A landmark study has revealed that more than half of cancer patients beginning treatment in England have a history of obesity, indicating that its role in the deadly disease is far more significant than previously believed. Prior estimates suggested obesity fueled four in ten cancer cases, but new research from experts at the University of Oxford highlights how relying solely on a single weight measurement at the start of treatment can profoundly underestimate a patient's lifetime exposure to obesity, which can critically impact their chance of survival.

The comprehensive study, spanning over a decade and published in the journal ESMO Real World Data and Digital Oncology, found that when a patient's past obesity was considered alongside their current health, its prevalence exceeded 50% across all 13 types of cancer examined. In stark contrast, when only a single Body Mass Index (BMI) measurement at treatment initiation was used, just 25% of patients were classified as clinically obese. For example, while only 14% of pancreatic cancer patients were obese at the start of treatment, a staggering 56% had been obese at some point in their lifetime, demonstrating that current weight alone does not provide a complete health picture.

This research, led by Professor Simon Lord, an expert in cancer treatment, analyzed past BMI data from the digital health records of 79,271 patients undergoing systemic treatments, such as chemotherapy. The findings underscore the critical need to consider patients’ history of obesity in broader clinical decision-making and in understanding cancer outcomes, especially for precision medicine. Dr. Helen Crocker from the World Cancer Research Fund noted the opportunity to improve clinical insights, while Dr. Victoria Perletta of the university's oncology department emphasized that understanding a patient's obesity history could build a fuller health picture, potentially informing personalized care, including chemotherapy dosing.

The study also revealed variations in obesity rates across different cancer types. Lower obesity rates at first treatment were observed for cancers often associated with unexplained weight loss and lack of appetite, such as pancreatic, gastroesophageal, bowel, lung cancer, and non-Hodgkin lymphoma. Conversely, obesity was more common at treatment start for uterine cancer, breast cancer, and malignant melanoma. Further analysis indicated that older patients (75 and older) had lower obesity rates, while individuals residing in more deprived areas were more prone to obesity. Scientists suggest that obesity may escalate cancer risks through biological mechanisms like chronic inflammation, metabolic alterations, and hormonal changes. Additionally, reduced participation in and efficacy of cancer screening programs among obese individuals, particularly in deprived areas, may contribute to these patterns.

Obesity has been definitively linked to at least 13 types of cancer by the International Agency for Research on Cancer (IARC) in 2016, including breast, bowel, womb, kidney, pancreatic, oesophageal, gallbladder, liver, upper stomach, myeloma, meningioma, and thyroid cancer. In 2024, Lund University, Sweden, identified a further 19 types, adding gastric tumors, cancers of the small intestine, pituitary glands, and certain heart and neck, vulval, and penis cancers to the list. It is crucial to note that being overweight or obese does not guarantee cancer development, but it significantly increases the risk. Obesity is recognized as the second biggest cause of cancer in the UK, accounting for over one in 20 cases, with Cancer Research UK affirming that maintaining a healthy weight reduces the risk of 13 specific cancers.

Looking ahead, the experts stressed that the increasing use of weight loss medications, such as GLP-1s like Wegovy and Mounjaro, will likely alter obesity patterns in cancer patients. This development makes longitudinal tracking of weight and BMI even more vital for understanding the complex relationship between obesity and cancer. The research team highlighted a future challenge in comprehending how expanded GLP-1 use might reduce obesity in patients receiving systemic anticancer therapy, and whether these drugs positively impact cancer outcomes when used in conjunction with systemic therapy.

Health charities have welcomed these timely findings, especially given the minimal past exposure of the population to weight loss interventions. Dr. Crocker underscored that this research addresses crucial questions where data on the role of BMI in cancer outcomes had previously been lacking, providing a clearer rationale for integrating both current and past BMI into clinical decision-making.