CT scans of 750 individuals show that people who are obese have larger organs and thus more cells. This could explain why people who are obese have a higher risk of many kinds of cancers.
鈥淲hile obesity is a complex disease that may affect cancer risk in several other ways, the increase in the size of an organ, and in the number of its cells, must increase the risk of cancer in that organ,鈥� states the team, which is led by Cristian Tomasetti at the Johns Hopkins University School of Medicine.
Others say the idea is plausible, but far from proven.
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Obesity is one of the biggest risk factors for cancer after smoking. Around , according to some estimates. Numerous , but why obesity increases the risk of certain cancer types, such as those of the kidney, remains unclear.
What we do know is that cancers are caused by mutations that disable the mechanisms that control cell growth. In theory, then, the more cells in any particular organ, the greater the risk of some of those cells becoming cancerous.
Read more: Obesity increases the risk of cancer
Tomasetti and his colleagues used CT scans to measure the volume of the kidneys, pancreas and liver in 750 people. The team found that for every 5-point increase in body mass index (BMI), the volumes of kidneys, liver, and pancreas increase by 11 per cent.
People with a BMI of around 50 have organs that are between 50 and 100 per cent larger than people with a healthy BMI. 鈥淭he effect is very large and unexpected, with severely obese patients having organs that can be double than normal in volume,鈥� the study states.
Only a very small fraction of these increases are due to an increase in the volume or number of fat cells in these organs, the study says, meaning the increases are mainly due to larger numbers of normal cells.
What鈥檚 more, the observed size increases in each organ correspond with the reported increase in cancer risk for each organ, the study says. 鈥淎lmost 80 per cent of the variation in cancer risk attributable to the effect of obesity can be explained by the variation that obesity induces in the volume of the three organs.鈥�
鈥淭his hypothesis is plausible,鈥� says Maria Dalamaga at the National and Kapodistrian University of Athens, Greece, . 鈥淲e already know that tall individuals are more prone to develop cancer.鈥� However, the study does not control for other factors known to affect the risk of cancer, such as age, sex and family history, she says.
Neil Iyengar at the Memorial Sloan Kettering Cancer Center in New York, whose work suggests , is more sceptical. The idea cannot explain why obesity seems to reduce the risk of a few cancer types, he says. 鈥淚 believe the conclusions are deeply flawed.鈥�
In principle, if more cells means a higher cancer risk, large animals like whales and elephants should be riddled with tumours. But they have evolved better mechanisms to protect against cancers, says Joshua Schiffman at the University of Utah.
鈥淲e have shown that elephant cell biology protects against mutation-causing damage better than human cells,鈥� he says.
Having more cells is a 鈥渧ery plausible鈥� explanation for why obesity increases cancer risk in people, says Schiffman. 鈥淏ut further experiments still need to be performed to prove that additional cells truly increase the risk of cancer in oversized organs.鈥�
Tomasetti wouldn鈥檛 discuss his team鈥檚 findings until the results are published in a peer-reviewed journal.
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