Is surge in maternal obesity behind the mystery rise in bowel cancer among young people?

Of the 68 cases diagnosed in offspring between 1986 and 2017, half had fallen ill before the age of 50, suggesting that "in utero events may contribute to increasing incidence rates of [colorectal cancer] in younger adults".

In the US, colorectal cancer rates are increasing in under-50s but decreasing in over-50s. Similar patterns are occurring in the UK and Europe (Source: The Reusch Center for the Cure of Gastrointestinal Cancers)

A disproportionate weight gain during early pregnancy was also linked to an increased risk of the disease, indicating that "the timing of weight gain may be most important".

Children whose mothers' overall weight gain was low - under 20Ibs - but concentrated in the first 32 weeks of gestation had the highest incidence of bowel cancer.

Higher birthweight babies - which were more common in obese mothers - were also more likely to go on to develop bowel cancer as adults.

READ MORE: Majority of expectant mothers in Scotland now overweight or obese 

Rates of maternal obesity have increased roughly six-fold in Western nations since the 1960s.

The findings, published today in the BMJ journal Gut, also come as new cases and deaths from bowel cancer have nearly doubled in younger adults but are falling or plateauing among over-50s in many high income countries.

Scotland has one of the worst rates of maternal obesity in Europe, with a record-breaking 25 per cent of women who gave birth in 2019/20 classed as obese.

Meanwhile, the rate of bowel cancer has more than doubled over the past 20 years in adults aged 35 to 39, from 4.8 cases per 100,000 in 1999 to 11.6 per 100,000 by 2019.

Diagnoses are also up by 89% in the 40 to 44 age group and by 45% among adults aged 45 to 49, but have been falling in all age groups over 55.

READ MORE: Inflammatory bowel disease rate in Scotland 'rising since 1960s'

Previous research in BMJ Gut found that the UK had one of the fastest growing rates of bowel cancer incidence in the world among 20 to 49-year-olds, after South Korea, New Zealand, and Cyprus. 

Between 2003 and 2012, incidence rose 3.3% per year on average in the UK compared to 0.9% for over-50s. 

The researchers, based at the University of Texas, caution that the results could be due to factors shared by mother and child - such as diet and gut bacteria - as well as independent factors, such as the offspring's own weight in adulthood.

However, they add that their findings suggest “the well-established relationship between obesity and colorectal cancer may have origins in periods that begin before birth”.

Average annual percentage change in colorectal cancer incidence by country in most recently available 10-year period (2003-2012 for UK) by age group. Blue for countries where incidence is falling/stable in over-50s age group; red for countries where it is rising in 50-plus age group

This might include nutrients received in the womb which permanently alter the structure and function of adipose (fat) tissue, appetite regulation and metabolism, while excess exposure to insulin and growth hormone may affect insulin sensitivity.

READ MORE: Children as young as 10 diagnosed with bowel cancer amid spike in cases in under-50s

They write: “Our results provide compelling evidence that in utero events are important risk factors of [colorectal cancer] and may contribute to increasing incidence rates in younger adults.

"There may also be other as yet unknown exposures during gestation and early life that give rise to this disease and warrant further study."

They add: "Given population trends in maternal obesity, which has multiplied in prevalence by nearly six since the 1960s, we may see a growing burden of early-onset [bowel cancer] for decades to come."