The longer a woman has been overweight or obese, the higher her cancer risk: study

A longitudinal study featuring nearly 74,000 US women has found that the longer a woman has been overweight or obese during her adult life, the higher her risk of developing cancer.

The finding highlights the importance of preventing excess weight and obesity at any age, the study’s authors said.

The lead authors from the World Health Organisation’s International Agency for Research on Cancer (IARC) and the University of California, Irvine, assessed overweight and obesity duration using Body Mass Index (BMI) data at different time points during the participant’s lives. BMI is calculated by dividing weight in kilograms by height in metres squared.

The study, published in the journal PLOS Medicine, found that being overweight for long periods of time is linked to a higher risk of several types of cancer.

“We also observed that, in women after menopause, for some cancer sites the increase in risk with increasing overweight duration is ‘steeper’ than for others,” said Melina Arnold, lead author of the study and a cancer epidemiologist at the WHO’s IARC.

“For example, while we observed that the risk of postmenopausal breast cancer increased by 7% for every ten years spent with obesity, this was 23% for endometrial cancer.”

Approximately two-thirds of the women in the study had been overweight or obese at some point during their adult lives. The health outcomes of these women were compared to those who had never been overweight or obese. The researchers took into account important information on other factors related to obesity, such as physical activity, diet, smoking, hormone use, and diabetes history.

The authors used data collected through the Women’s Health Initiative, which recruited postmenopausal US women aged between 50 and 79 and followed their health outcomes over time.

“We wanted to answer questions like ‘Does it matter how many years you have been obese during your life?’ and ‘Does the degree of overweight and obesity play a role in this?’ In epidemiology we call this ‘dose-response relationship’ or the ‘impact of cumulative exposure’.”

The study found that risks of postmenopausal breast and endometrial cancer related to overweight duration were much more pronounced in women who never used postmenopausal hormones.

Being overweight for longer periods of time increases the risk and severity of hypertension, insulin resistance, chronic inflammation, DNA damage and changes in hormone metabolism – key mechanisms that also increase the risk of cancer, said Dr Arnold.

Future research aims to examine whether it matters if you become obese during young adulthood or only later in life and how duration of obesity affects cancer prognosis, she said.

Dr Arnold warned that the overall conclusions from this study cannot be directly translated to the individual level.

“This means that not every woman who has been overweight for some time in her life will develop cancer at some point. The cause of cancer is always multi-factorial, meaning that both genetic and multiple environmental components might play a role here. Their exact interplay remains to be explored,” she said.

Professor Ian Olver, Chair of Translational Cancer Research at the University of South Australia, said the study findings were significant.

“There are many other factors that are associated with cancer where both intensity and duration are important. This adds persuasive evidence about the importance of the duration of obesity independent of other risk factors, like the use of hormone replacement in this group,” he said.

“Although cause cannot be inferred from a population study and the study should not be used alone to determine management, one conclusion of the study is that it is worth looking further into controlling obesity to reduce the risk of cancers that have been associated with being overweight or obese.”

Professor Olver noted that the study used BMI data calculated from information collected in retrospect, in some cases self-reported by study participants. This may allow for errors to be introduced, he said.

“Also, BMI is not the best measurement to assess body fat and its association with cancer,” he said.