An observational study using data covering more than half of the US population suggests that incidence rates of cancers linked to obesity are increasing most rapidly in young adults. The findings, published in The Lancet Public Health journal on World Cancer Day, could indicate that an even greater disease burden is on the horizon.
The study suggests that the sharpest increase in obesity-related cancer incidence is in adults aged 25 to 49. This was true for half of the 12 cancers classified as related to obesity: colorectal, uterine corpus, gallbladder, kidney, pancreatic, and multiple myeloma (a type of bone marrow cancer). Patients were divided into five-year age groups from 25-29 to 80-84 years old. Each successively younger age group experienced a greater percentage increase. Of the 18 other types of cancers included in the study only two showed a similar trend while eight - mostly related to smoking, infection, or both - showed a decline, and the rest remained stable.
Despite these increases in risk, the authors note that the overall incidence of these cancers remains higher in older age groups than in younger age groups.
"Our findings expose a recent change that could serve as a warning of an increased burden of obesity-related cancers to come in older adults," says Dr Ahmedin Jemal from the American Cancer Society, USA. "Most cancers occur in older adults, which means that as the young people in our study age, the burden of obesity-related cancer cases and deaths are likely to increase even more. On the eve of World Cancer Day, it's timely to consider what can be done to avert the impending rise." 
Most epidemiological studies have focused on older populations so the effect on cancer risk of excess bodyweight in early life, or of weight gain in young adulthood, is not well understood. In some cancers, excess bodyweight during early adulthood could be a more important influence on cancer risk than weight gain in later life . This is the first study to examine contemporary incidence trends in young adults for a comprehensive list of cancers in the United States.
The researchers analysed incidence data from 25 state cancer registries, covering 67% of the US population. They considered 30 of the most common cancer types, including 12 obesity-related cancers  in people aged 25 to 84 years, diagnosed between 1995 and 2014. During this timeframe, there were 14,672,409 cases of the 30 types of cancer. 
Incidence of multiple myeloma and cancers of the colorectum, uterine corpus, gallbladder, kidney, pancreas, and thyroid increased in younger adults (25 to 49 years). Incidences also rose in older adults, except for colorectal cancer, but by a smaller percentage.  The younger the age group, the greater the size of the increase - in all seven of the cancer types except for thyroid cancer. For example, in pancreatic cancer the average annual change was equal or less than 1% in people aged 40 to 84, 1.3% in those aged 35 to 39, and 2.5% in 30 to 34-year-olds. In the youngest age group (of 25 to 29 years), it was 4.3%. Across the six cancer types, the annual increase ranged from 0.4% in uterine corpus cancer to 3.0% in kidney cancer amongst 45 to 49-year-olds, and from 1.4% for multiple myeloma up to 6.2% in kidney cancer in 25 to 29 year-olds (see Figure 3 for trends in age-specific incidence rates for the 12 obesity-related cancers between 1995-2014).
While the rates are increasing more quickly in younger adults, it is important to note that overall incidence is lower than in older adults where the majority of cancers are diagnosed. For example, around two pancreatic cancer cases were diagnosed in every 100,000 25-49 year-old persons per year between 2010-2014, compared with around 37 in every 100,000 50-84 year olds .
In contrast to the other obesity-related cancers, the risk of developing breast cancer generally remained unchanged in younger generations. This could reflect the fact that some types of breast cancer are on the rise while others in decline. The authors suggest that other factors could also be at play, such as changes in screening practice and women's reproductive factors including age at first full-term pregnancy and number of children.
Overall, the findings could reflect the obesity epidemic of the past 40 years. Obesity rates have more than doubled in the US between 1984 and 2014. Obesity is now one of the most preventable causes of cancer in both the US and the UK - around 1 in 12 cases in the US are caused by excess weight and more than 1 in 20 in the UK.
"Over the past few decades, death rates have been in decline for most cancers, but in the future obesity could reverse that progress, barring any interventions. Younger generations are experiencing earlier and longer-lasting exposure to excess fat and to obesity-related health conditions that can increase cancer risk." says Dr Jemal. 
Lead author Dr Hyuna Sung of the American Cancer Society, USA, says: "Obesity is associated with health conditions that can contribute to the risk of cancer. For example, diabetes, gallstones, inflammatory bowel disease, and poor diet can all increase the burden of cancer. The quality of the American diet also has worsened in recent decades. More than half of adults who were 20 to 49 years old between 2010 to 2012 reported poor dietary habits, such as eating little fruits, vegetables, whole grains, fish and shellfish at the same time as eating too much salt, fast food, and sugary drinks." 
The authors call for increased obesity screening. Fewer than half of primary care physicians regularly assess body mass index in their patients - despite national recommendations for screening. Only a third of people who are obese report receiving a diagnosis or weight loss counselling. The authors suggest that restrictions on advertising calorie-dense food and drinks, taxes on sugary drinks and urban planning that promotes physical activity could be effective strategies to stem the emerging trend.
The researchers note that they were not able to quantify the impact of new modes of detection - such as highly sensitive diagnostic imaging - on emerging cancer incidence trends in young generations. However, screening is less common in younger adults, while increases in cancer incidence were still greater. The study does not provide evidence of a causal relationship between obesity and cancer. Additional experimental and population-based studies will be needed to test the hypothesis more directly.
Peer reviewed / Observational study / People
NOTES TO EDITORS
This study was funded by the Intramural Research Department of the American Cancer Society and the Intramural Research Program of the National Cancer Institute. It was conducted by researchers from Surveillance and Health Services Research Program at the American Cancer Society and the National Cancer Institute at the National Institutes of Health.
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 12 cancer types were chosen on the basis of their association with obesity: colorectal, oesophageal (adenocarcinoma), gallbladder, gastric cardia, kidney, liver and intrahepatic bile duct, multiple myeloma, pancreatic and thyroid cancer and, in women, uterine corpus, breast and ovarian cancer.
 For incidence rates and frequencies for all cancer types examined in the study, please see Table S2 the appendix. For incidence rate ratios please see Tables S5 (12 obesity-related cancers) and S6 (18 additional cancers) in the appendix.