Valorization of Beetroot and Watermelon by-Products as Functional Ingredients Targeting Nitric Oxide Pathways: Implications for Cardiovascular Health

Abstract Purpose of the Review Food loss and waste represent major challenges for food systems, while fruit and vegetable by-products generated during processing are underused sources of bioactive compounds with potential applications in functional food development. The present review aims to examine the valorization of beetroot and watermelon by-products as functional ingredients associated with nitric oxide (NO) bioavailability, vascular function, and cardiovascular health. Given that impaired NO signaling is a hallmark of endothelial dysfunction and cardiometabolic disease, nutritional strategies capable of modulating NO-related pathways are of growing interest. Recent Findings Beetroot by-products, including peels and pomace, contain betalains, phenolic compounds, dietary fiber, and other constituents of potential functional relevance. However, nitrate content is rarely measured in these materials, and direct evidence that their consumption increases nitrate bioavailability or improves vascular function is currently lacking. Their proposed contribution to the nitrate–nitrite–NO pathway is therefore based mainly on evidence from conventional nitrate-rich beetroot products. In contrast, watermelon rind is a natural source of L-citrulline, which is bioavailable after ingestion and may support NO production through the citrulline–arginine–eNOS pathway. Human studies have demonstrated the bioavailability of L-citrulline derived from watermelon rind, while studies using a microencapsulated rind juice providing 4 g of L-citrulline have reported improvements in conduit artery endothelial function. Antioxidant compounds present in these by-products may also help limit oxidative degradation of NO, although this effect has not been directly demonstrated. Summary Beetroot and watermelon by-products are linked to distinct pathways of NO generation but are currently supported by different levels of evidence. Future studies should quantify nitrate in beetroot by-products and L-citrulline in watermelon rind, including in the corresponding final food products, and evaluate changes in circulating nitrate, nitrite, and amino acids together with vascular function in well-controlled clinical studies.