Physiological fatty acid uptake reveals spatial and systemic constraints on nutrient accessibility in vivo

Immune cells rely on exogenous fatty acids (FA) for membrane synthesis, bioenergetics and signalling, yet current approaches cannot accurately quantify physiological FA uptake in vivo. Here, we use cyclopropene-tagged fatty acids (cpFA) that, unlike existing FA-uptake tools, are taken up by physiologically relevant mechanisms. We measure FA uptake at single-cell resolution in vivo and uncover a previously unappreciated distinction between nutrient uptake capacity and nutrient accessibility. Although arachidonic acid exhibits the highest uptake capacity ex vivo across immune populations, it displays limited tissue accessibility in vivo, whereas palmitate is broadly accessible. In vivo nutrient-uptake measurements reveal that tissue architecture shapes nutrient accessibility, with spatial constraints in the spleen and exclusion of circulating FA, but not amino acids, from the thymus. Together, these findings identify nutrient accessibility as a distinct layer of metabolic regulation and reveal that immune-cell metabolism is shaped by spatial and systemic constraints on nutrient access HighlightsO_LINutrient accessibility is a distinct layer of metabolic regulation C_LIO_LIPhysiological FA uptake differs from ex vivo uptake capacity C_LIO_LISpatial and systemic factors govern fatty-acid accessibility C_LIO_LITissue context shapes immune-cell metabolism in vivo C_LI In briefUsing bioorthogonal FA to quantify physiological nutrient uptake in vivo, Wang et al. show that nutrient accessibility is distinct from nutrient uptake capacity. Tissue architecture and systemic FA distribution create spatial constraints on nutrient access, revealing an underappreciated layer of metabolic regulation in immune cells.