Larimichthys crocea Swim Bladder Polysaccharides Attenuate 5-Fluorouracil-Induced Intestinal Injury by Modulating the Gut–Metabolic Axis

5-Fluorouracil (5-FU) is a first-line chemotherapeutic agent for solid tumors, but its clinical application is severely limited by dose-dependent intestinal injury that impairs patient quality of life and compromises therapeutic efficacy. Natural polysaccharides, especially marine-derived ones, have become safe and multi-targeted gut-protective candidates due to their excellent biocompatibility and prebiotic-like activities. Larimichthys crocea swim bladder is a characteristic marine biological resource, and its polysaccharides (CIPs) have shown potential bioactivities, yet their protective mechanism against 5-FU-induced intestinal injury remains unclear. Our study explored the protective effects of Larimichthys crocea swim bladder polysaccharides (CIPs) against 5-FU-induced intestinal injury in mice. Following 14-day preventive administration, CIPs alleviated 5-FU-induced body weight loss, diarrhea, colonic shortening, and mucosal injury, and restored goblet cell function. Mechanistically, CIPs enhanced intestinal barrier integrity by upregulating ZO-1, Occludin, and MUC2, suppressed the MyD88/NF-κB pathway to balance inflammatory cytokines, and ameliorated oxidative stress by regulating MDA, GSH, SOD, and CAT. CIPs also restored gut microbial diversity and the Firmicutes/Bacteroidota ratio, and modulated retinol and arginine metabolism. In vitro, CIPs reduced inflammation and oxidative damage in Caco-2 cells and promoted M2 macrophage polarization. Thus, CIPs alleviate 5-FU-induced intestinal injury via multi-targeted regulation of the gut–metabolic axis, showing great potential as a dietary intervention and gut health support agent in food science and oncology nutrition, and boosting the high-value utilization of marine resources.