Abstract
Research investigating substrate metabolism during exercise with female participants is limited, especially regarding effects of cold stress. The aims of this study were to determine whether exercising in cold ambient temperature affects substrate metabolism in females and whether the addition of a thermal layer alters the metabolic response in the cold. Trained female cyclists (n = 11, age 26 ± 7 y, height 1.69 ± 0.08 m, body mass 63.9 ± 7.2 kg, ∑8 skinfolds 111.6 ± 22.5 mm, body fat 20 ± 3%, O2 max 48 ± 8 ml.kg-1·min-1, mean ± SD) performed three trials in pseudo-random order; two at 5 ºC (Cold and Clothing (C5 °C)) and one at 15 ºC (Temperate), 93 and 79% relative humidity (RH), resp. Exercise comprised warm-up (30 min ~30% VO2 max) in 17.8 °C (43% RH), 75 min cycling at ~74% VO2 max, and a 4 km time trial in the test environment. The same attire was worn in each trial, with the addition of an extra thermal layer in the second 5 °C trial (Clothing; C5 °C), all with a fan at 3.9 m·s-1. Total carbohydrate (CHO) and fat oxidation (indirect calorimetry every 15 min) were calculated and corrected for protein oxidation (from sweat and urine urea). CHO oxidation was partitioned using [13C6]glucose tracer methodology, measuring enrichment in breath (CO2) and blood (glucose). Heart rate (HR), skin temperature (Tsk) (nine sites), core temperature (Tc) (rectal), and subjective measures of thermal sensation, discomfort and rating of percieved exertion (RPE) were recorded every 15 min. Protein oxidation was not different between conditions (p < 0.05). Neither total CHO, fat oxidation, muscle glycogen or liver-derived glucose oxidation were different (p > 0.05) between Cold and Temperate. Reduced fat oxidation in Clothing versus Cold (5 °C: 0.33 ± 0.15 g·min-1; C5 °C: 0.25 ± 0.21 g·min-1, mean ± SD) was observed (p = .031), and a trend of incresed CHO oxidation in Cold (5 °C: 1.96 ± 0.44 g·min-1; C5 °C: 2.19 ± 0.48 g·min-1; p = .058). Tc increased across exercise (p < .001) but was not different between Cold and either Temperate or Clothing. sk was ~6 °C and ~1 °C lower in Cold compared to Temperate (p < .001) and Clothing (p < .05). Thermal sensation was lower and discomfort was higher in Cold than Temperate (both p < .001). Only thermal sensation was modified by Clothing (p < .001). There were no differences in time trial performance between Cold and Temperate (p = .23), and Cold and Clothing (p = .60). In conclusion, there is no large difference in metabolism between moderately intensive cycling at 5 °C vs. 15 °C (with substantial facing air velocity) in women. The women in this study may be resilient to temperature variation possibly due to body composition and or sex hormone influences. Interestingly, the addition of clothing tends to stimulate CHO metabolism in the cold, decreases fat metabolism, and may reflect heat stress rather than a lack of cooling.