A Comprehensive Review on Medium- and Long-Chain Fatty Acid-Derived Metabolites: From Energy Sources to Metabolic Signals

Medium- and long-chain fatty acids (MLFAs) are increasingly recognized not only as metabolic substrates but also as precursors of diverse bioactive metabolites generated through host and microbial transformations. Recent advances in analytical chemistry and microbiome research have revealed that gut microorganisms catalyze extensive modifications of dietary MLFAs—producing hydroxylated, conjugated, and keto-fatty acids with enhanced potency toward host receptors. These metabolites exhibit dual activity on classical metabolic receptors, including FFAR1/4 and PPARα/γ, as well as ectopically expressed chemosensory receptors such as olfactory receptors (ORs) and bitter taste receptors (TAS2Rs). This expanded receptor landscape establishes a previously unrecognized chemosensory–metabolic axis that integrates dietary signals, microbial metabolism, and host physiology. Microbial MLFA derivatives such as 10-hydroxyoctadecenoic acid and conjugated linoleic acid regulate incretin secretion, adipogenesis, macrophage polarization, and intestinal barrier function through coordinated activation of FFARs and PPARs. Concurrently, dicarboxylic acids such as azelaic acid activate Olfr544 to modulate lipolysis, ketogenesis, GLP-1 release, and feeding behavior. TAS2Rs also sense oxidized lipids, linking lipid metabolism to immune regulation and enteroendocrine signaling. Collectively, these pathways highlight the microbiome as a metabolic transducer that converts dietary lipids into signaling molecules influencing endocrine, immune, and gut–brain circuits. Understanding the mechanisms governing MLFA bioconversion and receptor engagement provides new opportunities for therapeutic and nutritional intervention. Targeting ORs and TAS2Rs, engineering probiotics to enhance beneficial FA-derived metabolites, and developing receptor-selective synthetic analogs represent promising strategies. Future progress will require integrative approaches combining physiology, biochemistry, metabolomics, and microbial genomics to elucidate receptor specificity and host variability.