Oral niacin mitigates heat-induced reproductive impairments in male mice

Heat stress is a major environmental factor that impairs male fertility by disrupting hormonal regulation, increasing oxidative stress, and inducing germ cell apoptosis. Niacin (vitamin B3) has antioxidant and thermoregulatory properties that may counteract these effects. This study investigated whether oral niacin could protect against chronic heat stress-induced reproductive damage in adult BALB/c male mice. Mice were randomly assigned to Control, Heat Control, and two heat-stressed groups receiving niacin at 100 or 200 mg/kg/day for 30 days, with heat stress applied daily at 36 ± 2 °C for 4 h. Heat stress reduced body weight gain, testis and accessory gland weights, sperm count and motility, acrosome and DNA integrity, testosterone and thyroid hormones, and increased lipid peroxidation and pro-apoptotic gene expression. Niacin supplementation dose-dependently mitigated these effects: total sperm count increased from 9.8 × 10⁵ ± 1.85 × 10⁵ to 3.66 × 10⁶ ± 4.62 × 10⁵, total motility from 49.0 ± 13.6% to 81.7 ± 5.8%, testosterone from 0.89 ± 0.21 to 4.44 ± 0.70 ng/mL, and Bcl2 expression increased 2.7-fold. Fertility improved, with pregnancy risk rising from 50% to 100%, litter size from 3.7 ± 0.3 to 7.2 ± 0.3 pups, and blastocyst formation from 37.8 ± 2.7% to 68.0 ± 2.6%. Niacin also normalized stress-related gene expression, preserved testicular histology, and did not affect liver enzyme activities, highlighting its potential as a dietary supplement to protect against heat-induced male reproductive dysfunction.