Impact of Short-Term Gala Apple Intake on the Human Faecal Metabolome Assessed by ¹H NMR Spectroscopy

Background/Objectives: Apples are nutritionally valued for their dietary fibre and polyphenols, which influence gut microbial metabolism. However, the metabolic consequences of short-term apple consumption on the human gut environment have not yet been thoroughly investigated. This study aimed to investigate the impact of short-term supplementation of the habitual diet with Gala apples on the human faecal metabolomic profile and to identify metabolite changes reflecting microbial fermentation processes. Methods: A pilot dietary intervention was conducted in 15 healthy adults (6 females, 9 males; age 18-30 years). Participants consumed three Gala apples per day as part of their habitual diet for three consecutive days, one with each main meal. Faecal samples were collected before and after the intervention. Metabolic profiling was performed using ¹H NMR spectroscopy. Results: Multivariate analysis showed no clear clustering between pre- and post-intervention samples, suggesting that inter-individual variability exceeded the overall intervention effect. Univariate analysis showed a nominal increase in faecal methanol levels post-intervention (β = +0.72, FC ≈ 2.05, p = 0.012); however, this change did not remain statistically significant after correction for multiple testing (q = 0.27). Several other metabolites showed nominal decreases following the intervention, including amino acids, branched-chain fatty acids, short-chain fatty acids, and aromatic microbial metabolites. Conclusions: Short-term Gala apple consumption was associated with nominal, metabolite-specific changes in the faecal metabolome without clear global shifts. These findings may show immediate metabolic responses to increased intake of apple-derived substrates; however, given the exploratory design, small sample size, and high inter-individual variability, causal relationships cannot be established. Further studies in larger, controlled cohorts incorporating complementary approaches are required for confirmation.