Obesity associated with higher early-onset cancer risk among young Korean adults

1. In this cohort study, obesity, measured by body mass index (BMI) and waist circumference, was associated with higher early-onset cancer risk among Korean adults aged 20 to 39.

2. Thyroid, endometrial, and ovarian cancer had the highest 10-year cumulative incidence but endometrial cancer had the strongest relative association with obesity.

Evidence Rating Level: 2 (Good)

Study Rundown: The global prevalence of obesity has increased sharply in recent decades, reaching around 15% in men and 21% in women in 2021. Around 10% of cancer incidence worldwide in 2021 could be attributed to overweight or obesity, making it 1 of the 3 biggest contributors to cancer risk, after smoking and infection. The increasing prevalence of obesity and incidence of early-onset cancer among young people poses new challenges for public health. This study aimed to assess dose-response relationships between obesity and overall cancer incidence among Korean adults aged 20 to 39 and evaluate how they may be affected by age, sex, and smoking status. Both class I and class II obesity were associated with higher absolute and relative cancer risk compared with normal weight. The relative association between overweight or obesity and overall cancer was more apparent in men, while the absolute risk difference was higher in women. Abdominal obesity was also more strongly linked to cancer risk across all BMI categories, with the absolute risk difference associated with abdominal obesity being larger at higher BMI levels. Relative associations between obesity and cancer were more apparent among people with a smoking history, whereas absolute risk differences were similar between ever-smokers and never-smokers. Cancer risk was progressively higher with greater obesity for obesity-related cancers with consistent definitions. The highest 10-year cumulative incidence and risk differences were seen for thyroid, endometrial, and ovarian cancer, whereas endometrial cancer had the highest relative risk in people with obesity. This study has a few limitations. Due to its retrospective nature, a causal relationship between obesity and early onset cancer cannot be established. Factors such as dietary patterns, physical activity, family medical history, consumption of alcohol could affect the observed associations and were not accounted for. Last but not least, since the population consisted of young Korean adults, the findings may not be widely generalizable to other ethnic populations. Nevertheless, this study suggests that obesity, especially abdominal obesity, is associated with increased cancer risk among young people, thus highlighting obesity as a potentially important factor in early-onset cancer risk.

Click to read this study in the Annals of Internal Medicine

Relevant Reading: Worldwide trends in underweight and obesity from 1990 to 2022: a pooled analysis of 3663 population-representative studies with 222 million children, adolescents, and adults

In-Depth [retrospective cohort]: This cohort study aimed to assess associations between obesity and early-onset cancer risk in young Korean adults. Data for 6,891,401 people aged 20 to 39 years who underwent a national health examination between January 1, 2009, and December 31, 2012, were initially included. People diagnosed with cancer prior to the index date (n = 23,070) and people with missing data on key variables (n = 554,993) were excluded. The primary outcome was development of newly diagnosed cancer, whereas secondary outcomes included the incidence of site-specific cancer. A final cohort of 6,313,338 Korean adults was included, with a mean age of 30.9 years and with men making up 59.3% of the study population. A total of 151,893 incident cancer cases (2.4%) were reported during a mean follow-up of 11.2 years. Both class I and class II obesity were associated with higher absolute and relative cancer risk compared with normal weight. Cumulative incidence increased with higher BMI, ranging from 1.84% (95% CI, 1.83% to 1.86%) in the normal-weight group to 2.24% (CI, 2.22% to 2.27%) in class I obesity and 2.62% (CI, 2.56% to 2.68%) in class II obesity. Cancer risk was consistently higher with abdominal obesity across all BMI categories. People with class I obesity based on BMI and with abdominal obesity had higher 10-year cumulative incidence (2.45% [CI, 2.41% to 2.50%]) than those without abdominal obesity (2.13% [CI, 2.10% to 2.16%]), exceeding that of those with class II obesity without abdominal obesity (2.20% [CI, 2.03% to 2.36%]). Cancer risk was progressively higher with higher adiposity for obesity-related cancers with consistent definitions, with 10-year cumulative incidence ranging from 1.15% (CI, 1.14% to 1.16%) in the normal-weight group to 1.57% (CI, 1.55% to 1.60%) in class I obesity and 1.95% (CI, 1.89% to 2.00%) in class II obesity. The individual cancers with the highest 10-year cumulative incidence and risk differences were thyroid, endometrial, and ovarian cancer. Overall, this study suggested that obesity may be associated with increased cancer risk among young Koreans.

Citation: Yun JS, Mun H, Jung JH, Park Y, Kim SY, Kim K, et al. Obesity and Cancer Risk in Young Adults in Korea : A Nationwide Health Screening Cohort Study. Ann Intern Med. 2026 Oct 26. doi: 10.7326/annals-25-02612. Epub ahead of print. PMID: 42932883.

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