Whole wheat steaming bun matrix enhances folic acid bioavailability: protein drives systemic uptake, soluble fiber directs brain delivery.

The bioavailability of dietary folic acid (FA) is strongly influenced by the food matrix, yet the specific components within complex cereal-based products that influence FA absorption and tissue distribution remain poorly understood. This study employed a holistic-to-component deconstruction strategy to investigate how whole wheat steaming bun (WWSB), a widely consumed staple food, affects FA bioavailability and to identify the responsible macromolecular constituents. First, female C57BL/6 mice were fed WWSB with FA for 48 days in a dose-dependent manner. Compared with FA alone, the high-dose WWSB group (Hbun-FA) significantly increased plasma 5-methyltetrahydrofolate (5-MTHF) by 1.9-fold (P < 0.05), hepatic 5-MTHF by 48.6% (P < 0.05), and cerebral 5-MTHF by 61.9% (P < 0.05). To test the generalizability of these findings and to pinpoint the active matrix components, male Kunming mice were then administered FA together with isolated protein, starch, soluble dietary fiber (SDF), or insoluble dietary fiber (IDF) from WWSB. Protein was identified as the primary contributor to systemic bioavailability: the protein-supplemented group (P-FA) elevated hepatic 5-MTHF by >33.15% relative to FA alone. Remarkably, SDF showed a distinct brain-biased biodistribution of 5-MTHF, with its contribution to cerebral 5-MTHF increase being substantially greater than its effect on peripheral plasma levels. The whole-component reconstituted group (AC-FA) and intact WWSB group (B-FA) exhibited highly similar tissue distribution profiles of 5-MTHF on radar chart analysis, validating the deconstruction strategy. These findings indicate that the WWSB matrix enhances FA bioavailability predominantly through protein, while SDF selectively promotes brain folate accumulation. This work provides a theoretical and experimental basis for designing FA-fortified whole grain products with improved systemic bioavailability and enhanced cerebral accumulation.