Study finds being overweight is likely to cause cancer

A QIMR Berghofer study has found people who are overweight could reduce their risk of dying from cancer later in life by up to 30 per cent, by shedding some kilograms and maintaining a healthy body mass index (BMI).

The study also found that by reducing BMI by five points, an overweight person can cut their risk of developing any form of cancer by retirement age by 10 per cent.

Senior author and head of QIMR Berghofer’s Statistical Genetics research group, Associate Professor Stuart MacGregor, said the Mendelian randomisation study specifically looked at genetic markers related to BMI to explore the relationship between obesity and cancers.

“We found the risk of developing any form of cancer by age 65 escalated with every extra five-unit-point increase in BMI,” Associate Professor MacGregor said.

“The strength of this study is that it lets us extrapolate what the relationship is between BMI and cancer risk. By examining how genetic predisposition to obesity was related to cancers, we could also explore how other people who were overweight – due to lifestyle and other factors – were also at higher risk.

Associate Professor MacGregor said it was important for people to realise they could change their cancer risk because only a third of weight gain was attributable to a person’s genetics while two thirds was due to lifestyle and environment.

“We also observed that people who were genetically predisposed to being overweight had a higher risk of developing aggressive cancers that could result in death,” he said.

“This study adds to the growing evidence that obesity causes cancer, and might explain why all cancer rates have increased as populations have become heavier.

“These findings also provide evidence that cancer prevention strategies targeting weight control ought to be continued, given the high prevalence of obesity in many western countries.”

The World Health Organization (WHO) guideline for ideal weight is a BMI of 18.5 to 24.9.

“While some studies have shown a high BMI may help prevent some types of breast cancer, the risk of developing the vast majority of cancers seems to be made worse by excess weight,” Associate Professor MacGregor said.

“Worryingly, excess weight has the biggest impact on some of the most aggressive and deadly cancers such as those of the oesophagus and endometrium.

“Our study raises some important issues for public health and highlights the need for more research into whether this BMI-cancer relationship varies by different forms of weight gain (fat, muscle mass, etc.).

“To do that we will need larger genetic studies to identify genes that may cause different types of fat. This will be an interesting direction to take this work further.”

The Mendelian randomisation study examined genetic data from more than 46,000 Caucasian British participants in the UK Biobank project who were aged between 40 and 69 and who had developed any kind of cancer, and compared it to data from more than 390,000 cancer-free participants in the same cohort.

The study was funded by the National Health and Medical Research Council and the findings have been published in the British Journal of Cancer.

Quick facts:

  • The study results were similar for males and females, and smokers and non-smokers.
  • The Australian Institute of Health and Welfare (AIHW) says one in every two Australians will be diagnosed with cancer by age 85.
  • The Australian Institute of Health and Welfare defines people with a body mass index between 20 and 25 as being of healthy weight, while those between 25 and 30 are classified as overweight and those with a BMI of 30 and over are considered obese.
  • According to the Australian Bureau of Statistics, in 2015 almost two-thirds (63%) of Australians aged 18 and over were overweight or obese.

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