Cancer is, by and large, a disease of old age. In Scotland, your odds of being diagnosed with any form of the disease are roughly 127 times higher if you are aged 80 to 84 than if you are a teenager.

With the exception of cervical cancer, most screening services worldwide tend to target adults in middle age - between around 50 and 70 - who are more likely than younger people to develop cancer, but more likely than the elderly to get a meaningful survival gain from early diagnosis and treatment.

Against this backdrop, scientists have been puzzling over a strange trend in recent decades that has seen the prevalence of cancer creeping up among the under-50s.

The picture was brought into sharp focus this week by the first ever study to pull together global statistics on the issue.

Scientists from Edinburgh University's Usher Institute, Colon Cancer Genetics Group, and Institute of Genetics and Cancer were among those to provide input.

READ MORE: Diet Coke, aspartame, and why we struggle to understand cancer risk 

The paper, published in the journal BMJ Oncology, found that the number of new cancer cases detected in the under 50s had increased by 79% - to 3.26 million - between 1990 and 2019.

They forecast that this will increase by a further 31% by 2030, with adults in their 40s most affected.

This signals a major challenge for healthcare systems which must already grapple with an increased cancer toll from an ageing population, but now have to contend with the pressure to diagnose and treat a growing incidence of the disease in their younger populations.

The Herald: The steepest increase has occurred in the 35-39 age groupThe steepest increase has occurred in the 35-39 age group (Image: BMJ Oncology)

The American Cancer Society (ACS) has recommended initiating colorectal cancer screening from 45 years of age instead of 50.

For breast cancer, in countries with favourable healthcare conditions, it is recommended that women between the ages of 40 and 49 undergo a screening test for breast cancer every one to two years.

In NHS Scotland, both screening programmes still commence at 50.

In 2019, the highest age-standardised incidence rate for "early-onset" cancers was found in North America, at 273 cases for every 100,000 people under 50.

It was lowest, at 37.4 per 100,000 in Western sub-Saharan Africa, although the authors caution that in the majority of African countries cancer reporting systems "are poorly organised".

Overall, however, incidence rates for cancer in young adults were consistently highest in North America, Western Europe, and Australasia.

Globally, prostate cancer and cancer of the windpipe - nasopharyngeal - showed the fastest year-on-year rise in incidence rates, but there were also sharp increases in the rates of breast and bowel cancer in the young.

This is echoed by statistics in Scotland which show that rates of breast cancer in women aged 25 to 49 increased by 10% between 1997 and 2021, from 40 per 100,000 to 44 per 100,000.

The change has been especially pronounced in the 25 to 34 age group, where incidence rates have climbed by 22% over the past two decades.

Similarly, for adults aged 25 to 49 colorectal cancer rates have increased by 18% since 1997 - from 10.4 to 12.3 cases per 100,000 - but have nearly doubled, from 2.8 to 5.2 cases per 100,000 in the 30 to 34 age group.

READ MORE: Cancer screening and an ageing population - the dilemma for the NHS

Prostate cancer remains almost non-existent for men under 40, but in the 40-49 age group case rates in Scotland have gone from 3.2 in the late Nineties to 6.6 per 100,000 now.

Public Health Scotland does not provide specific data on nasopharyngeal cancer, but this would be included within head and neck cancers.

The statistics for head and neck cancer indicate that case rates are up by around 5.2% among people aged 40 to 49, compared to 20 years ago.

There is no single explanation for these trends among young people, but plenty of hypotheses.

The Herald:

The rise in breast cancer, for example, may be a hangover from an era in the 1990s and early 2000s when alcohol became much more affordable than it had been in previous decades and young women were encouraged to embrace a "ladette" culture of binge drinking.

Alcohol is known to increase levels of oestrogen and other hormones associated with breast cancer.

It is estimated that women who consume three alcoholic drinks per week have a 15% increased risk of developing breast cancer compared to women who do not drink at all.

In addition, people in their 30s and 40s today have grown up with a very different diet compared to previous generations.

They are roughly twice as likely to be obese compared to someone of the same age 20 to 30 years ago.

Beyond obesity itself - which is associated with a number of cancers, including breast and bowel - researchers have become increasingly interested in whether ultraprocessed food itself is inherently harmful.

This could be due to complex manufacturing processes or specific ingredients, such as additives, sweeteners or emulsifiers which may disrupt the gut microbiome to increase a person's risk of cancers, diabetes or weight gain.

However, the authors of the BMJ Oncology paper note that the main known risk factors underlying the most common cancers among the under-50s are "diets high in red meat and salt, and low in fruit and milk; alcohol consumption; and tobacco use".

Physical inactivity, excess weight, and high blood sugar are "contributory factors".

READ MORE: What's driving obesity rise - junk food, gut bacteria, or genes?

A full understanding of the reasons driving the trends "remains elusive", they stress, but add that "novel areas of research such as antibiotic usage, the gut microbiome, outdoor air pollution and early life exposures" - such as infections - are also being explored.

In a linked editorial, academics from Queen's University Belfast, argued that "early detection measures for this age group, including the potential expansion of screening, should be considered".

The Herald:

Professor Stephen Duffy, who heads up the Centre for Prevention, Detection and Diagnosis at Queen Mary University of London (QMUL), said "considerable time and thought" is needed to interpret the findings.

He said: “Overall, there are many interesting results here, but they are complicated, and the cancer prevention and control community will need to take a long look at them over the next few weeks to consider exactly what they mean and what we can do to reverse some of the increasing trends.”