Vitamin A as an essential nutrient: advances in biosynthesis.

Vitamin A and its derivatives (retinoids) are essential for vision, immunity, and cellular differentiation. Microbial biosynthesis offers a sustainable alternative to chemical synthesis for producing retinoids and their precursors. This review summarizes the recent advances in metabolic engineering of microbial cell factories for enhanced production of β‑carotene, retinol, retinal, retinoic acid, and retinyl esters. Key strategies, including strengthening precursor supply, cofactor and transporter engineering, and the identification and engineering of key enzymes such as β-carotene 15,15'-oxygenase, retinol dehydrogenase, retinal dehydrogenase, and retinol acyltransferase, were summarized. Additionally, the cultivation optimization approach addressing the oxidative instability of vitamin A is introduced. Future strategies for advancing metabolic engineering toward industrial-scale production are also discussed.