Feet-heating and calf-heating have opposing effects on glucose tolerance and heart rate variability: a randomized, controlled, crossover trial.

Heat exposure's effect on glucose tolerance depends on the amount of body exposed, likely relating to autonomic nervous system balance. We assessed how partial-body heat exposure at two different levels of the lower extremities affects glucose tolerance and autonomic nervous system balance, measured via heart rate variability. We hypothesized feet-heating would improve glucose tolerance without affecting heart rate variability, while calf-heating would worsen glucose tolerance and decrease heart rate variability compared to a thermoneutral control condition. In a randomized, controlled, crossover trial, healthy participants' (N = 31, 23(3) years, 45% male) glucose tolerance was measured in (A) thermoneutral; (B) feet-heating; and (C) calf-heating conditions. Every 30 min for 2 h, blood glucose, heart rate, heart rate variability, tympanic temperature, thermal comfort scores, and blood pressure were measured. There were significant interactions between condition and time for blood glucose (F (4.6,72.6) = 2.6, p = 0.036), heart rate (F (3.4, 54.5) = 3.5, p = 0.017), heart rate variability (F (4.3,63.2) = 7.5, p < .0001), tympanic temperature (F (8, 268) = 2.4, p = 0.014), and thermal comfort scores (F (8, 248) = 22.1, p < 0.0001). Calf-heating increased 90 min glucose (+12 (95% confidence interval, CI: 3-21) mg/dL, p = 0.013) and decreased heart rate variability throughout (mean decrease: 13%-22%, p < 0.007), while feet-heating lowered 90 min glucose (-7 (95% CI: -16 to +1) mg/dL, p = 0.090) without affecting heart rate variability (p = 0.14-0.99). Blood pressure and body temperature were similar between conditions, but heart rate and thermal comfort scores increased with heating. Calf-heating worsens, while feet-heating may improve, glucose tolerance. Changes in heart rate variability coincided with changes in glucose tolerance despite unchanged body temperature. Whether heart rate variability can be used to monitor autonomic nervous system balance during heating to optimize its acute effect on glycemic indices should be further explored.