In the presence of light, a multivitamin preparation is the main source of peroxides in solutions of total parenteral nutrition (TPN). This preparation contains two photosensitive products, 5′-phosphate flavin mononucleotide(FMN) and polysorbates (PS), as well as electron donors such as ascorbate(AH). We hypothesized that the admixture of FMN or PS with electron donors generates peroxides in TPN and alters the quality of nutrients. Using xylenol orange, peroxide concentrations were measured in solutions containing AH, FMN, and/or PS in water, a dextrose solution, an amino acid preparation, and a lipid emulsion. Thiol functions were evaluated by reduction of 5,5-dithiobis(2-nitrobenzoic acid) in the amino acid preparation. After 24-h light exposure, dextrose solutions with admixtures of AH + FMN or AH + FMN + PS generated peroxides at concentrations similar to those observed in a 1% multivitamin solution, and over three times higher than those observed with FMN, PS, or AH alone. However, in the presence of amino acids, FMN alone induced a generation of peroxides comparable to that observed with FMN + AH. In the lipid emulsion, peroxides increased over 3-fold in the presence of FMN or FMN + AH. The addition of catalase suggested that lipid peroxides and H2O2were produced, and the loss of thiol function suggested that an oxidation of amino acids occurred. When exposed to light, FMN induces reactions with amino acids, polyunsaturated fatty acids, and even AH, altering the quality of nutrients. Paradoxically, AH without FMN has a protective effect on peroxide generation in TPN.Abbreviations: AH,ascorbate;A&OV0254;,radical ascorbate;A,dehydroascorbate;Rf,riboflavin;FMN,5′-phosphate flavin mononucleotide;PS,polysorbates;TPN,total parenteral nutrition;TBH,tert-butylhydroperoxide;IL,Intralipid 20%