TL;DR
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A large observational study in South Korea linked obesity at ages 20 to 39 with higher rates of cancer over the following decade, with the association persisting into middle age. Absolute cancer rates remained low, and the findings do not show that obesity directly caused the cancers or that weight loss would prevent them.
A study of 6.3 million South Korean adults found that obesity between ages 20 and 39 was associated with a higher likelihood of developing cancer over the next decade, and the association persisted into middle age. The observational findings, published in the Annals of Internal Medicine, show a pattern of increasing risk with higher body mass index (BMI), but do not establish that obesity caused the cancers.
Researchers used national health insurance data to identify adults who were 20 to 39 years old and had at least one national health examination between 2009 and 2012. Their mean age was 30.9, and 59.3% were men. During an average 11.2 years of follow-up, the study recorded 151,893 cancers.
The 10-year cumulative incidence of any cancer was 1.84% among people in the normal-weight group, compared with 2.24% among those with class I obesity and 2.62% among those with class II obesity, using BMI categories based on criteria for Asian populations. Compared with normal-weight participants, the hazard of cancer was 10% higher for people classified as overweight, 22% higher for class I obesity, and 43% higher for class II obesity.
The authors also examined waist circumference and abdominal obesity. Among people with class I obesity, the 10-year cancer incidence was 2.45% for those with abdominal obesity, compared with 2.13% for those without it. Across BMI categories, the association was stronger among men than women. Rates were highest for thyroid, endometrial and ovarian cancers; the analysis also reported associations for colorectal, kidney, liver and pancreatic cancers. Breast cancer rates were lower among participants with obesity than among normal-weight participants in this study.
Cancer Risk Across Young Adulthood
The results add evidence on a population often less represented in research on obesity and cancer: young adults. The study authors said the association across multiple cancer types and the age of the participants point to the possible value of addressing weight earlier in life. The findings may also inform discussion of whether cancer prevention and screening research should give more attention to younger adults with obesity.
The absolute differences were modest, and the overall cancer incidence was low in this age group. The study does not show that an individual with obesity will develop cancer, nor does it establish that losing weight will reduce a person’s cancer risk. Its results describe population-level associations, not a prediction or diagnosis for any one person.
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How Researchers Tracked Cancer
Kyungdo Han of Soongsil University in Seoul and colleagues analyzed BMI alongside waist circumference, grouping participants under World Health Organization criteria adapted for Asian populations. They adjusted their analysis for several factors, including smoking status, alcohol intake, physical activity, income, residential area and existing health conditions. The data covered health examinations from 2009 to 2012, with cancer outcomes tracked through 2021.
Alpa Patel, a researcher at the American Cancer Society who was not involved in the study, told MedPage Today that the results add information about early-onset cancers and reflect a more contemporary study population than many earlier investigations. She also pointed to a limitation: the analysis recorded whether participants smoked, but not how much or for how long. Those details can affect cancer risk and complicate interpretation of the association.
The report notes that more than a dozen cancers have been associated with overweight and obesity in previous research. It also describes a broader rise in obesity prevalence and studies reporting increases in some cancers diagnosed at younger ages. The new study focuses specifically on the relationship in people who were young adults at the start of follow-up.
“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 reported in the study
Limits of the Risk Estimates
Because this was an observational study, it can identify associations but cannot determine whether obesity itself caused the cancers. Adjustments for measured factors reduce some potential confounding, but they cannot rule out the influence of unmeasured factors. Patel specifically noted that smoking intensity and duration were not reported.
The study also does not establish whether intentional weight loss in early adulthood lowers later cancer risk, or whether additional screening would benefit young adults with obesity. Its results come from a South Korean population and should not automatically be treated as identical to risk estimates in other countries. The reported cancer rates are averages for study groups, not individual forecasts.
Research on Prevention and Screening
Han and colleagues called for prospective studies to test whether intentional weight loss in early adulthood is followed by lower cancer risk. They also said future work should identify whether there are clinical settings in which earlier cancer screening for young adults with obesity might be warranted. The current report does not announce a screening recommendation or a change to clinical guidance.
Until those questions are studied, the findings provide evidence of an association rather than proof of a prevention strategy. Further research will need to clarify how weight, abdominal fat, smoking and other factors relate to specific cancers over time, and whether the results hold in populations beyond South Korea.
Key Questions
Does the study prove that obesity causes cancer?
No. It was an observational study, which found an association between weight categories and cancer incidence but cannot establish cause and effect.
How large was the difference in 10-year cancer incidence?
The reported incidence was 1.84% for normal-weight participants, 2.24% for those with class I obesity and 2.62% for those with class II obesity. These are group-level estimates from the study.
Which cancer types had the highest rates?
Rates were highest for thyroid, endometrial and ovarian cancers. The researchers also reported associations involving colorectal, kidney, liver and pancreatic cancers.
Does the research show that losing weight prevents cancer?
No. The study did not test whether weight loss reduces cancer risk. Its authors called for future research to examine that question.
Could the results change screening recommendations?
The study does not recommend earlier screening. Its authors said more research is needed to determine whether any clinical settings warrant earlier cancer screening for young adults with obesity.
Source: medpagetoday.com
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