Is weight gain a common side effect of stomach cancer treatment?

Weight change during stomach (gastric) cancer treatment varies some people gain weight, while others lose weight. This can happen because treatments and their side effects often change activity levels, appetite, and food choices. Fatigue can lower daily movement and calorie burn, while eating more starches or sugary drinks to soothe nausea can add extra calories, leading to weight gain. ^{[1] [2]} In contrast, stomach cancer therapies more commonly cause side effects like nausea, diarrhea, and taste changes that may lead to weight loss, but individual responses differ. ^{[1] [3]}

How treatments impact weight

• Chemotherapy: While many people experience nausea, diarrhea, and fatigue, which often lead to weight loss, reduced activity and comfort eating can contribute to weight gain in some cases. ^{[1] [2]}
• Targeted therapy: Side effects such as nausea, vomiting, diarrhea, and mouth sores may affect intake, but overall weight effects vary by person and regimen. ^[3]
• Immunotherapy: Common effects include rash, fatigue, diarrhea, and joint pain; these are more likely to affect appetite and activity rather than directly cause weight gain. ^{[4] [5]}

Why weight gain can happen

• Fatigue lowers activity, so you burn fewer calories. ^[1]
• Eating more carbohydrates to calm nausea can increase total calories. ^[6]
• Drinking juices and sugary beverages adds hidden calories. ^[6]

Is weight gain harmful?

A modest, intentional return toward a healthy weight after unintentional weight loss may be reasonable, but rapid or ongoing weight gain can increase fatigue, joint pain, and metabolic risks. It’s best to aim for steady, healthy weight maintenance during treatment and adjust based on your care team’s guidance. ^{[7] [8]}

Practical strategies to manage or prevent weight gain

• Balance your plate: Emphasize vegetables, lean proteins, whole grains, and limit added sugars and ultra-processed snacks. ^[7]
• Watch liquid calories: Choose water, unsweetened tea, or broth; keep juices and sugary drinks to small portions, if any. ^[6]
• Plan for nausea: Instead of only crackers or refined carbs, try small, frequent snacks with protein (e.g., yogurt, eggs, tofu, nut butter) to steady appetite and blood sugar. ^[7]
• Move gently every day: Short walks or light resistance exercises can counter fatigue and help maintain muscle. ^[9]
• Track portions and patterns: A simple food and symptom journal helps identify triggers for overeating or inactivity, so you can adjust. ^[7]
• Get tailored support: Ask for a referral to a registered dietitian experienced in oncology to personalize calories, protein, and symptom management. ^[8]

When to contact your care team

• Rapid weight gain (for example, more than 1–2 kg in a week) or new swelling could signal fluid retention and should be checked. ^[4]
• New or worsening fatigue, diarrhea, or appetite loss may require treatment adjustments or supportive medicines. ^{[1] [4]}

Bottom line

Weight gain can happen during stomach cancer treatment, often due to fatigue and changes in eating patterns, though many people experience weight loss from side effects. A balanced diet, mindful beverage choices, gentle daily activity, and guidance from an oncology dietitian can help you maintain a healthy weight during treatment. ^{[1] [2] [3] [7] [8]}