Cancer rates have been rising in adults under 50 across the developed world, and for years the trend has confounded researchers. A major new study has now produced what its authors call the strongest lead yet — rising obesity — while making clear that most of the increase remains a mystery.

The analysis, published in BMJ Oncology by scientists at the Institute of Cancer Research (ICR) and Imperial College London, examined national cancer registry data for England from 2001 to 2019. It identified 11 cancer types increasing in people aged 20 to 49: bowel, breast, thyroid, ovarian, kidney, liver, pancreatic, gallbladder, endometrial, oral, and multiple myeloma.

Nine of those cancers are also rising in older adults. Only bowel and ovarian cancer are increasing exclusively in the young — a distinction that makes them the hardest to explain.

A pattern that doesn’t fit the usual suspects

The researchers cross-referenced cancer trends against six behavioural risk factors identified by the International Agency for Research on Cancer: smoking, alcohol consumption, physical inactivity, body mass index, fibre intake, and red or processed meat consumption.

Five of the six have been stable or improving over the study period. Smoking rates have fallen. Alcohol consumption has held steady or declined. Diets have marginally improved.

The sole exception is BMI. Overweight and obesity levels have been climbing since the mid-1990s, and that trajectory aligns — at least partially — with the rise in cancer diagnoses.

Extra fat tissue is understood to alter hormone levels, particularly insulin, which functions as a growth factor linked to cancer development. Prof Marc Gunter of Imperial College London noted that a recent study from his team found insulin specifically may be playing a role in early-onset bowel cancer.

But obesity is far from a complete answer. In bowel cancer, the researchers estimate that rising BMI accounts for roughly 20 per cent of the additional cases. The other 80 per cent — and the majority of the increase across all 11 cancers — remains unexplained.

“Our main conclusion is that, although BMI is our best clue, much of the increases still remain unexplained,” Prof Montserrat García Closas of the ICR said.

Context matters

The numbers, while concerning, need perspective. In England, roughly one in 1,000 people aged 20 to 49 receives a cancer diagnosis each year, compared with roughly one in 100 among those aged 50 to 79. Of the roughly 31,000 cancers diagnosed in younger adults in 2023, breast cancer accounted for 8,500 cases, bowel cancer for 3,000.

A 3 per cent increase in bowel cancer cases in young people amounts to roughly 100 extra diagnoses annually. As Prof Amy Berrington of the ICR put it: “Although rates have been increasing, cancer in young people is still a rare disease.”

There are also early signs the trend may be moderating. Berrington said she has been examining more recent data, up to 2023, and “the trends that were increasing have begun to flatten a bit.”

The hunt for what’s missing

The study’s conclusion — that multiple factors are likely acting in combination — opens far more questions than it closes. Researchers are now investigating ultra-processed foods, so-called “forever chemicals” (PFAS), antibiotic use and its effects on gut bacteria, air pollution, inflammation, and even weedkillers as possible contributors.

Improvements in cancer detection may also be playing a role, with more tumours being caught at younger ages that would previously have gone undiagnosed until later in life.

One notable success sits outside the study’s scope: cervical cancer rates are plummeting among women who received the HPV vaccine in childhood, a reminder that population-level cancer prevention works when the cause is understood.

What individuals can do right now

Despite the unanswered questions, the researchers were emphatic on one point: lifestyle choices still matter. Nearly 40 per cent of cancers worldwide are considered preventable through measures like avoiding smoking, maintaining a healthy weight, staying physically active, and limiting alcohol.

García Closas stressed that while the findings are worrying, “there are things that can be done to reduce the risk of cancer by having a healthy lifestyle — for instance, being physically active and maintaining a healthy weight.”

Gunter also noted that if the new generation of GLP-1 weight-loss drugs, such as semaglutide, succeed in reducing obesity rates, “that should have an effect on some of the obesity-related cancers in the future.”

The study calls for large, long-term research to untangle the remaining causes. In the meantime, its authors argue the evidence on obesity is already strong enough to make reducing it a public health priority — particularly among children and young people.

That is a first clue, not a final answer. But after years of watching cancer rates climb without understanding why, it is the most significant lead scientists have found.

Sources