Detrimental Reductions In Evaporative Heat Loss During Cycling Following Ice Slurry Ingestion In The Heat

Abstract

PURPOSE: To compare the magnitude of reduction in evaporative heat loss from the skin (Esk) with different levels of internal heat loss (Hint) induced by ice slurry and 1.5°C fluid ingestion relative to 37°C fluid ingestion during cycling in a hot environment. METHODS: Nine men RESULTS: All data are expressed relative to the NEU trial. Hprod was similar to NEU in both trials (COL: +34±154 kJ; P=0.52, ICE +17±210 kJ; P=0.81). Hint was progressively lower in the COL (-105±18 kJ) and ICE (-200±20 kJ) trials relative to NEU. In parallel, Esk was also progressively lower in the COL (-137±165 kJ; P=0.037) and ICE (-381±199 kJ; P<0.001) trials. However while evaporative reductions were equal to Hint in the COL trial (P=0.64), they were greater than Hint in the ICE trial (P=0.05), leading to a greater heat storage with ICE ingestion. Mean local sweat rates (COL: -0.06 ±0.09 mg•min-1•cm-2; P=0.09, ICE: -0.16±0.14 mg•min-1•cm-2; P=0.009) and whole-body sweat losses (COL: -56±69 g; P=0.04, ICE: -191±122 g; P<0.001) were lower compared to NEU. Conversely, changes in Tre (COL: +0.04±0.24°C; P=0.63, ICE: +0.07±0.12°C; P=0.11), Tsk (COL: -0.07±0.35°C; P=0.55, ICE: -0.04±0.38°C; P=0.74) and local skin blood flow (COL: -2.9±10.9%max, P=0.45; ICE: -6.8±13.7%max, P=0.18) were similar to the NEU trial. CONCLUSIONS: The greater internal loss with 1.5°C fluid, relative to 37°C fluid, was balanced by an equal reduction in Esk. However the internal heat loss with ice slurry ingestion, relative to 37°C fluid, was paralleled by a disproportionately greater reduction in Esk. These results indicate that the temperature of an ingested fluid has no net influence on human heat balance, but ice slurries should potentially be avoided.