Heat exposure increases circulating fatty acids but not lipid oxidation at rest and during exercise

Alterations in lipid oxidation during exercise have been well studied, but limited data exists on the effects of passive heat exposure and exercise in the heat on changes in lipid oxidation. This study was designed to examine: (1) the effects of heat exposure on lipid metabolism during passive heating and subsequent exercise in the heat by focusing on changes in whole-body lipid oxidation and plasma lipid concentrations, and (2) the effects of extended passive pre-heating on exercise performance in the heat. Male participants (n=8) were passively heated for 120min at 42°C, then exercised on a treadmill in the same temperature at 50% V̇O2max for 30min (HEAT). This same procedure was followed on a separate occasion at 23°C (CON). Results showed that lipid oxidation rates were not different between HEAT and CON during passive heating or exercise. However, non-esterified fatty acid (NEFA) concentrations were significantly higher following passive heating (618µM, 95% CI: 479–757) compared to CON (391µM, 95% CI: 270–511). The same trend was observed following exercise (2036µM, 95% CI: 1604–2469 for HEAT and 1351µM, 95% CI: 1002–1699). Triacylglycerol, phospholipid and cholesterol levels were not different between HEAT and CON at any point. Four of 8 participants could not complete 30min of exercise in HEAT, resulting in a 14% decline in total external work. Rate of perceived exertion over the final 5min of exercise was higher in HEAT (9.5) than CON (5). We conclude that: (1) heat exposure results in increased circulating NEFA at rest and during exercise without changes in whole-body lipid utilization, and (2) passive pre-heating reduces work capacity during exercise in the heat and increases the perceived intensity of a given workload. •Plasma NEFA elevated by heat exposure at rest.•Not matched by increase in lipid oxidation.•Subsequent exercise in the heat following passive heating maintains increase in NEFA.•Exercise in heat after passive heating increases perceived exertion of a given exercise intensity.