Folic acid supplementation does not attenuate thermoregulatory or cardiovascular strain of older adults exposed to extreme heat and humidity

New Findings: What is the central question of this study? Does folic acid supplementation alleviate thermoregulatory and cardiovascular strain of older adults during exposure to extreme heat and humidity? What is the main finding and its importance? Folic acid supplementation for 6 weeks did not affect whole-limb blood flow/vasodilatation, core and skin temperatures, heart rate, blood pressure and cardiac output. Thus, 6 weeks of folic acid supplementation does not alleviate thermoregulatory or cardiovascular strain of healthy older adults exposed to extreme heat and humidity. Abstract: Folic acid supplementation reverses age-related reductions in cutaneous vasodilatation during passive heating. However, it is unknown if folic acid supplementation alleviates thermoregulatory and cardiovascular strain experienced by older adults during heat exposure. We evaluated the effect of folic acid supplementation on thermoregulatory and cardiovascular responses of nine healthy older adults (61–72 years, 3 males/6 females) exposed to extreme heat and humidity. Participants rested at 42°C while relative humidity was increased from 30% to 70% in 2% increments every 5 min. The protocol was performed before (CON) and after (FOLIC) 6 weeks of folic acid supplementation (5 mg day⁻¹). Local cutaneous vascular conductance (CVC, laser-Doppler flowmetry), forearm vascular conductance (FVC, Doppler ultrasound), mean skin and oesophageal temperatures, heart rate, blood pressure and cardiac output were measured. Folic acid supplementation increased fasting serum folate concentrations (P < 0.01). Absolute CVC was greater throughout the protocol following supplementation (CON: 1.29 ± 0.16 units mmHg⁻¹ vs. FOLIC: 1.65 ± 0.24 units mmHg⁻¹, P < 0.01). However, normalized CVC (CON: 54 ± 8% vs. FOLIC: 59 ± 7%, P = 0.22), FVC (CON: 3.47 ± 0.76 ml mmHg⁻¹ vs. FOLIC: 3.40 ± 0.56 ml mmHg⁻¹, P = 0.93), mean skin (P = 0.81) and oesophageal (CON: 36.87 ± 0.28°C vs. folic: 36.90 ± 0.25°C, P = 0.98) temperatures, heart rate (CON: 83 ± 10 beats min⁻¹ vs. FOLIC: 84 ± 8 beats min⁻¹, P = 0.64), blood pressure (P = 0.71) and cardiac output (P = 0.20) were unaffected by folic acid supplementation. These results demonstrate that 6 weeks of folic acid supplementation does not alleviate thermoregulatory or cardiovascular strain of healthy older adults exposed to extreme heat and humidity.