The Latest on Obesity and Cancer: An Interview with a BCRF Expert

‘It’s a major driver of cancer risk that we see in clinic nearly every day,’ says BCRF investigator Dr. Neil Iyengar.

Dr. Neil Iyengar has been a Breast Cancer Research Foundation (BCRF) investigator for more than a decade. In that time, he has seen his areas of research — metabolic health and lifestyle medicine — undergo massive transformations thanks to the expansion of bariatric surgery and the advent of weight-loss medications like semaglutide (Ozempic®) and tirzepatide (Zepbound®).

For his latest study, Dr. Iyengar and his colleagues issued a review of the science behind obesity and cancer risk, explaining how excess weight drives cancer and exactly how much people should lose for protective benefits. Here, he shares important insights.

How do excess body weight and obesity fuel cancer?

Obesity isn’t just about weight — it’s about the biologic state that comes with it, which promotes cancer growth.

We used to think that body fat is an inert or passive tissue that simply accumulates with obesity; however, we have learned through decades of research that it’s actually a very biologically active organ that maintains energy balance in our bodies. Too much body fat can create an environment that not only disrupts this energy balance, but also helps cancer develop and grow.

Too much body fat increases inflammation, lowers immunity, raises hormone levels like estrogen, disrupts how the body processes insulin and other growth signals which can damage our DNA, and changes the type of bacteria in the gut, which can lower our defenses against cancer. Over time, those changes can make it easier for abnormal cells to survive, multiply, and form tumors. This matters because excess weight is now linked to at least 13 different cancers, including postmenopausal breast cancer, and those cancers together account for about 40 percent of all cancers diagnosed in the U.S.

You mention that 10 percent of all new cancers are caused by excess body weight/obesity. Is that a high number?

It really is. It places it among the most common preventable causes of cancer. For comparison, smoking remains the leading cause at about 1 in 5 cancers, but excess body weight is consistently ranked second, even ahead of other factors like alcohol.

Cancers linked to obesity represent a substantial share of what oncologists treat every day. This is not a rare or niche issue; it’s a major driver of cancer risk that we see in the clinic nearly every day.

Is there a preferred method for losing weight, or does it not matter as long as the weight is coming off? For example, does it matter if someone loses weight quickly with a GLP-1 or bariatric surgery vs. slowly via lifestyle modifications?

This is one of the most common questions we get, and the transparent answer is that we don’t yet know if one method is clearly better than another specifically for cancer prevention. What we do know is that larger, sustained weight loss often in the range of 15-25 percent with bariatric surgery has been associated with meaningful reductions in cancer risk for people with high body mass index in observational studies. Newer medications like GLP-1 agonists can also achieve 10–20 percent weight loss in many patients, making this a promising approach to cancer prevention, but long-term data are still emerging. What I would stress the most, though, is that lifestyle remains foundational. For example, physical activity alone has been associated with 20-30 percent lower breast cancer mortality in observational studies. No other interventions have demonstrated as much efficacy for lowering cancer risk or improving cancer outcomes. Overall, the focus should be less on how the weight is lost, and more on achieving long-lasting improvements in overall metabolic health and lifestyle.

You say that a weight loss of at least 10 percent is needed to lower cancer risk. If someone isn’t at that goal yet, is something better than nothing?

The 10 percent threshold likely reflects the amount of weight loss needed to meaningfully reverse some of the underlying biology, such as inflammation, hormone levels, insulin resistance, and other pathways that link obesity to cancer. We know that 5 percent weight loss can improve metabolic health markers like blood sugar and cardiovascular risk, but those improvements may not fully translate to changes in cancer-related biology. In contrast, weight loss of 10 percent or more is more consistently associated with reductions in inflammation and hormone levels that are relevant to cancer risk. That said, even 3-5 percent is not trivial — it’s a meaningful and important start. I often tell my patients: Don’t let the perfect be the enemy of the good. Every step in the right direction counts.

What’s your takeaway for patients?

My biggest takeaway is that this information should feel empowering, not overwhelming. We now understand that metabolic health, which is impacted by body weight, body fat levels, physical activity, and nutrition, is a central part of cancer prevention and survivorship. Even modest improvements in metabolic health can have real impact on overall health. Naming obesity as an important cancer risk factor isn’t about blame or shame. We now recognize that patients have more agency than ever before, whether through lifestyle changes, medications, or other approaches. The key message is that this is an area where patients can take an active role in their health, and where we as clinicians and researchers are continuing to learn how to better support the metabolic health of our patients.