Obesity, Weight Loss, and Cancer: What We Know So Far

Obesity is a chronic disease arising from complex interactions between genetics, neurobiology, eating behaviors, and the environment. It is characterized by an unhealthy amount or distribution of body fat and defined by body mass index (BMI), a measure of how a person’s weight relates to their height. 

The World Health Organization has reported that more than 1 billion people are living with obesity, and its prevalence is rising across the globe. 

People with obesity are at a higher risk of developing many diseases, including diabetes, high blood pressure, cardiovascular disease, and stroke. Having obesity is also associated with a greater risk of developing at least 13 types of cancer, with the risk of cancer increasing the longer an individual is overweight and with the amount of excess weight they have.  

Several mechanisms have been proposed to link obesity with increased cancer risk, including higher blood insulin levels, chronic inflammation, impaired tumor immune responses, and increased estrogen and adipokines produced by fat cells.

Research suggests that obesity may also worsen cancer recurrence, progression, prognosis, and the risk of secondary primary cancers. While observational studies have found that cancer risk is lowered among people with obesity who have lost weight, more research is needed to confirm this and understand whether weight loss improves cancer outcomes. 

Dr. Jennifer Ligibel, a professor at Harvard Medical School and senior physician in the Breast Oncology Center at the Dana-Farber Cancer Institute, researches the impact of physical activity and body weight on cancer risk and outcomes.  

Ahead of her talk at The American Association of Cancer Research Annual Meeting 2026 (AACR 2026), Technology Networks spoke with Ligibel to learn more about the relationship between obesity and cancer. 

Katie Brighton (KB): What do you see as the biggest misconceptions—among clinicians or the public—about the relationship between obesity and cancer? 

Dr. Jennifer Ligibel (JL): I think there is still a lack of awareness that obesity is linked to an increased risk of many cancers, including common cancers like breast and colon cancer. Although the International Agency for Research on Cancer published a definitive report ten years ago in the New England Journal of Medicine, there is still a lack of awareness that obesity (as well as other lifestyle factors, such as lack of exercise) is linked to increased cancer risk. 

KB: Can you outline what we currently know about the biological pathways linking obesity to increased cancer risk and poorer outcomes? 

JL: There is still a lot that we do not know about the biological pathways linking obesity to cancer risk. We do know that obesity impacts the body broadly, increasing inflammation, weakening the immune system, leading to metabolic dysregulation and insulin resistance. Obesity also has a direct effect on many tissues where cancers form, like the adipose tissue in the breast and surrounding the colon, as well as effects on the microbiome. Cancer likely arises due to the interplay of the effects of obesity on these overlapping pathways, creating a systemic and local environment favourable to cancer formation and progression. 

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KB: What have your studies revealed about the ways physical activity and body weight influence cancer risk, treatment outcomes, and quality of life for people living with or beyond cancer? 

JL: Studies show that obesity and inactivity increase an individual’s risk of developing more than a dozen cancers. Observational evidence also shows that these factors are linked to poor outcomes in some cancers, especially breast, colon, and prostate cancers. Data from the bariatric surgery literature show that patients with obesity who undergo bariatric surgery are at much lower risk of cancer as compared with matched patients (by age, gender, BMI, and number of other medical problems) who do not undergo surgery.  

Our ongoing work through the Breast Cancer Weight Loss (BWEL) trial will test whether a structured, lifestyle-based weight loss program can reduce the risk of cancer recurrence and mortality in 3180 women with breast cancer and overweight or obesity, to determine whether obesity is a modifiable risk factor for breast cancer recurrence and mortality. 

KB: As new metabolic therapies (including GLP‑1s) become more widely used, how do you see them fitting into the broader landscape of obesity management in oncology? 

JL: GLP-1s lead to significant and sustained weight loss. These drugs have made weight loss more achievable for a much larger segment of the population with obesity. Although data are very early, a number of observational studies have started to show a decreased risk of cancer in patients treated with GLP-1s for diabetes or obesity, as has been seen with bariatric surgery. Much more evidence is needed, but these data offer the tantalizing possibility that use of these drugs could be an important part of a cancer prevention strategy for patients with obesity. 

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KB: Looking ahead, what are the most urgent research questions that need answering to better understand—and potentially intervene on—the obesity–cancer connection? 

JL: I think the most important question is whether losing weight will reduce an individual’s risk of developing cancer and, for individuals who have already developed cancer, will weight loss after diagnosis reduce the risk of cancer recurrence and improve survival?  

If the answer to those questions is yes, how much weight do you need to lose, and does it matter how the weight is lost (through surgery, medication, or lifestyle)? Does a “healthy lifestyle” matter, or is weight loss the key to reduced cancer risk and improved survival? 

KB: Is there anything you can tell us about your talk at AACR 2026? 

JL: I will be speaking about the connection between obesity and cancer and what we know so far about the impact of weight loss, achieved through lifestyle and surgery, on cancer risk. I will also share preliminary findings from our Phase 3 weight loss trial in breast cancer survivors, showing the impact of the weight loss program on body weight and metabolic biomarkers.