Cancer and weight gain: why it happens and what to know

Weight gain can be an unexpected and frustrating part of the cancer experience, especially when patients feel like they’re doing “everything right.” For some, the gain is subtle; for others, it can be more significant, affecting both physical and emotional well-being. While much attention is given to cancer-related weight loss, it’s equally important to acknowledge the causes of weight gain across different types of cancer, how it affects patients, and what can be done about it.

This article explores how ovarian, breast, and thyroid cancers can contribute to weight gain and offers insight into why it happens - often as a side effect of the very treatments designed to help.

Ovarian cancer and weight gain

Why it happens

• Reduced physical activity: Fatigue and low energy are common during treatment, often leading to decreased movement and lower calorie expenditure.

• Side effects of treatment: Chemotherapy and hormone therapy may directly cause weight gain or lead to fluid retention (edema). Certain medications can also slow down metabolism, making weight management more difficult.

• Changes in eating habits: Some patients may eat more due to stress, anxiety, or cravings. Others find that eating helps with nausea, leading to increased calorie intake.

What to keep in mind

Weight gain in this context is not about lack of willpower. It's a response to a complex mix of physical and emotional changes. Movement, even light walking or stretching, when possible, may help balance energy levels. Nutritional guidance from a specialist can also be helpful in navigating appetite changes.

Breast cancer and weight gain

Why it happens

• Hormonal treatment effects: Anti-estrogen therapies like tamoxifen or aromatase inhibitors can lead to fat accumulation, especially around the abdomen and hips, and can reduce muscle mass - changing body composition and contributing to weight gain.

• Reduced physical activity: Treatment-related fatigue, pain, or recovery from surgery often leads to less movement, lowering metabolism.

• Changes in eating habits: Some patients eat more due to emotional stress, to cope with nausea, or as part of comfort-seeking behaviors.

• Metabolic changes: Chemotherapy may reduce metabolic efficiency, making it harder to burn calories even with normal activity levels.

What to keep in mind

Understanding that treatment-related weight gain is a common and biologically driven outcome can ease some of the guilt or frustration patients may feel. Supportive interventions - like strength-based physical therapy, dietary support, and fatigue management - can help restore balance over time.

Thyroid cancer and weight gain

Why it happens

• Hormone disruption: The thyroid gland plays a central role in metabolism. Treatments for thyroid cancer - including thyroidectomy (removal of the gland) and radioactive iodine - often suppress or eliminate thyroid function, slowing metabolism.

• Thyroid hormone replacement challenges: After treatment, patients typically take thyroid hormone replacement. If the dosage isn’t properly adjusted, it may not fully restore normal metabolic function, leading to weight gain.

What to keep in mind

Post-treatment weight changes are often linked to hormone regulation rather than lifestyle alone. Regular follow-ups with endocrinologists to monitor thyroid hormone levels are essential, and even small medication adjustments can make a significant difference in energy and weight management.

Final thoughts

Weight gain during or after cancer treatment can feel confusing, especially when you're trying to take care of your health. But these changes often reflect complex biological processes, not personal failure. Whether driven by hormones, fatigue, medication, or emotional stress, your body is doing its best to respond to a very difficult situation.

Support exists, from medical professionals to nutritionists to community groups, and with time, many patients find ways to regain balance and confidence in their bodies again.

Every body changes during cancer. What matters most is how we care for it, and for ourselves, in the process.