Does Weight Gain After a Breast Cancer Diagnosis Affect Survival?

When people think of cancer, many still picture patients who have lost weight, appetite, and strength. However, this picture has changed substantially, particularly among patients diagnosed at an early stage and with modern treatment protocols. Recent research shows that a substantial proportion of patients with cancer gain weight during treatment, potentially affecting treatment success and long-term survival.

For example according to one study following a breast cancer diagnosis, 67% of patients were overweight or obese within two years of diagnosis .

Reasons for this weight gain include hormonal treatments, particularly corticosteroid use, treatment-related increases in appetite, and reduced physical activity. Corticosteroids can mimic cortisol's effects in the body, increasing appetite and promoting fat storage.

But is weight gained after diagnosis merely a cosmetic issue, or does it also increase the risk of death from breast cancer? New findings presented at the European Society for Medical Oncology (ESMO) 2025 Breast Cancer Congressprovide striking answers.

Study Background: 6803 Women, 8.5 Years of Follow-Up

This prospective study, led by Dr. Sixten Harborg at Aarhus University, used data from the internationally recognized Nurses’ Health Study to examine changes in body weight before and after diagnosis. It included women with nonmetastaticbreast cancer, totaling 6803 women .

Participants were classified by their prediagnosis weight, and postdiagnosis weight changes were categorized as follows:

• Stable weight: A change between –2% and +2%

• Moderate weight loss: –2% to –5%

• Major weight loss: Beyond –5%

• Moderate weight gain: +2% to +5%

• Major weight gain: More than +5%

During follow-up, 1179 women died from breast cancer.

📊 Main Findings: Gaining More Than +5% Increases Risk

Women with major weight gain after diagnosis had a significantly higher risk of breast cancer death than those whose weight remained stable:

➤ Among women who gained excess weight after a breast cancer diagnosis, the overall risk of death was 26% higher .

➤ If a woman had a normal weight before diagnosis, the weight gained after treatment was associated with a 32% increase in risk.

➤ Women who were overweight before diagnosis (carrying excess weight but not obese) had a risk increase reaching 37%.

➤ However, in women who were already obese, additional weight gained after diagnosis did not significantly increase this risk.

📌 In other words: Weight gain most strongly affects women who are normal weight or overweight. Weight management during treatment is therefore an important factor in addressing the disease.

What About Weight Loss?

Women with major weight loss after diagnosis also appeared to have a somewhat higher risk of death, but this was not statistically significant:

• Major weight loss: A 14% higher risk, but not statistically significant (95% CI: 0.96–1.34)

Moderate weight changes, either loss or gain, had no significant effect.

Commentary: “Prevention Is More Effective Than Treatment”

The findings are an “important warning,” particularly for normal-weight or overweight women. He said the pattern probably reflects the already elevated risk among women who are obese at diagnosis.

Furthermore, a recent meta-analysis of 64 studiesfound associations between higher postdiagnosis body mass index (BMI) and:

• Increased overall risk of death

• Increased breast cancer-specific risk of death

• Increased risk of a second primary breast cancer .

Promising Interventions: The BWEL Study

What Can Be Done?

The BWEL (Breast Cancer Weight Loss) study offers an example. It included women who had completed treatment within 14 months of diagnosis: more than 3100 women with a body mass index ≥27 .

Through a telephone-based weight-loss program, participants achieved:

• Clinically meaningful, sustained weight loss over 12 months during the program.

• Improved insulin resistance and metabolic markers as well.

In addition, according to another large study on diet and cancer, even modest weight loss in women older than 50 reduced breast cancer risk .

Conclusion: Preventing Weight Gain May Matter More Than Losing Weight

Weight management after breast cancer is more than a cosmetic issue: it is a vital factor in the disease course. These new findings deliver a clear message:

👉 Major weight gain after diagnosis increases the risk of death, particularly in normal-weight or overweight women.

Patients and healthcare professionals should therefore maintain attention to nutrition, physical activity, and lifestyle after treatment.