Losing weight without trying or any particular reason is a major red flag regarding one's health, and it shouldn't be taken lightly. In case of stomach cancer, it may be due to a loss of appetite, low nutrient absorption in the body, or an increased metabolic rate due to the tumour. Sudden and unexplained weight loss is also a classic sign of many different types of cancers, and so, it shouldn't be taken lightly.
A 2019 study titled 'Predicting unintentional weight Loss in patients with gastrointestinal cancer' concluded that "Weight loss in patients with GI cancers has implications for survival. Patients with upper GI cancers experienced more weight loss and decreased survival rates compared with patients with lower GI cancers. Patients with a combination of upper GI cancer (oesophagogastric or pancreatic) and low baseline BMI had the fewest survival days and worst patient outcomes. Early intervention for weight management plays a critical role for improving the health outcomes and fatality rates of these patients."