Yet here the story turns. Apart from excess weight, most established risk factors either remained stable or improved over the previous one to two decades. Red meat consumption showed some of the largest gains: average daily intake among younger men fell from 38 grams in 2008 to 17 grams in 2018, and from 22 grams to 10 grams among younger women. Fiber intake, with more than 90% of younger adults falling short in 2018, was nonetheless stable or slightly improving between 2009 and 2019.

The numbers on attributable risk sharpen the paradox. In 2019, established behavioral factors accounted for an estimated 68% of mouth cancer cases in younger men, 49% of bowel cancer cases in the same group, and 42% of liver cancer cases. The shares were broadly similar in older adults. These factors matter enormously, but their improving trajectory sits alongside worsening cancer rates, and that gap is what the study cannot yet fill.

"These patterns suggest that while similar risk factors across ages are likely, some cancers may have age-specific exposures, susceptibilities, or differences in screening and detection practices," the researchers write.

What could be driving the unexplained rise?

The study was observational, meaning it can identify associations but cannot prove that any particular factor caused the increases. The authors also note real constraints: reliable long-term national data were missing for some risk factors, the analysis covered England rather than the whole United Kingdom, and the calculations assumed a 10-year delay between exposure to a risk factor and the appearance of cancer.

Candidate explanations remain on the table. The researchers point to reproductive history, early-life or prenatal exposures, and changes in diagnostic and screening practices as factors they did not evaluate. Other researchers have proposed greater consumption of ultra-processed foods and sugary drinks, childhood obesity, sedentary behavior, antibiotic exposure, and air pollution. But most of these suspects, the authors note, have shown stable or declining trends over the last decade, weakening the case that any single one is responsible.

One avenue deserves closer scrutiny: disruption of the gut microbiome, the community of microorganisms living in the digestive system. Changes in these microbial populations have been suggested as a possible influence on cancer risk, although further research is needed to establish their role.

"Although overweight and obesity are linked to 10 of the 11 cancers evaluated and account for a substantial proportion of cancer cases, both BMI-attributable and BMI-non-attributable incidence rates have increased, though the latter more slowly, suggesting other contributors," they add.

How worried should younger adults be?

The researchers urge perspective. Cancer remains overwhelmingly a disease of older age, and the absolute number of cases in adults over 50 far exceeds those in younger groups.

"Although increases in cancer in younger adults are concerning, the absolute burden remains far higher in older adults, underscoring the public health and clinical importance of studying risk factors across all ages," they conclude.

The study's deeper implication spans a lifetime: exposures during childhood, or even before birth, may shape cancer risk decades later. Until those threads are traced, the rise in early-onset cancer remains an open question, one measured in registry data and, for now, answered only in part.