For decades, colorectal cancer was treated as a disease of older age. Screening programmes were built around that assumption, and so was clinical instinct. A 34 year old with rectal bleeding was reassured and sent home.
That assumption no longer holds. Incidence in adults under 50 has been rising steadily for roughly three decades, in country after country, while incidence in older adults has fallen or stayed flat. Understanding why matters less, for most people, than understanding what to do about it.
What the data shows
An analysis of cancer registries across 50 countries found that early onset colorectal cancer, diagnosed between the ages of 25 and 49, is rising in 27 of them. In 14 of those countries, rates rose in young adults at the same time as they remained stable or fell in adults aged 50 to 74.
In Europe specifically, an analysis covering 20 countries and nearly 188,000 cases found the rise was steepest in the youngest group:
- Ages 20 to 29: an average increase of 7.9% per year
- Ages 30 to 39: an average increase of 4.9% per year
- Ages 40 to 49: an average increase of 1.6% per year
The pattern is not random. Work by the International Agency for Research on Cancer, modelling data from Australia, Canada, England and the United States, found that people born in 1990 carry at least four times the risk of those born in 1960, and in English men the difference reaches eightfold. This is what epidemiologists call a birth cohort effect, and it points to exposures that began early in life.
No population level data exists for the trend in Greece specifically.
What is driving it
No single cause explains the rise. The evidence points to several factors acting together.
A pooled analysis of more than a million participants in three large European cohorts found the following associations with early onset disease:
- Body mass index, in men, for each 5 kg/m² increase: 33% higher risk, rising to 55% for colon cancer specifically
- Alcohol, per 30 g per day: 15% higher risk
- Current smoking versus never smoking: 24% higher risk
- Being physically active versus inactive: 29% lower risk of colon cancer
Sedentary behaviour appears to act independently of both weight and exercise. In a cohort of nearly 90,000 women, those watching more than 14 hours of television per week had a 69% higher risk of young onset colorectal cancer than those watching 7 hours or less, after adjusting for physical activity and BMI.
Diet pattern matters more than any single food. A Western dietary pattern is associated with higher risk, while Mediterranean and DASH patterns are associated with lower risk. One large cohort study found that women in the highest quintile of ultra-processed food intake had a 45% higher risk of developing early onset conventional adenomas, the precursor lesions, though not of the serrated type.
The most striking recent finding is bacterial. Analysis of 981 colorectal cancer genomes from 11 countries found mutational signatures caused by colibactin, a toxin produced by certain strains of E. coli, were 3.3 times more common in patients diagnosed under 40 than over 70. Those mutations were timed to the first ten years of life. This is a mechanism, not yet a proven cause, and it does not translate into anything a patient can act on today. But it fits the birth cohort pattern.
How much of it is inherited
Less than most people assume, and more than the traditional criteria would catch.
In a study of 450 patients diagnosed under 50, around 16% carried a pathogenic germline variant in a cancer susceptibility gene, and 8.2% had Lynch syndrome. Notably, one third of the patients found to have a mutation did not meet the traditional criteria for genetic testing.
Put plainly: roughly one in six cases under 50 involves an inherited predisposition, about half of those Lynch syndrome, and the large majority of cases are sporadic. This is the argument for offering genetic testing to everyone diagnosed young, rather than only to those with a striking family history.
The symptoms, and the delay
A systematic review of 81 studies covering nearly 25 million patients under 50 found the following symptoms at presentation:
- Rectal bleeding: 45%
- Abdominal pain: 40%
- Change in bowel habit: 27%
Rectal bleeding carried by far the strongest association with a colorectal cancer diagnosis, followed by anaemia and abdominal pain.
The same review found the problem that matters most. The mean time from first presentation to diagnosis was 6.4 months, with a median of four months. Delays of four to six months were described as common.
That delay has consequences. According to the American Cancer Society, three in four colorectal cancers in adults under 50 are diagnosed at an advanced stage, including 27% with distant metastases.
The delay is rarely the patient’s fault alone. Rectal bleeding in a young person is usually attributed to haemorrhoids, and changed bowel habit to irritable bowel syndrome, and most of the time that attribution is correct. The problem is that “usually correct” is not a diagnostic strategy when the alternative is a cancer that doubles its stage while everyone waits.
What to do about it
If you have symptoms, age is not reassurance. Rectal bleeding, a persistent change in bowel habit, iron deficiency anaemia without an obvious cause, or unexplained weight loss warrant examination at any age. A diagnosis of haemorrhoids or IBS should be made after the alternatives are excluded, not instead of excluding them.
Know your family history, and be specific about it. Guidance from the American College of Gastroenterology is that if you have one first-degree relative diagnosed with colorectal cancer or an advanced polyp before the age of 60, or two first-degree relatives at any age, colonoscopy should begin at age 40, or ten years before the youngest affected relative’s age at diagnosis, whichever comes first, and be repeated every five years.
A first-degree relative diagnosed under 50 raises your own risk of early onset disease by two to four times.
Understand where screening currently stands. The EU target population for colorectal screening is 50 to 74, and the Greek national programme covers ages 50 to 69. The United States lowered its starting age to 45 in 2021. No European national programme has yet followed. That means that in Greece, between roughly 45 and 50, the route to a colonoscopy is through symptoms or family history rather than through a screening invitation.
When to see a doctor
Do not wait for a screening invitation if you have:
- Blood in the stool, whether bright red or dark
- A change in bowel habit lasting more than a few weeks
- Unexplained weight loss
- Persistent abdominal pain or cramping
- Iron deficiency anaemia with no clear source
- Persistent fatigue alongside any of the above
- A family history of colorectal cancer, particularly at a young age
The single most useful thing a young patient can do is to describe symptoms accurately and to return if they do not settle. The single most useful thing a clinician can do is to take them seriously the first time.