Every individual harbours a complex, diverse and mutualistic microbial flora in their intestine and over the time it became an integral part of the body, affecting a plethora of activities of the host. Interaction between host and gut-microbiota affects several aspects of host physiology. Gut-microbiota affects host metabolism by fermenting unabsorbed/undigested carbohydrates in the large intestine. Not only the metabolic functions, any disturbances in the composition of the gut-microbiota during first 2–3 years of life may impact on the brain development and later affects cognition and behaviour. Thus, gut-dysbiosis causes certain serious pathological conditions in the host including metabolic disorders, inflammatory bowel disease and mood alterations, etc. Microbial-metabolites in recent times have emerged as key mediators and are responsible for microbiota induced beneficial effects on host. This review provides an overview of the mechanism of microbial-metabolite production, their respective physiological functions and the impact of gut-microbiome in health and diseases. Metabolites from dietary fibres, aromatic amino acids such as tryptophan, primary bile acids and others are the potential substances and link microbiota to host physiology. Many of these metabolites act as signalling molecules to a number of cells types and also help in the secretion of hormones. Moreover, interaction of microbiota derived metabolites with their host, immunity boosting mechanisms, protection against pathogens and modulation of metabolism is also highlighted here. Understanding all these functional attributes of metabolites produced from gut-microbiota may lead to the opening of a new avenue for preventing and developing potent therapies against several diseases.

Vitamin D has long been recognized for its role in calcium homeostasis and bone metabolism; however, it is now emerging as an important regulator of central nervous system (CNS) function. Recent evidence suggests that vitamin D signaling contributes to the pathogenesis and progression of several neurodegenerative disorders. Vitamin D exerts neuroprotective effects through multiple mechanisms, including regulation of calcium homeostasis, modulation of immune responses, reduction in oxidative stress, stimulation of neurotrophic factors, and maintenance of blood–brain barrier (BBB) integrity. Vitamin D receptors and metabolizing enzymes are widely distributed across several brain regions, highlighting their direct involvement in neuronal function. This review summarizes the biosynthesis, metabolism, and signaling pathways of vitamin D. It explores its role in neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), stroke, and traumatic brain injury (TBI). Evidence from experimental and clinical studies indicates that vitamin D deficiency is associated with an increased risk and severity of these conditions, while supplementation may provide therapeutic benefits.