Where You Carry Your Weight Matters More for Cancer Risk Than the Number on the Scale

When it comes to cancer risk, body weight has long been part of the conversation. But a significant new study suggests that the connection between excess weight and cancer may be considerably stronger than previous estimates have indicated, and that the way we have been measuring that risk may be a large part of why we have been underestimating it.

The research, published in Cancer Communications, used data from more than 458,000 participants in the U.K. Biobank followed for a median of nearly 12 years, found that the proportion of cancers attributable to excess weight could exceed 10% when researchers accounted for several factors that prior studies routinely overlooked. Previous estimates in the U.K. had put that figure at around 6.3%.

The Problem With BMI

Much of the discrepancy comes down to how we measure body fat. Most prior research on weight and cancer risk relied on body mass index, or BMI, a simple calculation based on height and weight. But BMI has well-documented limitations. It does not distinguish between fat mass and lean muscle mass, does not account for where fat is distributed in the body and does not reflect important variations by age, sex, race or ethnicity.

This study compared BMI against two alternative measures of body fat: waist circumference and waist-to-hip ratio. Both of these measures capture central adiposity, meaning fat concentrated around the abdomen, which is more closely tied to metabolic dysfunction and, as this research confirms, more closely tied to cancer risk. When investigators used waist circumference quartiles instead of standard BMI categories and accounted for other methodological factors, the estimated proportion of cancers attributable to excess weight more than doubled, rising from 5.5% to 11.5%.

The Hidden Factor: Prediagnostic Weight Loss

One of the more nuanced findings of the study involves what team calls reverse causation. Cancer can cause weight loss before a diagnosis is ever made, sometimes years before symptoms appear, through a process called cachexia. This means that in studies tracking body weight and cancer outcomes, some people who go on to develop cancer may already be losing weight at the time their measurements are taken, making the apparent relationship between higher weight and cancer risk look weaker than it actually is.

When investigators excluded the first several years of follow-up data to account for this effect, the association between excess weight and cancer risk grew stronger across all measures, particularly for waist circumference and waist-to-hip ratio.

Who Is Most at Risk

The study found that the cancer burden attributable to excess weight appeared higher in women than in men and higher in younger participants under 60 than in older ones. Cancer risk also increased steadily across the full range of body measurements, not just at the thresholds conventionally defined as overweight or obese, suggesting that conventional cutoffs may be leaving meaningful risk unaccounted for.

What This Means for Your Health

The team is clear that these findings point to an urgent need to address the ongoing obesity epidemic, which they describe as likely to drive a major rise in the global cancer burden in the years ahead. For patients, the practical takeaway is that excess body weight, and particularly excess abdominal fat, carries cancer risk that may be more significant than most people realize.

“Since the prevalence of obesity continues to rise in most countries and the population is aging at the same time, the number of cancer cases caused by obesity is likely to continue to rise significantly in the future,” explained co-investigator professor Hermann Brenner, of the DKFZ German Cancer Research Center. “Effective strategies for the prevention and treatment of obesity could therefore make a much greater contribution to cancer prevention than previously assumed.”