Eat small meals regularly. Low appetite is a common side effect of cancer treatments. Oftentimes, you may lack the appetite to finish a full meal all at once. You can try to combat this by eating smaller meals throughout the day. Opt to increase calories when possible. Whenever you have the chance, add more calories to a dish. There are many ways to add a few hundred more calories to an existing meal. Seek nutrient dense foods. When trying to gain weight with cancer, you need to do so in a healthy manner. Nutrient dense foods have a lot of nutrients. While they tend to be low calorie, you can mix nutrient-dense foods with other high calorie options to help you gain weight and get necessary calories.