Chemotherapy uses powerful drugs to target and destroy rapidly dividing cells, making it a standard treatment for many cancers. While designed to eliminate cancer cells, these drugs also affect healthy, fast-growing cells in the gut, hair follicles, and bone marrow. A common side effect is an unpredictable change in body weight. Chemotherapy’s impact is not uniform; it can cause significant weight loss in some patients and weight gain in others, due to various biological and physiological responses.
Why Chemotherapy Can Lead to Weight Loss
Weight loss during chemotherapy is linked to reduced caloric intake and altered metabolism driven by side effects. Gastrointestinal distress, including chemotherapy-induced nausea and vomiting (CINV), severely limits the ability to eat consistently. This often leads to anorexia, a sustained loss of appetite, even with anti-nausea medications.
Chemotherapy can damage the digestive tract, causing painful mucositis (sores in the mouth, throat, and esophagus). These sores make chewing and swallowing difficult, causing patients to avoid food or gravitate toward bland liquids. Furthermore, damage to the intestinal lining also reduces the body’s ability to absorb nutrients.
Chemotherapy frequently alters the senses of taste and smell (dysgeusia), making food taste metallic or unappealing. This reinforces reduced food intake. The underlying cancer can also trigger cancer cachexia, a metabolic wasting syndrome characterized by systemic inflammation and loss of muscle and fat mass.
Why Chemotherapy Can Lead to Weight Gain
Weight gain occurs in a substantial number of patients, largely due to medications prescribed alongside chemotherapy. Corticosteroids, such as dexamethasone, are frequently administered to manage allergic reactions and control nausea. These potent drugs promote weight gain by stimulating appetite and causing the body to retain salt and water.
The resulting higher caloric intake and fluid retention contribute directly to weight gain. Steroids also cause fat redistribution, often leading to increased storage around the abdomen, neck, and face. Some chemotherapy drugs can also cause the body to retain excess fluid, manifesting as edema.
Treatment-related fatigue is another contributor, as it drastically reduces physical activity and energy expenditure. Less mobile patients burn fewer calories, resulting in muscle mass loss and a lowered resting metabolic rate. Emotional stress can also lead to changes in eating behavior, resulting in higher intake of calorie-dense meals.
Patient-Specific Variables Influencing Weight Change
The direction and severity of weight change depend on patient and treatment factors. Cancer type and stage play a role; for example, gastrointestinal cancers often predispose patients to weight loss due to obstruction or malabsorption. Conversely, regimens for breast, prostate, and ovarian cancers frequently involve high-dose steroid or hormone therapies linked to weight gain.
The specific chemotherapy regimen is a key factor, as some drugs are highly emetogenic (nausea-inducing), while others require intensive steroid pre-medication. Patients starting with a lower baseline body weight or pre-existing malnutrition are more susceptible to severe weight loss. In contrast, younger patients closer to their ideal body weight may be at higher risk of treatment-related weight gain.
Metabolic status and age also influence the outcome, as a slower metabolism makes it easier to gain weight when activity is reduced. The cumulative effect of multiple treatments, including surgery or radiation, adds further complexity.
Strategies for Managing Chemotherapy-Related Weight Changes
Managing weight requires a proactive approach and close collaboration with the oncology care team and a registered dietitian. For patients experiencing weight loss, the focus is on maximizing calorie and protein intake to prevent muscle mass loss. This is achieved by consuming small, frequent meals throughout the day, which are better tolerated than three large ones.
Foods can be enriched with high-calorie ingredients like butter, cream, or healthy oils to increase caloric density without increasing portion size. Liquid nutritional supplements, such as meal replacement shakes, provide concentrated calories and protein when solid food is difficult to manage. The care team may also prescribe appetite-stimulating medications.
For managing weight gain, especially that caused by corticosteroids, dietary adjustments can mitigate fluid retention. Following a lower-sodium diet helps reduce water retention and lessen edema. Gentle, medically approved physical activity, such as short, regular walks, helps maintain muscle mass and combat fatigue. Open communication with the oncologist is important for adjusting steroid dosage.