TL;DR
A large South Korean observational study found that obesity among adults ages 20 to 39 was associated with higher cancer incidence over the following decade, with risk increasing across BMI categories. Absolute rates remained low, and the study cannot show that obesity caused cancer or that weight loss reduces the risk.
A study of 6.3 million South Korean adults ages 20 to 39 found that higher BMI in young adulthood was associated with a greater likelihood of developing cancer over the following decade, with the association continuing as participants aged. The observational findings, published in Annals of Internal Medicine, show an association rather than proof that obesity causes cancer.
Researchers used national health insurance data for adults who had at least one national health examination between 2009 and 2012. They tracked cancer diagnoses through 2021, with a mean follow-up of 11.2 years. The study population had a mean age of 30.9, and 59.3% were men. Investigators accounted for factors including smoking status, alcohol intake, physical activity, income, residential area and existing health conditions.
The reported 10-year cumulative incidence of any cancer was 1.84% among people classified as normal weight, compared with 2.24% for class I obesity and 2.62% for class II obesity. Relative to normal-weight participants, the estimated cancer hazard rose with BMI: 1.10 for overweight, 1.22 for class I obesity and 1.43 for class II obesity. These are relative hazard estimates, not the percentage of people who developed cancer.
Associations were stronger among men than women across the reported BMI categories. The authors also found that abdominal obesity was associated with additional risk beyond BMI alone in some comparisons. Thyroid, endometrial and ovarian cancers had the highest 10-year rates in analyses by cancer type, followed by colorectal, kidney, liver and pancreatic cancers. Breast cancer rates were lower among participants with obesity than among those of normal weight; that finding does not establish a protective effect.
Risk Patterns Before Middle Age
The study addresses a gap in evidence: many earlier analyses of obesity and cancer focused on older people, while this research followed a large group whose weight was measured in their 20s and 30s. The findings suggest that differences in cancer incidence associated with BMI can be observed in young adulthood and remain relevant during later follow-up.
However, the absolute cancer rates were low in this age group, and the difference between normal weight and class II obesity was 0.78 percentage points in the reported 10-year cumulative incidence. The results do not show that an individual with obesity will develop cancer, nor do they establish that weight reduction would prevent cancer. The authors said further research is needed before conclusions can be drawn about weight-loss interventions or changes to screening practices.
How Researchers Tracked Risk
The analysis classified BMI using categories adapted for Asian populations and also considered waist circumference and abdominal obesity. Over the follow-up period, researchers documented 151,893 cancers among participants. Because the study was observational, its statistical adjustments can account for measured factors but cannot rule out all differences between groups or establish cause and effect.
Alpa Patel, a researcher at the American Cancer Society who was not involved, told MedPage Today that the study adds information about early-onset cancers and reflects a more recent population, with follow-up starting between 2009 and 2012. Patel also pointed to a limitation: the researchers recorded smoking status but not smoking intensity or duration, which can affect cancer risk. The report notes that smoking patterns across BMI groups may differ from those in older studies.
“Although the absolute risk differences were modest, the consistency of associations across multiple cancer types and the young age of the population underscore the potential importance of early-life weight management.”
— Kyungdo Han and colleagues, as quoted in the study report
Limits of the Evidence
The study shows an association, not that obesity caused the cancers recorded during follow-up. Although investigators adjusted for several health and social factors, unmeasured influences may remain. Smoking intensity and duration were not reported, a limitation Patel highlighted.
The findings also do not establish whether losing weight in early adulthood reduces later cancer risk, or whether young adults with obesity should receive different screening. The study was conducted in South Korea, and the extent to which its results apply to populations with different demographics, health systems and BMI patterns is not established by this analysis.
Research Needed on Prevention
The authors called for prospective studies to test whether intentional weight loss in early adulthood is associated with lower subsequent cancer risk and to identify whether any young adults might benefit from earlier screening. Until such evidence is available, this study alone does not support a change in cancer-screening guidance. Its main contribution is a large, long-term account of how BMI and cancer incidence were associated in a relatively young population.
Key Questions
Does the study prove obesity causes cancer in young adults?
No. It was an observational study and found an association between BMI and cancer incidence. It cannot establish that obesity caused the cancers.
How much did cancer incidence differ by weight category?
The reported 10-year cumulative incidence was 1.84% for normal-weight participants, 2.24% for class I obesity and 2.62% for class II obesity. These figures describe the study population, not an individual’s personal risk.
Which cancers had the highest reported rates?
Thyroid, endometrial and ovarian cancers had the highest 10-year rates in the cancer-type analyses, followed by colorectal, kidney, liver and pancreatic cancers.
Does the research show that weight loss prevents cancer?
No. The researchers said further studies are needed to determine whether intentional weight loss in early adulthood reduces later cancer risk.
Should screening guidance change based on these findings?
The study does not establish that screening recommendations should change. The authors called for research to determine whether earlier screening might be warranted in particular clinical settings.
Source: medpagetoday.com
