Your past might come back to bite you in the butt — literally.
A new study suggests that a key factor in early life could increase your risk of developing colorectal cancer as an adult.
The findings add to a growing body of research trying to pinpoint what’s fueling the alarming rise in colorectal cancer among Americans under 50 over the last three decades.
“Although the relationship between adult body size and colorectal cancer risk is well-documented, the potential influence of measures of body size during early life is less understood,” said Dieuwertje Kok, associate professor of nutrition and cancer at Wageningen University & Research in the Netherlands.
To get to the bottom of it, Kok and her colleagues analyzed 37 studies tracking how weight, height and body size in childhood and young adulthood could impact the risk of colorectal cancer later in life.
They found that for young adults aged 18 to 25, every five-point increase in body mass index above the healthy range raised colon cancer risk by 12%.
For teens between 10 and 19, that risk climbs between 5% and 18%.
Even toddlers aren’t safe — a high BMI in kids as young as 2vwas also linked to greater risk.
And here’s the kicker: Every extra kilo (2.2 pounds) at birth over “normal” birth weight — that’s 2.5 to 4 kilograms, or 5.51 to 8.82 pounds — was tied to a 9% greater chance of developing colorectal cancer down the road.
“Cancer is a complex disease which develops over several decades, so better understanding of its early origins is critical for more effective prevention efforts and understanding the research gaps,” said Helen Croker, assistant director of research and policy at World Cancer Research Fund International.
“The results of this study show that raised body mass index across childhood through young adulthood is an important risk factor for colorectal cancer,” she added.
Colorectal cancer has long been associated with older adults, especially people over 65. But rates among those under 50 have been creeping up since the ’90s.
In fact, if you were born between 1981-1996, you face twice the risk of colorectal cancer as compared to people born in 1950, according to the University of California Davis Health.
Scientists suspect that ultra-processed foods and certain diets are major drivers behind the rise in colorectal cancer in young people.
One study linked Western diet — which is high in fat and low in fiber — to an imbalance of gut bacteria, triggering inflammation that ages cells faster and makes them more cancer-prone.
Another found that low calcium intake is tied to one in five colorectal cancer deaths in people 35 and younger.
Ultra-processed junk like ice cream, chips and soda have also been shown to boost the risk and weaken the body’s ability to fight the disease.
Our couch potato habits aren’t helping. A 2019 study found young adults who watch more than two hours of TV a day have a 70% higher risk of developing colon cancer than those who watch less.
Chronic conditions play a role, too. Research suggests people with nonalcoholic fatty liver disease — mainly caused by obesity, diabetes, high blood pressure and high cholesterol — are nearly 24% more likely to develop colon cancer.
Inflammatory bowel disease, Crohn’s disease and ulcerative colitis have also been shown to increase the risk.
But here’s the scary part: Colorectal cancer is showing up in young, healthy and active people with no obvious risk factors. For them, the culprit might be genetics or hidden environmental triggers.
One study found that a toxin from certain E. coli bacteria called colibactin leaves a unique DNA mark on colon cells in kids exposed to it. These mutations were 3.3 times more common in early-onset cases than in older patients.
In another study, researchers found a fungus called cladosporium — which usually causes skin and nail infections — in the tumors of young colorectal cancer of patients. Scientists aren’t sure how it triggers the disease, but it might damage DNA.
A gene called HMGA1 may also hold the answer. Research suggests it acts like a “key” that turns on DNA regions controlling colon stem cells. When overactivated — due to stress, mutations or poor diet — it can lead to tumor growth and help cancer hide from the immune system.
As scientists race to uncover the causes behind early-onset colorectal cancer, young Americans keep getting diagnosed at alarming rates.
Worse still, young adults tend to be diagnosed later — when the disease is more advanced and the tumors more aggressive.
That’s helped make colorectal cancer the leading cause of cancer death in US men under 50, and the second deadliest for women in the same age group.
In response, the US Preventive Services Task Force lowered the recommended screening age from 50 to 45 in 2021 for average-risk adults.
In 2025, the American Cancer Society estimates about 154,270 US adults will be diagnosed with colon or rectal cancer — and roughly 52,900 will die from it.
