The findings could explain why obese young people do worse on initial chemotherapy – called induction therapy – than their peers who aren't obese.
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"Induction chemotherapy provides a patient's best chance for remission or a cure," principal investigator Dr. Steven Mittelman, of The Saban Research Institute of Children's Hospital Los Angeles, said in a hospital news release.
"Our findings indicate that a patient's obesity negatively impacts the ability of chemotherapy to kill leukaemia cells, reducing the odds of survival."
However, the current study was only designed to find a connection between pre-treatment obesity and leftover cancer cells. It wasn't designed to prove that obesity was the definitive cause of the poorer outcomes.
For the study, the researchers focused on nearly 200 patients between the ages of 1 and 21 years. About one-third were overweight or obese, as defined by the U.S. Centres for Disease Control and Prevention.
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Researchers found that those with so-called "minimal residual disease" – leukaemia cells that can be detected by sensitive methods – were more likely to have poor results over the next two to five years.
The patients who were obese and had residual disease had the worst outcomes of all in the study.
"In addition to increasing a patient's likelihood of having persistent disease following treatment, obesity appears to add a risk factor that changes the interaction between chemotherapy and residual leukaemia cells," said the study's co-lead investigator Dr. Hisham Abdel-Azim of The Saban Research Institute in the release.
Read: Today's parents blind to kids' obesity
Gaining weight during the first month of treatment didn't appear to worsen outcomes, the study found.
The study was published online in the journal Blood.
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Image: Chubby boy from Shutterstock.