A Polymeric Nanocarrier Platform for Rapid and Precise Fatty Acid Tracing in Cells.

Stable isotope tracing provides insights into metabolism by tracking the movement of isotopically labeled precursors through metabolic networks. Fatty acid tracers, such as uniformly labeled 13C-palmitate, are used to study lipid biosynthesis, energy storage, and/or signaling. These tracers are complexed with BSA to improve solubility; yet, this approach is limited by transport bottlenecks, toxicity, and immunogenicity. Here, we developed biodegradable nanocarriers that improve hydrophobic tracer delivery and benchmarked performance against BSA with metabolomics and lipidomics. Nanocarriers accumulated U-13C-palmitate to higher intracellular levels, and more rapidly, than BSA-conjugated controls. Once inside the cell, nanocarrier-delivered tracers exhibited first-order depletion kinetics, ensuring predictable and efficient metabolism. In contrast, BSA produced delayed or biphasic tracer depletion due to transport limitations, which hindered the bioavailability. Entrance of nanocarrier-delivered U-13C-palmitate into the cellular metabolic network manifested through 13C-labeled desaturated and elongated fatty acids and incorporation into complex lipids without material-mediated aberrations. Our results demonstrate that nanocarrier-assisted tracing captures key metabolic trends with enhanced labeling while overcoming limitations of BSA-mediated delivery. This versatile, customizable platform enables opportunities for metabolic tracing in complex systems.