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The identification of child development milestones, specifically cognitive abilities, requires parental/caregiver awareness and knowledge. With the existence of multiple cognitive abilities such as attention and focus, memory, language, psychomotor skills, logic, reasoning, decision-making, problem-solving, social/emotional, there are multiple indicators to measure children's cognitive abilities. Pediatricians are the most knowledgeable and trusted authority to routinely screen and monitor the development of healthy children. Therefore, it is crucial that parents understand their children's developmental milestones so if they encounter anomalies, they can immediately consult with the professionals. This review compiles recently published research and current knowledge to examine potential key indicators of cognitive development in infants and children that can be utilized by parents and communities in an easily understood manner.

Immune cells in the central nervous system (CNS) of the fetus are essential for normal neurodevelopment. Innate immunity-related molecules, including cytokines, toll-like receptors and complement family, are known to be expressed in the brain. Microglia, macrophage-like immune cells that reside in the brain and spinal cord, constitute 80% of all immune cells in the brain, making them the most abundant immune cell type. Acquired immunity-related molecules, such as major histocompatibility complex and antibody receptor, are also known to be expressed in the brain. In addition to this, research has demonstrated that they play important functions in the development of the brain. Neurodevelopmental diseases, including schizophrenia, autism spectrum disorders, autism-like obsessive-compulsive behaviours and social impairment, are characterized by a disruption of a wide variety of processes in the developing brain that depend on the normal function of microglia. Enteric infections and malnutrition in the first two years of life are linked to later cognitive impairment. Multiple studies have shown that bacterial and viral illnesses have direct or indirect impacts on cognitive performance in children. The immune system is in constant communication with the central nervous system and participates in the control of behaviour and a range of other essential neurological activities throughout the lifespan.

The interaction of micronutrients and macronutrients has been shown to have a significant impact on children's cognitive development. Furthermore, proper nutrition helps a child's immune system avoid infection and disease, which can impair nutrient absorption and lead to deficiency. Microbiotas in the gut play critical roles in body physiology, including nutrient absorption, infection resistance and immune system development. Furthermore, it is known that the gut microbiota influences immune cell maturation. Finally, these three factors are thought to influence neurogenesis and cognitive development. A virtual meeting was held with five invited experts to gain a better understanding of the relationship between nutritional factors, the immune system and cognitive development.

The concept of the gut (microbiota)-brain axis, which influences the development and function of the gastrointestinal, immune, neuroendocrine, and metabolic systems, is gaining popularity. Recent animal studies have demonstrated that the gut (microbiota)-brain axis also plays a role in establishing cognitive function. It is known that a disruption in the environment's microbiota balance can increase disease susceptibility in children. Historically, it has been hypothesized that neurodevelopmental disorders are the results of a disruption in children’s health. However, it is becoming clear that the gut microbiota and the central nervous system communicate in both directions, which could explain how microbiota affects cognitive function. Dietary factors also play important role in the central nervous system via the gut (microbiota)-brain axis, demonstrating the importance of nutrition in optimizing cognitive function. This narrative review of recently published studies and current knowledge aims to elucidate the relationship between the gut (microbiota)-brain axis and cognitive function, as well as the variables that may influence it.