Background: The impact of overweight and adipocyte size on the development of type 2 diabetes mellitus (T2DM) even remains unclear. Aim: To study: 1) the relationship between the state of adipocytes and/or overweight/obesity, the development of T2DM and its clinical and laboratory signs; 2) the effect of weight loss on glycemic level, hyperinsulinemia (HI), insulin resistance (IR), and T2DM status. Methods. Design: a systematic review. Settings: Web of Science, EBSCO, Scopus/ Science-Direct, Google Scholar, PubMed, Cochrane, и Wolter Kluwer were searched for articles published for 26 years (2000-2026). The study bases on a systematic review of 3853 articles published worldwide. Results. In total, 142 full-text articles were assessed for eligibility. With an increase in overweight, the size of adipose tissue increases, adipocytes increase, the cell radius increase. All it leads to an overload of intracellular transport and internal organs. Increase in cell size triggers intracellular mechanisms to limit further nutrient supply. When cell size increases excessively, conformational changes in cellular receptors are activated, resulting in the development of IR. The increase in cell size and the maximum accumulation of overweight, as parallel processes, lead to hyperglycemia and HI with gradual development of IR and T2DM. Any type of i ntentional weight loss leads to a decrease in IR, HI and the disappearance of T2DM. Targeted weight loss in patients with T2DM improves metabolic and cardiovascular health, reduces blood pressure and blood sugar, reduces doctor visits, normalization of HbA1c, HI, IR. Conclusions. IR is a protective reaction of the cells, preventing its oversaturation and overflow. Overweight is an independent risk factor for the development of T2DM and its clinical and laboratory manifestations. Targeted weight loss leads to disappearance of symptoms of HI, IR and T2DM.