Introduction::
The increase in the number of children affected by obesity raises serious concerns, especially in terms of long-term health consequences. The purpose of this study was to assess the nutritional status and metabolic complications, depending on the degree of obesity.

Material and methods::
This retrospective observational study analyzed the medical records of patients from the Interclinical Childhood Obesity Treatment Center at the University Children’s Hospital in Kraków at the beginning of the obesity treatment. The study enrolled 129 obese patients, aged 6–18 years. The study group was divided depending on the level of obesity into either a severely obese group (SO) or a moderately obese group (MO). Severe obesity was defined as a body mass index (BMI) > 35 kg/m² (children aged 6–14 years) and a BMI > 40 kg/m² (older than 14). Based on these criteria, 60 children were SO and 69 were MO.

Results::
The mean BMI Z-score in the study group was 3.5 ±0.8. Severe patients had a significantly higher % of body fat (48.3 vs. 33.2, p < 0.001). In the study group, 67% of patients were diagnosed with vitamin D₃ deficiency, 7% had vitamin B12 deficiency, 3% had anemia, and 16% had iron deficiency based on their total iron binding capacity (TIBC) level. The subgroups differed significantly in their TIBC and vitamin B12 level. Both moderate and severe obesity showed a positive correlation with vitamin D deficiency and TIBC, with the association being more pronounced in individuals with severe obesity.

Conclusions::
Obesity is a form of malnutrition. Poor diet quality causes nutrient deficiencies, which, among other factors, affect the development of metabolic complications. The micronutrient and vitamin deficiency underscores the need for targeted nutritional interventions that prioritize not only the reduction of calorie intake but also the replenishment of essential vitamins and minerals to improve metabolic health and mitigate the adverse effects of obesity.