While most people associate a cancer diagnosis with weight loss, gaining weight is also a possibility for some individuals with colon cancer. This outcome can feel confusing, but there are specific reasons it can occur. The weight gain may be a direct result of the cancer’s effects on the body or a side effect of the treatments used to fight the disease.
Weight Gain as a Direct Symptom
Although less common than treatment-related causes, colon cancer can directly lead to an increase in body weight. One mechanism is the development of ascites, an abnormal buildup of fluid in the abdominal cavity. This condition is often associated with advanced cancers that have spread to the peritoneum, the tissue lining the abdomen. Cancer cells can irritate this lining, causing it to produce excess fluid.
This fluid accumulation can cause significant abdominal swelling, bloating, and a rapid increase in weight. The pressure from the fluid can also lead to other symptoms, such as shortness of breath, nausea, and a feeling of fullness after eating very little. In some instances, a large tumor within the colon can create a blockage, leading to severe constipation and bloating.
Treatment-Related Causes of Weight Gain
More frequently, weight gain during colon cancer is a consequence of the treatments. Therapies designed to eliminate cancer cells can affect the body’s metabolism and daily habits. These factors often work in combination, making it a common experience for many patients.
Steroid Medications
A contributor to weight gain is the use of corticosteroid medications, such as dexamethasone. These steroids are administered alongside chemotherapy to help manage side effects like nausea and reduce inflammation. While effective, they are also known to increase appetite, making it difficult for patients to manage their caloric intake.
Steroids can also alter how the body stores fat and manages fluid. They often cause the body to retain water, leading to puffiness and swelling in the hands, feet, and face—a condition sometimes described as a “moon-shaped” face. Furthermore, these medications can cause fat to be redistributed to areas like the abdomen, the back of the neck, and the upper back, changing body composition.
Changes in Diet and Appetite
The emotional and physical stress of a cancer diagnosis and treatment can impact eating habits. For some, anxiety and stress lead to “comfort eating,” where high-calorie foods are consumed more frequently. Nausea from chemotherapy can sometimes be relieved by snacking on simple carbohydrates, which can increase overall calorie consumption.
Food cravings can also develop as a response to treatment. Some chemotherapy drugs can alter taste and smell, making certain foods less appealing while increasing the desire for others, often those that are sweet or salty.
Reduced Physical Activity
Cancer-related fatigue is a side effect of both chemotherapy and radiation therapy. This is not ordinary tiredness, but a persistent lack of energy that is not relieved by rest. This fatigue often leads to a more sedentary lifestyle, as patients may not have the energy for their usual activities or exercise.
This decrease in physical activity lowers the number of calories the body burns each day. When caloric intake remains the same or increases due to steroid-induced appetite or comfort eating, the energy imbalance results in weight gain.
Managing Weight After Colon Cancer
Once active treatment is complete, addressing any weight gained is a focus for long-term health, as maintaining a healthy weight may reduce the risk of cancer recurrence. Research shows that being physically active after treatment is associated with a lower risk of colorectal cancer returning.
Lifestyle changes should be approached carefully and with professional guidance, so it is important to consult with your oncology team before starting a new diet or exercise program. They can provide personalized advice and may refer you to a registered dietitian or a physical therapist who specializes in working with cancer survivors.
The focus should be on creating gradual, sustainable habits rather than pursuing rapid weight loss. A dietitian can help develop a nutritional plan centered on whole foods like fruits, vegetables, and lean proteins. A physical therapist can design a gentle exercise program to help rebuild strength and stamina, counteracting treatment-related fatigue.