Association Between Body Mass Index (BMI) and Prognosis in Cancer Patients: Analysis of Baseline, Trajectories, and Mediating Effects

Obesity means having too much body fat, which is now a major health concern worldwide. A common way to measure obesity is the Body Mass Index (BMI), which is calculated from a person’s height and weight. BMI is often used in medical research as a rough measure of body fat. In the past 30 years, obesity rates have increased by more than a quarter in adults and nearly half in children. Obesity can cause serious health problems and often requires lifestyle changes and medication to manage.
Cancer is the second leading cause of death worldwide, with nearly 20 million new cases and 9.7 million deaths reported in 2022. Lung cancer is the most common and deadliest form, followed by cancers of the breast, bowel, prostate, liver, and stomach. The number of new cancer cases is expected to rise to 35 million by 2050.

Obesity has been linked to a higher risk of developing many cancers, such as non-small cell lung, breast, prostate, bowel, and stomach cancer. However, studies have found a surprising pattern known as the “obesity paradox.” In some cases—such as bladder cancer, non-small cell lung cancer, and advanced skin cancer—people with obesity appear to live longer than those with lower body weight. The reasons for this are unclear, and associations with tumor type and cancer therapy need to be further validated. It may be linked to research design problems, such as:
• Reverse causation – where weight loss is caused by illness rather than the other way around.
• Confounding factors – not accounting for other variables that may influence the results.
• Selection bias – when the study group is not representative of the wider population.
• Ignoring mediators – not accounting for factors that explain how or why BMI affects cancer outcomes.

Studies may only record BMI before treatment begins (baseline BMI). This may not show the full picture because weight can change during treatment and illness. We do not yet know if BMI directly affects cancer survival or if it influences how patients respond to treatment.

In this study, we will use both baseline BMI and repeated BMI measurements during treatment. This will allow us to track changes over time (BMI trajectories) and see how they relate to cancer outcomes. We will group patients with similar BMI patterns and study whether these patterns affect how long they live, how well treatments work, and whether they experience treatment side effects (adverse events). We will also explore whether side effects play a role in the link between BMI and cancer outcomes.

Our goal is to provide clearer evidence on the role of BMI in cancer treatment and survival. This could help doctors tailor treatment plans, improve patient care, and highlight the importance of good nutrition and healthy weight management during cancer treatment.