Background: Osteocalcin is considered the specific marker of osteogenesis. However recently more and more researcher’s attention is drawn to osteocalcin as a possible regulator of adipose tissue. Material and methods: 56 children aged from 3 to 12 months with clinical signs of vitamin D-deficient rickets were under the supervision. The main group was formed of 28 children with physical development that exceeded the age limit. The comparison group consisted of 28 children with the normal physical development. The control group was formed of 20 apparently healthy children of similar age. According to the aim of the research we determined serum concentrations of osteocalcin and lipids for all of them. Results: Statistically lower levels of osteocalcin were recorded in children of the main group (p < 0.05). There was a positive correlation between the concentration of serum osteocalcin with HDL (r = 0.86, p < 0.001) for children of the first year of life. A strong inverse correlation was found between the bone marker data and SC (r = -0.93, p < 0.001). A significant difference in the mean values of serum osteocalcin relative to the comparison group was observed in children with obesity and overweight. Conclusions: The results of our research suggest a causal relationship between obesity and impaired bone metabolism in children during the first year of life.