Why one cancer type keeps spreading among young people while others decline

As a general rule, people living today are healthier and live longer than humans in previous generations, with dramatically lower rates of infectious disease and declining mortality from many chronic illnesses when differences in age are taken into account.

Cancer, however, does not fit neatly into that picture. While overall colorectal cancer rates have been falling in many countries, the Science Alert outlet reports on an unexpected trend that has emerged among younger adults: people under 50 are increasingly being diagnosed with the disease.

The phenomenon has puzzled researchers because many of the factors known to increase colorectal cancer risk are actually more common among older people.

Colorectal cancer, which includes cancers of both the colon and rectum, remains predominantly a disease of older adults. Yet while incidence has generally declined among older age groups, rates among younger people have been rising in several countries since the 1990s.

And doctors still do not have a convincing explanation for why.

The environmental puzzle

One possible explanation involves exposure to pesticides and herbicides.

Chemicals such as picloram have been investigated for possible links to colorectal cancer. But the theory faces an immediate problem: many pesticides and other agricultural chemicals are used less today than they were during the 1980s and 1990s.

If declining exposure is occurring at the same time that colorectal cancer is increasing among younger people, it becomes difficult to explain why the two trends would move in opposite directions. There is another problem.

Older generations have generally had greater cumulative exposure to many environmental chemicals. Regulations governing pesticides and other substances were less stringent in previous decades, meaning that today's older adults may have encountered substantially greater levels of some chemicals during their childhood and working lives.

That makes it difficult to explain why colorectal cancer would be declining among older people while increasing among younger ones if a particular environmental exposure were the primary cause.

The same problem applies to another group of chemicals that has attracted considerable attention: per- and polyfluoroalkyl substances, better known as PFAS or "forever chemicals".

PFAS exposure has been investigated in connection with a range of health problems, including possible links to colorectal cancer. But exposure is not neatly divided along generational lines, and in some circumstances older people may have experienced greater cumulative exposure than the younger populations now seeing rising cancer rates.

Researchers are therefore left with a difficult question: what environmental factor could have become more influential for today's younger adults while simultaneously becoming less important for older generations?

Obesity offers only part of the answer

Another obvious suspect is obesity.

There is strong evidence linking obesity to an increased risk of colorectal cancer, and younger generations are substantially more likely to be overweight or obese than previous generations were at the same age.

That makes obesity a plausible contributor to the rise in colorectal cancer among younger adults.

But it does not appear to explain the entire phenomenon.

Obesity has also increased among older people, meaning that the trend does not correspond neatly with the age-specific pattern in colorectal cancer. Modelling studies suggest that weight gain may account for no more than around 15% of the increase in colorectal cancer, and potentially less.

Obesity may therefore be part of the explanation, but it is unlikely to be the missing piece that solves the puzzle.

A global pattern — but not everywhere

The mystery becomes even more complicated when researchers look beyond individual countries.

The rise in colorectal cancer among younger adults is not universal.

A 2019 study found that rates among younger people were stable or declining in nearly half of the countries examined. Research published in 2025 reached similarly mixed conclusions.

That variation makes it particularly difficult to identify a single environmental or lifestyle factor.

If the same underlying cause were driving the increase, researchers would expect to see broadly similar patterns across countries with comparable levels of economic development.

Instead, the picture is fragmented.

Colorectal cancer rates among younger people have increased in countries such as the UK and US, while declining trends have been reported in countries including Italy and Lithuania.

That raises an important question: what is different about the populations experiencing increases, and what protects those where rates are falling?

Could testing be part of the answer?

One possibility is that at least some of the apparent increase reflects changes in how frequently people are tested and diagnosed.

Screening recommendations are unlikely to be the primary explanation. In several countries, formal recommendations to begin routine screening at younger ages were introduced only recently, while the increase in colorectal cancer among younger people has been evident since the 1990s.

But screening guidelines are not the only thing that has changed in healthcare.

Young people may now be more likely to undergo diagnostic testing because of changes in medical awareness, access to healthcare, physician behaviour and the willingness of patients to seek medical attention for symptoms.

If more younger adults undergo colonoscopies, scans or other diagnostic procedures, more cancers that might previously have gone undetected could be identified.

That would not necessarily mean that the underlying incidence of cancer has increased by the same amount as the number of diagnoses.

It is an important distinction because the statistics researchers are trying to explain are ultimately based on diagnosed cases.

A mystery without a simple answer

The rise of colorectal cancer among younger people therefore remains one of the more perplexing developments in modern cancer epidemiology.

Environmental chemicals offer possible explanations but struggle to account for the generational pattern. Obesity is a recognised risk factor but appears insufficient to explain the scale of the increase. And the fact that trends differ substantially between countries makes a single global cause even harder to identify.

Changes in testing and diagnosis could explain at least part of the increase, although they are unlikely to account for everything.

The most frustrating possibility for researchers is that there may be no single culprit.

The rise could instead reflect a combination of factors — changes in diet, obesity, gut health, environmental exposures, medication use, lifestyle and healthcare-seeking behaviour — interacting in ways that researchers have yet to fully understand.

For now, the central question remains unanswered: why, at a time when overall colorectal cancer rates are falling, are younger people in some parts of the world increasingly developing the disease?

Finding that answer could be crucial not only for understanding colorectal cancer, but also for identifying why some of the health gains of previous generations may not be carrying over to today's young adults.

By Nazrin Sadigova