After running the London marathon for the second time aged 47, Alison Crooks felt she was in the best shape of her life.
But two days after crossing the finish line a phone call from her GP meant she had to dramatically re-evaluate that feeling.
While talking to him previously about a different concern Alison had casually mentioned that she was going to the loo more often than normal. Her GP suggested testing a stool sample – just to be on the safe side.
Alison was blindsided to receive a call 48 hours after the marathon to say that the test had detected blood in her sample – and further tests over the following two weeks revealed she had bowel cancer.
‘I felt disbelief that this was happening to me,’ says Alison, an environmental consultant from Southampton.
‘I was fit, vegetarian since my teens, never smoked and had no family history of the disease – so I didn’t have a typical risk profile for bowel cancer.’
Two months later Alison underwent surgery to remove part of her lower colon – this revealed the cancer was stage 3, meaning it had spread to a nearby lymph node.
One of her first questions on being told she had cancer was, ‘why me?’ – and Alison, now 50, says her ‘doctors couldn’t give any reason, other than saying that it was being noticed more in younger people’.
Alison Crooks shows off her medal after completing the London marathon in 2023 to raise money for charity Bowel Cancer UK
But new research might help explain it: this suggests that endurance runners, such as people who run marathons, might have an increased risk of getting the disease.
Dr Timothy Cannon, an oncologist at Inova Schar Cancer Institute in Virginia, who led the study, decided to investigate after he noticed a disturbing trend in his clinic of ultramarathon runners (50km or longer) aged under 40 who had advanced-stage colorectal cancer.
His team recruited 100 regular marathon and ultramarathon runners aged between 35 to 50, who agreed to have a colonoscopy – where a thin, flexible tube fitted with a tiny camera is used to explore the bowel.
None of the study participants had any known health complaints, but the colonoscopies revealed that almost half – 41 per cent – had at least one pre-cancerous growth in their colon (known as an adenoma, a type of polyp).
And 15 per cent of participants had advanced adenomas – meaning they were at least 1cm wide. This put them at greater risk of the growth becoming cancerous.
Normally, just 1 to 2 per cent of people in this age group would be expected to have advanced adenomas.
The researchers said ‘intensive long-distance running is a risk factor for advanced adenomas of the colon’ and suggested that colon cancer screening specifically for younger endurance runners ‘is warranted’.
The findings were presented at the American Society of Clinical Oncology conference earlier this year, but have yet to be published in a medical journal.
After her diagnosis in June 2023 Alison underwent surgery to remove the diseased section of her colon and subsequently began four rounds of chemotherapy which ended that November
‘While the results don’t prove causation, they strongly suggest that intense endurance exercise could be a meaningful risk factor for colorectal cancers and warrants further study,’ says Dr Cannon.
‘We believe this research may help define a new, at-risk population for earlier colorectal cancer screening. If we can identify these cancers earlier in people who otherwise wouldn’t meet current screening criteria, we can save lives.’
There are 44,000 new cases of bowel cancer each year in the UK, leading to more than 16,000 deaths – only lung cancer kills more.
In the UK, everyone aged between 50 and 74 is sent a home-testing kit, as part of a national screening programme. It checks for blood in your stool, which can be an early sign of bowel cancer.
Yet while 94 per cent of cases occur in people aged over 50, cases are rising sharply among the younger generation.
‘There’s a theory that it might be related to obesity or an unhealthy diet of too many calories and shifting away from a more fibre-rich diet to one with more processed foods, sugars, fats and refined grains [such as white rice or bread],’ says Professor Matt Rutter, a consultant gastroenterologist at North Tees and Hartlepool NHS Foundation Trust.
The suggestion is that this affects the microbiome – the community of microbes that inhabit the gut and plays a major role in health, from immunity to mental wellbeing: as a result ‘bad’ bacteria overwhelm the good bacteria ‘which might increase toxins, promote inflammation or be changing the metabolism and gut immune function’, says Professor Rutter.
With extreme exercise, such as marathon running, blood is directed away from the intestine – ‘if you’re doing an endurance sport, then obviously your muscles are demanding lots of oxygen, so the body diverts blood from non-essential organs like the bowel to muscles, so essentially the bowel becomes relatively starved of oxygen’, explains Professor Rutter.
And this may help explain the findings of the new study.
‘We call this ischemia – and in this scenario, that lack of oxygen can leave the bowel damaged and inflamed,’ adds Professor Rutter.
‘To my knowledge there’s no evidence that ischemic inflammation in the colon predisposes someone to cancer, but we do know that other forms of inflammation in the colon such as inflammatory bowel disease (IBD) can predispose to cancer.
‘It’s certainly plausible that repeated episodes of ischemic inflammation in the bowel may predispose to precancerous growths and even cancer.’
Professor Rutter, who is also a clinical advisor of the charity Bowel Cancer UK, described the US study as ‘intriguing’ but is cautious.
‘On the face of it, the 15 per cent with advanced adenomas certainly seems higher than you would expect.
‘But the study’s big flaw is that it doesn’t have a control group of those aged between 35 and 50 who don’t run, to compare the advanced adenoma rate in this group.
‘From these results it’s not possible to say whether this signals a genuine increased risk of extreme endurance sport or is it just a statistical quirk. At the moment I would very much encourage people to keep running and stay fit.’
Further research is underway to analyse any dietary factors or types of training which may have caused these surprising results – certainly the study is at odds with other research which has found that exercise can protect against cancer generally – Cancer Research UK for example says being active can reduce 13 different types of cancer.
‘We recommend that people should exercise at least twice a week for two or more hours to reduce the risk of bowel cancer,’ says Dr Kevin Monahan, a consultant gastroenterologist at St Mark’s Hospital in London, which is considered to be the leading bowel hospital in the country.
‘I’m one of those people who run or cycle to work every day, and there’s overwhelming evidence of the health benefits of maintaining cardiovascular fitness.’
Professor Trevor Graham, director of the Centre for Evolution and Cancer at the Institute of Cancer Research in London, adds: ‘Exercise is extremely well established to be protective against cancer – research shows that getting enough physical activity when you’re young can protect you from a number of cancers when you’re older.’
In fact the current evidence suggests exercise also improves the chances of those undergoing cancer treatment. A study published in June found that exercising after treatment for colon cancer reduces the risk of death by more than one third. The study, led by researchers at Queen’s University Belfast involved 899 people with the disease, and published in the New England Journal of Medicine, showed that after eight years, patients doing an individualised exercise programme (which included a brisk 40 minute walk every day) had a 37 per cent lower risk of dying than those simply given pamphlets about exercise.
Alison had been a regular runner since she was 30 and in 2006 ran her first London Marathon – ironically she was running it again in 2023 to fundraise for the charity Bowel Cancer UK, after a close friend died of the disease a year earlier.
It was only by chance that Alison’s colon cancer was picked up when she increased her marathon training.
‘Nothing significant had really changed with my health but I mentioned in passing that I was going to the loo more often, especially when doing long runs.’
The GP gave Alison a stool testing kit to us eat home which she returned to the surgery for it to be analysed only days before the marathon.
After her diagnosis in June 2023 Alison underwent surgery to remove the diseased section of her colon. She then began four rounds of chemotherapy which ended in November 2023.
She slowly got back to running at the start of 2024, even completing a 10km race that April.
‘Running saved me,’ says Alison. ‘My symptoms were so slight that I probably wouldn’t have seen my GP about it if it didn’t affect my running.
‘I was surprised when I read about this new study. But it won’t stop me and I will keep running for as long as I can.’