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A large South Korean observational study found that adults who had obesity at ages 20 to 39 had higher rates of cancer over the following decade, with the association persisting into middle age. Absolute cancer rates were low, and the study cannot show that obesity 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 cancer over the following decade, with the association continuing into middle age. The observational findings, published in Annals of Internal Medicine, also linked abdominal fat with additional risk, while researchers stressed that absolute cancer rates in this young population were low.
Researchers used national health insurance data to identify adults who received at least one national health examination between 2009 and 2012. Participants were followed for an average of 11.2 years; 151,893 cancers were recorded. The study accounted for factors including smoking status, alcohol intake, physical activity, income, residential area and existing health conditions.
The 10-year cumulative incidence of any cancer was 1.84% among people of normal weight, compared with 2.24% for class I obesity and 2.62% for class II obesity under the Asian-specific BMI categories used in the study. Relative to normal-weight participants, cancer hazards rose with BMI: the hazard ratio was 1.10 for overweight, 1.22 for class I obesity and 1.43 for class II obesity.
The researchers reported stronger associations among men than women. Abdominal obesity was also associated with higher incidence within BMI groups: among people with class I obesity, the 10-year rate was 2.45% with abdominal obesity and 2.13% without it. Rates were highest for thyroid, endometrial and ovarian cancers, followed by colorectal, kidney, liver and pancreatic cancers. Breast cancer rates were lower among participants with obesity than among those of normal weight, a finding that does not establish a protective effect.
How Weight and Cancer Rates Compared
The findings add evidence about a population often less represented in research on obesity and cancer: young adults. Cancer prevention programs have generally focused on middle-aged and older people, while the study suggests that weight-related associations can be observed from early adulthood and remain relevant later.
The absolute difference between the normal-weight and class II obesity groups was 0.78 percentage points over 10 years. That is a population-level difference, not an estimate of an individual’s personal outcome. The study does not establish that obesity caused any cancer, nor that losing weight would reduce a person’s risk. Its results can inform future research and discussions about prevention, but they do not by themselves establish a reason to screen young adults with obesity earlier or more often.
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A Large Study of Young Adults
Lead author Kyungdo Han, PhD, of Soongsil University in Seoul, and colleagues defined young adulthood as ages 20 to 39. The analysis included 6.3 million people with examinations in 2009–2012 and used BMI groupings based on World Health Organization criteria for Asian populations. Participants’ average age was 30.9, and 59.3% were men.
Alpa Patel, PhD, of the American Cancer Society, who was not involved in the research, told MedPage Today that the study adds information about early-onset cancers and uses a more contemporary population than many earlier studies. She also noted that the higher-BMI groups were more likely to currently smoke in this study, unlike some historical patterns. The authors and Patel have discussed findings from an observational analysis, not a trial of weight loss or cancer prevention.
More than a dozen cancers have been associated with overweight and obesity in prior research, according to the source report. Yet many earlier studies contained limited information about young adults. The South Korean analysis is large and lengthy, but its results come from one country and may not apply in the same way to populations with different health systems, demographics or obesity patterns.
“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, PhD, and colleagues
What the Study Cannot Establish
Because this was an observational study, it shows an association rather than proof that excess weight caused the cancers. Adjustments for measured factors cannot rule out the influence of unmeasured or imperfectly recorded factors. Patel specifically pointed to the absence of information on smoking intensity and duration, even though smoking status was included.
The study also does not show whether intentional weight loss in early adulthood changes later cancer risk. Nor does it establish which individuals might benefit from earlier screening. The source report gives no basis for applying the study’s absolute rates directly to a person’s individual risk, and findings from South Korea may not transfer directly to other populations.
Research Needed on Weight Loss and Screening
Han and colleagues called for prospective studies to test whether intentional weight loss in early adulthood is followed by lower cancer risk and to identify whether any clinical settings might warrant earlier screening for young adults with obesity. Those questions remain open; the study itself does not recommend a screening change.
Further work could also clarify how abdominal fat, BMI, smoking exposure and other factors relate to particular cancer types over time. Until such evidence is available, the reported results are best understood as a large-scale association that can guide research, not as proof of cause or a personal prediction.
Key Questions
What did the study find?
In a South Korean cohort of 6.3 million adults, higher BMI in people aged 20 to 39 was associated with higher cancer hazards during follow-up. The association was stronger among men, and abdominal obesity was associated with additional risk in some comparisons.
How large was the difference in cancer rates?
The study reported 10-year cumulative incidence of any cancer of 1.84% for normal-weight participants, 2.24% for class I obesity and 2.62% for class II obesity. These are group-level rates, not individual predictions.
Does the study prove obesity causes cancer?
No. It was an observational study, which can identify associations but cannot establish that obesity caused the cancers. Other factors may contribute, including smoking exposure not fully captured in the analysis.
Does this mean young adults with obesity need earlier cancer screening?
The study does not establish that earlier screening is beneficial or recommend a change to screening guidance. The authors said further research is needed to identify whether particular clinical settings might warrant earlier screening.
Did the researchers test whether weight loss lowers cancer risk?
No. The study examined weight categories and later cancer incidence; it did not test a weight-loss intervention. The authors called for prospective research to determine whether intentional weight loss in early adulthood changes subsequent risk.
Source: medpagetoday.com
