Complete caloric restriction is common to most humans. Whether characterised by regular religious practise, a surgical patient’s perioperative period or the trend of intermittent fasting for health (e.g., periodic fasting, time-restricted feeding) a large proportion of the global population experiences caloric restriction. These examples often accompany physical activity for sport, exercise, or occupation. Whilst the separate and combined effects of caloric restriction and exercise have been studied for several physiological consequences, few investigations have investigated the thermoregulatory outcomes. Previous research demonstrated that 48h of caloric restriction in both men and women increased metabolic rate and peripheral blood flow such that core temperature was reduced when subjected to cold stress. To our knowledge, no study has determined this for heat stress, the purpose of the current study.

Eight healthy and recreationally active adults (4 female, age: 31±8 y, weight: 69±15 kg, body fat: 15±8%, VO₂max: 3.8±1.1 L·min^-1) performed two treadmill trials in a heat chamber (30°C, 40% RH) that consisted of consecutive 20 min bouts at 40% (5.7±0.6 km·h^-1) and 70% (10.0±1.0 km·h^-1) of their individually-determined ventilatory threshold. Both trials were scheduled at ~10am and followed either 34-39h of complete caloric restriction or their usual diet. Measures of gastro-intestinal temperature (T_gi), heart rate (HR), blood pressure (MAP), skin blood flow (SkBF, laser Doppler flowmetry), fluid loss (body weight change) and sweat rate (technical absorbent), expired gases, urine specific gravity (USG) and blood biochemical markers were taken. Descriptive data are provided as mean±SD, with all analyses conducted using SPSS software for Windows and significance accepted at p<0.05. Data were analysed using either a paired samples t-test or ANOVA for repeated measures.

Baseline resting data for T_gi, HR, MAP, SkBF, USG and body weight were not different between trials (p≥0.22), whilst caloric restriction decreased blood glucose (4.7±0.3 vs. 5.4±0.3 mmol·L^-1, p<0.01) and insulin (28±15 vs. 89±53 pmol·L^-1, p<0.01) and increased free fatty acids (0.6±0.2 vs. 0.3±0.2 mEq·L^-1, p<0.01). Participants were exercising at 34±4 and 78±10 VO₂max respectively, with no difference between trials (p=0.60); although caloric restriction reduced the respiratory exchange ratio (0.77±0.04 vs. 0.81±0.03 a.u., p=0.02). Exercise increased T_gi (∆1.2±0.3 °C, p<0.01) reaching 38.5±0.4 °C with no difference between trials (p=0.56). Water consumed (0.35±0.43 L·h^-1, p=0.62), whole-body sweat loss (0.66±0.35 kg·h^-1, p=0.32) and local sweat rate (0.60±0.28 mg·min^-1·cm^-1, p=0.16) were not different between trials. Exercise increased SkBF and decreased MAP (both p<0.01) with no difference between trials (both p>0.60). Exercising HR continued to increase (p<0.01) and was higher with caloric restriction (∆3±3 beats·min^-1), reaching 90±8% VO₂max.

Complete caloric restriction lasting 34-39h had minimal physiologic impact during 40 min of exertional heat strain, with only heart rate mildly increased.