Background/Objectives: Intestinal diseases are a major global health problem due to their high prevalence, chronic course, and significant impact on quality of life. Increasing attention has been given to the extra-skeletal effects of vitamin D, particularly its role in immune regulation, maintenance of intestinal barrier integrity, and modulation of the gut microbiota. The aim of this review was to evaluate current evidence regarding the role of vitamin D in immune regulation, intestinal barrier function, and the pathogenic mechanisms of intestinal diseases, including inflammatory and functional gastrointestinal disorders. Methods: A structured narrative review was conducted using the PubMed, Scopus, and Web of Science databases to identify publications addressing the role of vitamin D in intestinal diseases. The literature search was updated in May 2026 and included studies published between 2004 and 2026. The aim of this review was to summarize current evidence on the effects of vitamin D on immune regulation, intestinal barrier integrity, gut microbiota, and their clinical relevance in intestinal diseases. Results: The included studies demonstrated that vitamin D plays an important role in maintaining intestinal homeostasis through regulation of innate and adaptive immune responses, preservation of epithelial barrier integrity, and modulation of gut microbiota composition. Vitamin D deficiency was associated with impaired VDR-dependent signalling, increased intestinal permeability, dysbiosis, and enhanced pro-inflammatory immune responses, which may contribute to the development and progression of inflammatory bowel disease, Crohn’s disease, ulcerative colitis, irritable bowel syndrome, and celiac disease. Conclusions: Current evidence supports the important role of vitamin D in immune regulation, intestinal barrier protection, and maintenance of gut microbial balance. Further clinical studies are required to better understand its therapeutic potential in intestinal diseases.

Acute intestinal infections remain one of the leading causes of morbidity among children under five years of age and represent an important public health challenge in both developed and developing countries. Although these infections often share similar clinical manifestations, their etiological structure, severity, diagnostic accessibility, treatment approaches, and prevention strategies differ considerably depending on socioeconomic conditions, sanitation, healthcare infrastructure, and vaccination coverage. Aim: This review aims to summarize current data on the epidemiology, etiology, clinical presentation, diagnosis, treatment, and prevention of acute intestinal infections in children aged 1–5 years, as well as to provide a comparative analysis of approaches in countries with different income levels. Methods: A review of scientific literature focused on acute intestinal infections in young children was conducted. Source selection included full-text assessment. The extracted data were summarized descriptively according to pathogen groups: viral, bacterial, and protozoal infections. A PRISMA flow diagram was used to present the source selection process. Results: The reviewed literature indicates that viral pathogens, particularly rotavirus and norovirus, are among the leading causes of acute gastroenteritis in children under five years of age. According to Global Burden of Disease 2021 estimates, approximately 392.8 million diarrhoeal episodes and 0.34 million deaths occurred among children under five years of age globally in 2021, with mortality disproportionately concentrated in sub-Saharan Africa and South Asia. Bacterial infections, including Shigella and diarrheagenic Escherichia coli, are less frequent but are more often associated with invasive disease, severe diarrhea, and systemic complications. Protozoal infections, including Giardia lamblia, may contribute to prolonged diarrhea, especially in children with malnutrition or weakened immune status. Substantial regional differences were identified in pathogen distribution, antimicrobial resistance, diagnostic capacity, access to healthcare, and vaccination coverage. In developing countries, the disease burden remains strongly associated with limited access to safe water, sanitation, timely diagnosis, and appropriate medical care, whereas in developed countries acute intestinal infections more commonly contribute to outbreaks, emergency department visits, and hospitalizations. Conclusions: Acute intestinal infections in children aged 1–5 years remain an important global health burden with substantial regional disparities. Effective prevention and control require context-specific strategies combining high vaccination coverage, improved water, sanitation and hygiene, timely pathogen-specific diagnosis, rational antimicrobial use, and strengthened surveillance, with particular priority given to high-burden and resource-limited settings.