Acute metabolite responses to swimming exercise of different intensities in highly trained male and female swimmers

We investigated metabolite responses to different swimming intensities in 16 highly trained swimmers (9 males, 7 females, aged 16–24 years). After determining critical swimming speed (CS) with a 12 × 25 m maximal effort test, participants completed three swimming trials at moderate (below CS), heavy (at CS), and severe (above CS) intensities on separate days. Capillary blood samples (1 mL) were collected before and after each trial for metabolite profiling via mass spectrometry. Orthogonal partial least squares analysis (OPLS-DA) revealed distinct metabolite changes between moderate and severe intensity trials [R²X (cum): 0.56; R²Y(cum): 0.95, pR²Y: 0.02, pQ²: 0.02]. Free fatty acids (FFAs) 18:0, 20:0, 20:4, and 22:4 showed higher log₂ fold changes (log₂FC) after moderate compared to heavy and severe trials (all p < 0.01). Plasma lactate, pyruvate, alanine, and HDL-4 cholesterol concentrations had greater log₂FC after heavy and severe trials than moderate (all p < 0.01). Males tended to show lower log₂FC in FFA than females in the severe trial, though this was not significant. These findings demonstrate that swimming intensity influences metabolite profiles, with reduced lipid metabolism and increased TCA cycle activity as intensity rises. A 1 mL capillary blood sample can effectively capture these metabolic shifts.