Flavin mononucleotide (FMN), also known as riboflavin-5'-phosphate, is a biomolecule derived from riboflavin (vitamin B2). It is an essential cofactor for various oxidoreductase enzymes involved in redox reactions within the cell. FMN’s role in supporting antioxidant defense is primarily indirect, as it is integral to the function of enzymes like glutathione reductase, which is crucial for maintaining cellular glutathione in its reduced, active state. Glutathione is one of the body's primary antioxidants, directly neutralizing reactive oxygen species (ROS) and repairing free radical damage.

Several clinical and preclinical studies have demonstrated that riboflavin deficiency impairs antioxidant defenses and increases susceptibility to oxidative stress. Supplementation with riboflavin (and by extension, FMN) restores normal enzyme activity and reduces oxidative damage in such contexts. However, most research focuses on riboflavin rather than FMN specifically, and there are limited direct studies on FMN supplementation in humans for free radical damage. The mechanistic rationale is strong due to its essential role in antioxidant enzymes, but direct clinical evidence for FMN supplementation as a treatment for free radical damage is moderate.

Therefore, the use of FMN for supporting the body’s defense against free radical damage is scientifically plausible and supported by some evidence, but more direct clinical trials are needed to establish its efficacy.