 0.02) and TTE (p = 0.04) studies. Six papers fulfilled the inclusion criteria for caffeine, three from hydration and only one paper in the electrolyte category. Only one individual caffeine paper showed a significant difference in athletic performance and none of the six studies showed any thermoregulatory benefit. The three hydration papers showed no significant performance effects and only one paper had a significant difference in Tc and hydration. There were no papers that fulfilled the fluid temperature category after a previous systematic review conducted by Burdon et al., (2010). The one study on electrolytes that fulfilled the strict inclusion criteria indicated no performance or thermoregulatory differences between groups. Conclusion Consuming carbohydrate during endurance exercise in the heat has beneficial effects in both performance and Tc enabling athletes to exercise for longer with higher Tc. There also appears to be benefit in both athletic performance and thermoregulation when athletes consume fluids or cool fluids when exercising in the heat. Further research is warranted to provide athletes and coaches with specific guidelines on the amount and types of carbohydrate and fluid to consume during endurance exercise to optimise performance and thermoregulation in the heat. Keywords: thermoregulation, nutritional strategies, carbohydrate, caffeine, hydration, fluid temperature, exercise and hea