The effect of whey protein isolate supplementation on cortisol awakening response, profile of mood states, and heart rate variability in recreationally active women

The hypothalamus-pituitary-adrenal (HPA) axis plays a major role during stress responses and is associated with the secretion of cortisol. Serum cortisol concentration peaks between 30 and 45 min after awakening, and is known as the cortisol awakening response (CAR). Decreases in heart rate variability (HRV), disruptions in CAR, and changes in Profile of Mood States (POMS), typified by disturbances in HPA activity, have been associated with fatigue due to repetitive strenuous physical exercise. Branched chain amino acids (BCAA), present in whey protein, compete for tryptophan transporters in the brain, potentially reducing fatigue associated with exercise, potentially affecting CAR, POMS, HRV, and fatigue index (FI). PURPOSE: To determine the effects of whey protein isolate supplementation on CAR, POMS, and HRV after strenuous exercise, in recreationally active women, on post-exercise days. METHODS: Eleven recreationally active women (19 ± 2 yrs; VO2peak = 31.6 ± 4.5 ml/kg/min) completed a double blinded, randomized, cross-over placebo trial, with a 7 day washout between trials. A supplement regimen (25 g of maltodextrin (PL) or 25 g of maltodextrin plus 25 g of whey protein isolate (WH)) on Day 1, 2, and 3 was used for each trial. Participants ingested supplements between 8am - 9am (Day 1, 2, 3) and 30 min prior to exercise (Day 2 & 3). On Day 2 and 3 of each trial participants walked for 30min on a treadmill at 70-75% VO2peak (21.7 ± 0.1 ml/kg/min), rested 5 min, and completed a 30 s Wingate anaerobic test (WAnT). Saliva (2ml) was collected on days 1-4 of PL and WH between 6am and 8am immediately upon waking and every 15 min for the next hour. Saliva samples were analyzed for cortisol concentration using an enzyme linked immunosorbent assay (ELISA) and the area under the curve (AUCg) was calculated for cortisol. HRV and POMS were recorded between 15 - 30 min and 30 - 45 min after waking, respectively. Repeated measures ANOVA (2 trial x 4 days) were used to determine significant differences (p < .05) in cortisol AUCg, POMS, and HRV. A repeated measures ANOVA (2 trial x 2 day) was used to determine significant differences (p < .05) in WAnT fatigue index (FI). Pearson correlation and multiple regression models were implemented to determine associations between CAR, POMS, and HRV to FI. RESULTS: Main effect means for cortisol AUCg were significantly different (p = 0.033) between PL (33.4 ± 2.0 µghr/dL) and WH (30.9 ± 0.8 µghr/dL). There were no significant main effect differences in POMS or HRV by day or condition. No significant difference in WAnT fatigue index means occurred between PL (20.79 ± 6.12% Day 2 & 21.97 ± 6.16% Day 3) and WH trials (22.52 ± 6.45% Day 2 & 22.78 ± 6.38% Day 3). The only significant Pearson correlation existed on Day 3 during the PL trial, between POMS and FI (r = -.582, p = 0.030). All other correlations were not statistically significant. Additionally, neither CAR, POMS, nor HRV was able to predict FI through multiple regression equations (all p > .05). CONCLUSIONS: Whey protein isolate may decrease CAR on post-exercise days, but may have no effect on POMS, HRV, or FI, in recreationally active women. As indicated by a decrease in CAR, results demonstrate a possible reduction in central fatigue associated with strenuous exercise. However with no effect on the short-duration sprint cycling performance, the physiological effect of the reduction in central fatigue may be minimal.