Due to the roles of oxidative stress, inflammation, and neurotransmitter imbalances in cognitive and mental dysfunction, we aimed to develop a functional drink with antioxidant and anti-inflammatory properties as well as the potential to support neurotransmitter balance for improved cognition and mental health. The Teng Mo, Fen Hong Mee, and Hong Chon Su guava varieties were screened for their polyphenol and flavonoid contents, antioxidant and anti-inflammatory effects, and suppressive effects on acetylcholinesterase (AChE), monoamine oxidase (MAO), GABA transaminase (GABA-T), and glutamate decarboxylase (GAD). Juice from the cultivar with the highest potential was selected and mixed with mint and honey syrups, pomelo-derived dietary fiber, ascorbic acid, agar, water, and fruit puree (pear/apple/orange) to create three guava jelly drink formulations. The formulation with pear puree showed the highest biological potential and was selected as the final product. It is rich in vitamin C, gallic acid, and dietary fiber, and provides approximately 37 Kcal/100 g. It also promotes the growth of lactic acid-producing bacteria in the culture. Thus, our drink shows the potential to reduce oxidative stress and inflammation, improve neurotransmitter regulation, and stimulate the gut–brain axis, thereby promoting cognition and mental wellness. However, clinical research is essential to confirm these potential benefits.

Given the lack of clinical data supporting the effects of a novel guava jelly drink on cognition and mood regulation, we aimed to explore these effects and their possible mechanisms in working-age volunteers. In an 8-week, three-arm double-blind, placebo-controlled trial, healthy males and females aged 20–40 years old (N = 25/arm) were randomly assigned to consume 86 g per day of either a placebo or guava jelly drink containing either a low (36.6%) or high (73.2%) dose of guava juice with mint syrup. N100 and P300 brain waves, working memory, Perceived Stress Scale (PSS), Hospital Anxiety and Depression Scale (HADS), and biomarkers related to oxidative stress, inflammation, neurotransmitters, and gut microbiota were assessed at baseline and after 4 and 8 weeks of consumption. The low-dose group showed improvements in N100 amplitude, P300 latency, and HADS anxiety score, whereas the high-dose group exhibited improved N100 amplitude, working memory, PSS score, and total HADS score. In addition, the high-dose group also exhibited increased GPx activity without a reduction in MDA. Overall, these results suggest that guava jelly drink positively modulates perceived stress, anxiety, selective attention, and working memory. However, the precise underlying mechanism requires further study.