Brain-derived neurotrophic factor response to daylong exposure to extreme heat in young and older adults: a secondary analysis.

Brain-derived neurotrophic factor (BDNF) is a growth factor associated with a range of neurological, cardioprotective, and metabolic health benefits. While passive heat stress has been observed to increase circulating BDNF, the BDNF response to a given stressor may be attenuated with increasing age. To investigate the influence of age on the BDNF response to heat stress, we compared BDNF responses to daylong (9 h) exposure to hot ambient conditions (40 °C, 9% relative humidity) between 19 young (range: 19-31 years; 9 women) and 37 older adults (61-78 years; 12 women). We also explored whether cumulative thermal strain (area under the curve of rectal and mean body temperatures) impacted comparisons. Serum BDNF concentrations were assessed at pre- and end-exposure using enzyme-linked immunosorbent assays. Circulating BDNF concentrations increased from baseline in both groups ( 0.001), but end-exposure concentrations were 2594 [1555-3633] pg/mL lower in older than young adults ( 0.001). This age-related difference persisted, albeit to a lesser magnitude, after accounting for the lower pre-exposure BDNF levels in older adults (baseline-adjusted between-group difference: 1648 [667-2630] pg/mL;  0.001). Additionally, the BDNF response was not related to indices of thermal strain ( ≥ 0.562), and baseline-adjusted between-group differences were not appreciably altered by adjusting for area under the curve of rectal (1769 [714-2825] pg/mL;  0.002) or mean body temperatures (1745 [727-2763] pg/mL;  0.001). Our study is the first to demonstrate an age-related reduction in the BDNF response to prolonged passive heat exposure, which informs our wider understanding of how environmental stressors influence BDNF responses in older adults.