While cancer is typically associated with cachexia, characterized by unintentional weight loss and muscle wasting, certain types of cancer and their clinical management can lead to unexplained weight gain. This increase in body mass results from two distinct mechanisms. The first is profound metabolic shifts caused by hormone-producing tumors, and the second is the physical accumulation of mass or fluid within the body. Understanding these specific pathways is necessary to accurately identify the underlying cause of the weight change.
Endocrine Cancers and Metabolic Changes
Some malignancies are capable of producing hormones, which fundamentally alters the body’s metabolism and redirects how it stores energy. A primary example involves tumors that lead to an excess of the stress hormone cortisol, causing a condition known as Cushing’s syndrome. This can occur with adrenocortical carcinoma, a cancer of the adrenal gland, or with pituitary tumors that overproduce adrenocorticotropic hormone (ACTH).
The resulting high cortisol levels trigger a significant metabolic shift in the body. Cortisol stimulates appetite and promotes the storage of fat, particularly in the central areas of the body. This leads to a characteristic pattern of fat redistribution, including visceral obesity around the abdomen, a rounded face often called “moon face,” and a fatty deposit behind the neck known as a “buffalo hump.”
This hormonal imbalance also slows down the body’s overall metabolism, making it more difficult to burn calories. Adrenal cancers that secrete cortisol can sometimes cause very rapid weight gain, with some patients gaining 10 to 30 pounds over just a few weeks or months. This is a direct consequence of the tumor chemically manipulating the body’s fat and glucose regulation systems.
Physical Mechanisms: Tumor Bulk and Fluid Retention
Weight gain can also result from the physical presence of the cancer and its secondary effects on fluid balance, independent of metabolic changes. This increase in body mass is often due to the sheer size of the tumor itself or the accumulation of excess fluid. Ovarian, liver, and gastrointestinal cancers frequently cause this type of physical weight gain.
A major contributor is ascites, which is the abnormal buildup of fluid in the peritoneal cavity, the space within the abdomen. Cancer cells can irritate the lining of the peritoneum, causing it to secrete large amounts of fluid. The tumor mass can also block the lymphatic system or the veins that drain the liver, preventing normal fluid reabsorption.
The fluid accumulation from ascites can be substantial, leading to noticeable abdominal swelling, distension, and a rapid increase in weight and waist size. While the actual tumor mass contributes to the weight, the fluid buildup is often the most significant and sudden factor.
Weight Gain Induced by Cancer Treatments
Weight gain is frequently a side effect of necessary medical interventions, rather than a direct result of the cancer itself. Corticosteroids, such as prednisone and dexamethasone, are widely used in cancer care to manage inflammation, reduce side effects of chemotherapy, and sometimes treat the cancer directly. These steroid medications are synthetic versions of cortisol and mimic its effects in the body.
The use of corticosteroids can lead to weight gain through three primary mechanisms. They increase appetite, which naturally results in a higher calorie intake if not carefully managed. They also cause the kidneys to retain sodium and water, leading to noticeable fluid retention and puffiness, particularly in the face and extremities.
Finally, steroids contribute to the same central fat redistribution seen in Cushing’s syndrome, depositing fat around the trunk and face. Certain other treatments, like some forms of hormone therapy for breast or prostate cancer, can also contribute to weight gain. Chemotherapy that induces severe fatigue is another element, as it slows down a patient’s activity level and decreases their metabolic rate.