A 39-year-old fitness coach mistook her colon cancer symptoms for IBS

From a very young age, Danni Duncan struggled with IBS symptoms

“I remember being maybe around 5 or 6 and really being in so much pain, literally on my hands and knees in the bathroom,” Duncan, 40, told Business Insider. She was put on elimination diets throughout primary school and diagnosed with IBS when she was around 11

Duncan, who lives in the Gold Coast of Australia, said she grew up eating McDonald’s, frozen pizzas, and Pop-Tarts. As an adult, her IBS symptoms improved as she filled her diet with whole foods and cut back on alcohol. She also began training for marathons and became a fitness and nutrition coach

Then, in her late 30s, she struggled to go to the bathroom again. She chalked it up to IBS and started taking Metamucil, a fiber laxative, to reduce constipation. She also felt persistent bloating for months after giving birth to her fourth child in September 2024

Eventually, at 39, she was diagnosed with stage 2 colon cancer

“It was like I was coming out of my own body,” Duncan said. “I kind of giggled and said, ‘This can’t be true. I’m supposed to live to be 100. That’s how healthy I am.'”

Severe anemia got her diagnosed

When Duncan was about 20 weeks pregnant with her fourth child in 2024, her iron levels were the lowest they’d ever been. Even after an iron infusion, her levels remained low. Still, she said her doctor explained it away as a pregnancy symptom and suggested iron supplements

In early 2025, about five months after giving birth, Duncan started training for a marathon that March. She felt winded like never before

Even when she had slept and fueled well, she couldn’t get her speed up. Come marathon day, she dropped out of the race when she was three-quarters finished. At the gym, she couldn’t finish her sets and experienced dizzy spells in her kitchen

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The breaking point was a month later, in April 2025, when she sensed she was going to faint one morning and stumbled onto her couch

A blood test at the doctor’s office revealed she had a hemoglobin level of 70 g/L, indicative of severe anemia (a normal level is considered 120 to 160 g/L for adult women). She was scheduled for another iron infusion the next day and a colonoscopy two weeks later, in May 2025, to rule out colon cancer

The colonoscopy showed a tumor in her colon

Duncan, whose 37-year-old friend died from colon cancer the year prior, initially thought of her kids growing up without her. “That thought broke me — I had a tear for a second, and then my mind just switched to like, ‘What are we doing?'” Duncan said. “‘I have to fight this because I have children.'”

Exercising through chemo side effects

Less than two weeks after learning she had colon cancer, Duncan underwent surgery to remove the tumor, including a right hemicolectomy in which part of her colon was taken out. She also had 17 lymph nodes removed outside her bowel wall, the tissue structure that forms the small and large intestines

While cancer wasn’t found in her lymph nodes, it was found in the lymph node channels, leading to her stage 2B diagnosis. It put her at higher risk of recurrence because she was closer to stage 3, when the cancer would have officially spread to the lymph nodes. Her doctor wanted her to do four rounds of chemotherapy, considered a standard amount of chemotherapy for earlier-stage cancers, to reduce that risk

She tried to stick to her routines, including regular walks, light bodyweight exercises, and runs on the weeks she didn’t have chemo. Still, extreme fatigue and nausea sometimes wore her down

She also developed cold sensitivity, a common side effect from oxaliplatin, her chemo medication. “Everything seizes up,” she said. “Your throat seizes up, you can’t breathe; your face seizes up like you’re having a stroke.” She wore gloves to open her fridge door or hold weights at the gym

The hardest part, she said, was not being able to breastfeed, as chemo drugs can be passed on to the baby through breast milk. Duncan was diagnosed when her daughter was 7 months old and had planned on breastfeeding for far longer

“I just loved it and we had such a great connection,” she said. “It’s broken my heart and it still breaks my heart.”

A major marathon to celebrate the end of treatment

As of November 2025, Duncan’s CT scans and colonoscopy no longer showed signs of cancer. Going forward, she has three more monthly blood tests and will have yearly CT scans and colonoscopies every two years

In some ways, Duncan said life after cancer has been more challenging than the treatment itself. “When you’re in the throes of cancer, you’ve got such a focus,” she said. “Once it’s finished, people stop checking in because you’re cancer-free now. That’s when the mental struggle is probably harder.”

Duncan, who turned 40 during treatment, is now training for the New York City Marathon, which she planned to run in 2025 but was forced to postpone. It’s a longtime dream of hers because she found out she was pregnant with her first child in New York City

Duncan will run it with her husband on behalf of the Jodi Lee Foundation, a colon cancer organization in Australia

From both her personal experience and advocacy work, Duncan said she’s become more aware of the rising rates among young people and the importance of getting symptoms like changes in bowel movements checked out if they’re unusual for you

“Stand up for yourself. Be like, ‘I’m not too young,'” she said. “If you’ve got symptoms, push, find another doctor.”