The process of maintaining the quality of life for oncology patients, among other things, is reflected in the care for their nutritional health. Timely and comprehensive nutritional assessment in oncology patients before the start of treatment is crucial for ensuring adequate nutritional intervention during therapy. Recent studies show that between 20% and 40% of oncology patients are malnourished, while malnutrition can affect up to 75% of cancer patients, depending on the localization and stage of the malignancy and the chosen oncological therapy. Data suggest that approximately 20% of oncology patients do not die from the disease itself but from malnutrition and its consequences. Early intervention can help preserve muscle mass, prevent sarcopenia, and reduce nutritional risk, which significantly improves treatment outcomes. The aim of this study was to assess and monitor nutritional status and body composition and to identify patients for whom treatment of sarcopenia as a separate clinical entity is indicated, using the NRS 2002 tool and bioelectrical impedance analysis, before and after four cycles of chemotherapy. The study included 30 patients with newly diagnosed lung or gastrointestinal cancer, who were scheduled to undergo chemotherapy. This prospective observational study was conducted at the Oncology Department of the General Hospital Zadar from April 1, 2024, to August 31, 2024. The results showed stability in ideal body mass, but revealed a reduction in muscle mass and sarcopenic index, particularly in patients with gastrointestinal cancer. There was an increase in the number of patients with a high nutritional risk, especially among those with gastrointestinal cancers. Despite stable BMI, the preservation of fat tissue and loss of muscle mass indicate a risk of developing sarcopenic obesity. The results emphasize the importance of early nutritional intervention, even in patients with mild nutritional risk.