Passive heat loading links lipolysis and regulation of fibroblast growth factor-21 in humans

• Passive heat loading was effective for expressing FGF21 and lipolysis.
• Under the identical type (leg immersion) of passive heat loading, 43 °C was more effective for expressing FGF21 and lipolysis compared to that at 39 °C and 42 °C.
• Half immersion was more effective than leg immersion at the same intensity (42 °C) for expressing FGF21 and lipolysis.

There is relativley little information on the serum biomarkers of heat stress. Therefore, the goal of this study was to verify the effect of passive heat loading (PHL) on the expression of fibroblast growth factor-21 (FGF21) and free fatty acids (FFAs). Four PHL protocols based on intensity (39 °C vs. 43 °C, leg immersion, 30 min) and type (leg vs. half immersion, 42 °C, 30 min) were used. Each protocol was applied on a 2 day cycle to 12 healthy adult males (age, 22.4±2.9 years; height, 174.1±4.6 cm; weight, 71.3±5.6 kg; body mass index, 23.1±3.0). The subjects were categorized into two groups according to the study design (randomized, with a parallel trial). Body temperature, FGF21 and FFAs were determined prior to PHL, immediately and 60 min after PHL. Body temperature was significant higher (43 °C) than the 39 °C measured under identical PHL type (leg immersion). PHL was effective for the expression of FGF21 and for lipolysis. The quantitative levels of FGF21 and FFA increased with increasing temperature (39 °C<42 °C<43 °C). A significant difference in the quantitative levels of FGF21 and FFAs was also evident based on the type of PHL (leg