Fresh and Fermented Beetroot: From Microbial Biotransformation to Nitrate–Nitrite–NO Signalling, Redox Homeostasis and Microbiome Interactions

Beetroot (Beta vulgaris L.) is a rich source of dietary nitrates, polyphenols, and other bioactive compounds that have potential cardiovascular, metabolic, and health-promoting effects. While previous reviews have largely focused on the nitrate-derived nitric oxide (NO) signalling pathway or the antioxidant properties of the pigments known as ‘betalains’, there is an increasing body of evidence that suggests microbial transformation and fermentation can substantially modify the biological properties of beetroot. This narrative review summarises the latest research on beetroot phytochemistry, nitrate–nitrite–NO metabolism, microbial biotransformation, fermentation and host physiological responses. Particular attention is given to the impact of fermentation on phenolic compounds, betalains, and the metabolism of nitrates and nitrites, as well as to its effects on antioxidant activity, bioaccessibility, and interactions with the gut microbiome. The available evidence suggests that fermentation can alter the beetroot matrix, increasing the accessibility or recovery of certain bioactive compounds while producing additional microbial metabolites with potential biological activity. These changes may complement the well-established effects of beetroot-derived nitrates and betalains on vascular function, redox homeostasis, inflammation and exercise-related physiology. However, evidence specifically linking fermented beetroot to clinical outcomes is limited, and the effects of fermentation depend heavily on microbial strains, processing conditions, the food matrix and host characteristics. Interindividual differences in microbiome composition may further contribute to variability in responses to beetroot-derived compounds. Thus, fermentation is a promising strategy for increasing the functional potential of beetroot by altering the bioaccessibility of its bioactive constituents through microbial biotransformation. Further well-designed human studies are needed to establish whether these compositional and mechanistic changes result in clinically significant benefits, and to clarify how individual microbiome profiles contribute to responses to fermented beetroot.