Biological factors play a crucial role in determining the effectiveness of dietary and nutrition strategies or interventions. These factors include age, sex, genetics, metabolic rate, hormonal balance, and the presence of chronic diseases, all of which influence how nutrients are absorbed, metabolized, and utilized in the body. For instance, genetic predispositions can affect individual responses to specific nutrients, such as lactose intolerance or gluten sensitivity, while hormonal fluctuations, such as those seen in thyroid disorders or diabetes, can alter metabolic demands and nutrient requirements. This chapter provides an in-depth analysis of how each of these biological dimensions plays a significant role in determining nutritional needs and responses, underlining the importance of adopting personalized approaches in nutritional planning. Age and sex also contribute to variations in nutritional needs, with children, pregnant women, and older adults requiring tailored dietary approaches. Moreover, chronic conditions like cardiovascular diseases, renal disorders, and gastrointestinal diseases often necessitate specific nutrient modifications to manage symptoms and prevent complications. Understanding these biological factors is essential for designing personalized nutrition interventions that enhance adherence, improve health outcomes, and support disease prevention. Integrating tools such as biomarkers, omics technologies, and wearable devices can further enhance precision nutrition by identifying individual needs in real time. Overall, a biologically informed approach to nutrition ensures that dietary recommendations are not only evidence-based but also context-specific, thereby maximizing their effectiveness in promoting long-term health and well-being.