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

Abstract

Brain-derived neurotrophic factor (BDNF) is a growth factor 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-hours) exposure to hot ambient conditions (40°C, 9% relative humidity) between 19 young (range: 19-31 years; 9 female) and 37 older adults (61-78 years; 12 female). 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 (P<0.001), but end-exposure concentrations were 2594 [1555, 3633] pg/mL lower in older than young adults (P<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; P<0.001). Additionally, the BDNF response was not related to indices of thermal strain (P≥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; P=0.002) or mean body temperatures (1745 [727, 2763] pg/mL; P=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.