The article describes a clinical example of the development of sarcopenic obesity in a child with type IXa glycogen disease. The reasons for the development of sarcopenic obesity were, on the one hand, the lack of control over the child’s nutrition: excess consumption of easily digestible carbohydrates, fats and lack of protein in the actual diet and inactivity due to a child’s motor activity restriction in view of surgical interventions for Perthes’ disease, on the other. The article provides practical recommendations on the example of prescribing diet therapy to a child with a complex combination of type IX glycogenosis, sarcopenic obesity, Perthes disease, and eosinophilic esophagitis.