How obesity increases cancer risk

Obesity increases cancer risk by causing organs such as the liver, kidneys, and pancreas to enlarge primarily through an increase in cell number (hyperplasia), not just cell size. This expansion raises the number of cells susceptible to mutations, thereby elevating cancer risk. Organ size may predict cancer risk more accurately than BMI, highlighting the importance of maintaining a healthy weight early in life.

The study, published in Cancer Research, reveals a major driving force behind how obesity increases cancer risk across multiple organs.

Image Credit: CDC

The findings emphasize the importance of maintaining a healthy weight from early childhood and propose a potentially more accurate way than body mass index (BMI) to predict the increase in cancer risk associated with obesity.
The study reveals that excess weight doesn't just affect metabolism or hormones—it can physically enlarge organs, creating more opportunities for cancer to take hold. Understanding that process matters because it helps explain how everyday health choices can shape cancer risk years or even decades down the line.

In other words, as a person gains weight, their organs also grow in size by accumulating more cells to meet the higher energy needs of a bigger body. Having more cells boosts the odds of more DNA errors as cells divide, increasing the likelihood of cancer.
To test this hypothesis, researchers conducted a two-pronged study.
First, the team evaluated 747 adults whose weight in relationship to their height spanned the complete BMI spectrum, from underweight (18.5 BMI) all the way to severely obese (40-plus BMI). Using CT scans, the researchers measured the size of each adult's liver, kidneys, and pancreas.

This study is the first to analyze the size of multiple organs in a large cohort of individuals across the full BMI spectrum.

The scientists discovered that the organs grew larger as body weight increased. For every 5-point increase in BMI, the liver grew by 12%, kidneys by 9%, and the pancreas by 7%.

Next, the research team counted the cells in samples of kidney tissue taken from autopsies and reanalyzed biopsy data from living patients. The lab showed that more than 60% of the kidneys' growth resulted from an increase in the number of cells in the organ, a process called hyperplasia. The rest was due to individual cells growing bigger, or hypertrophy.
The new finding corrects earlier theories that larger organ size in obese individuals resulted primarily from fatter cells. Rather, obesity mainly increases the number of cells at risk for copying errors, uncontrollable growth, and potential malignancy.

This increase in organ size has harmful consequences.
The more cells in an organ, the more mutations and the greater the risk of one cell going awry during division and becoming cancerous.
Overall, the study showed a strong link between organ size enlargement and cancer risk across all three organs, confirming the mathematical predictions. The finding provides evidence for this as a major mechanism of tumorigenesis induced by obesity, in addition to factors like inflammation and hormonal imbalances.

This newly discovered effect of obesity can be large—with organs even doubling in size.

"When an organ doubles in size, it is expected to roughly double its risk of developing cancer", say the researchers.
Noting the relationship between diet and cancer, the authors emphasized the importance of maintaining a healthy weight from a young age.

Sophie Pénisson et al, Hyperplasia Functions as a Link Between Obesity and Cancer, Cancer Research (2026). DOI: 10.1158/0008-5472.can-25-2487