Astaxanthin Supplementation Lowers Respiratory Exchange Ratio During Submaximal Cycling in Females: A Randomized, Double-Blind, Placebo-Controlled Crossover Trial

Astaxanthin (AX) is a lipid-soluble carotenoid that has produced mixed results in increasing fat oxidation during submaximal exercise in male cohorts. Females exhibit a higher reliance on fat oxidation, potentially making them a demographic more likely to benefit from AX supplementation. Following an initial VO2max test to determine maximum power (Wmax), twenty recreationally active females (age: 22 ± 2 y) participated in a two-arm double-blind, counterbalanced, crossover design study with a 2-week supplementation period of either AX (12 mg·day−1) or placebo (PLA) separated by a 2-week washout period. Participants cycled continuously for 30 min for six, 5-min stages with data collected over the last minute of each stage. The initial and final stages were 40 and 65% of Wmax, respectively with 5% increment increases between stages. There were no differences between AX or PLA for heart rate, blood lactate, or rate of perceived exertion at any intensity (all p > 0.05). For RER, there was no significant supplement × intensity interaction (p = 0.14), but a significant main effect was observed for both intensity (p < 0.001) and supplement (p = 0.04), with overall RER being significantly lower during the cycling bout following AX supplementation compared to PLA. Two weeks of AX supplementation lowered RER values in an active female cohort, indicating a shift toward a greater reliance on lipid-dominant substrate preference at specific submaximal intensities. The lack of change in cardiovascular or perceptual markers suggests that these shifts in RER occur independently of systemic physiological or psychological stress, though the exact underlying cellular mechanisms remain to be elucidated.