## 2 Methodology  
_Search Strategy and Inclusion Criteria_  
An exploratory review was carried out to gather and evaluate the breadth of existing evidence related to the topic, aiming to map out the current state of the research. The methodology was guided by the PRISMA Extension for Scoping Reviews (PRISMA-ScR) guidelines [23]. The process involved clearly stating a research question, searching for and identifying relevant studies, carefully selecting appropriate sources, organizing and extracting key data, and ultimately summarizing and presenting the findings as outlined below [24].  
The core question that guided this review was  
* What are the health implications for humans of consuming milk with the A1 \(\beta\)-casein protein variant compared to consuming milk that contains only the A2 \(\beta\)-casein variant?  
Secondary questions:  
* How do \(\beta\)CM-7 and the microbiota interact in gut health and disease regulation?
* How does individual susceptibility to \(\beta\)CM-7 exposure influence the observed effects of A1 \(\beta\)-casein consumption?
* How does the consumption of A1 \(\beta\)-casein versus A2 influence digestive health in different populations?
* Is there an association between A1 \(\beta\)-casein consumption and an increased risk of inflammation or cardiovascular disease?
* How does A1 \(\beta\)-casein affect muscle performance and recovery in athletes?
* How does A1 \(\beta\)-casein influence immune function?
* What is the impact of A1 \(\beta\)-casein on neurological and cognitive health?In March 2025, an extensive search for the relevant literature was conducted using databases such as PubMed, Scopus, and the Web of Science Core Collection. There were no restrictions placed on the publication dates. Both the keywords and standardized terms related to five primary concepts were applied to the titles, abstracts, and keywords of the articles identified:  
* Types of \(\beta\)-casein (A1 \(\beta\)-casein, A2 \(\beta\)-casein, \(\beta\)-casein variants, milk proteins);
* \(\beta\)CM-7 (\(\beta\)CM-7, BCM-7, \(\beta\)-cascomorphin-7, \(\beta\)-cascomorphin, opioid peptides);
* Human studies (clinical trials, intervention studies, observational studies, cohort studies, case-control studies);
* Microbiota perturbations and \(\beta\)CM-7 (gut-brain axis, intestinal barrier, gut microbiome, gut microbiota);
* Human health outcomes (chronic diseases, gastrointestinal health, cardiovascular health, diabetes, mental health, autism, autoimmune diseases, muscle performance and recovery);
* Population (children, middle-aged adults, athletes).  
Original research papers investigating associations between the effects of A1 \(\beta\)-casein or \(\beta\)CM-7 compared to A2 \(\beta\)-casein on health-related outcomes were included. Only full-length articles and patents were selected.  
Various types of studies involving human participants were considered, including randomized controlled trials, prospective and cohort studies, as well as cross-sectional observational and epidemiological research. Review articles were not included in the analysis. Information was gathered from the complete texts of the selected articles, which were then thoroughly examined. Each study was first evaluated on its own, and then the results were compared and synthesized.  
The selection of studies and the extraction of relevant data were carried out independently by two reviewers. The findings were presented both in tabular format and through descriptive summaries.