Being overweight for long periods as an adult significantly increased the incidence of all obesity-related cancers by 7 percent.
High body mass index (BMI) has become the leading risk factor of disease burden in high-income countries.
Worldwide, at least 2.8 million people die each year as a result of being overweight or obese. An estimated 35.8 million (2.3 percent) of global disability-adjusted life years (DALYs) - the sum of years of potential life lost due to premature mortality and the years of productive life lost due to disability - are caused by overweight or obesity.
Recent studies have suggested that the risk of cancer related to obesity is accelerated by time. The aim of the new study was to assess the impact of adulthood overweight and obesity duration on the risk of cancer.
The study was led by Melina Arnold, of the International Agency for Research on Cancer (IARC) - the specialized cancer agency of the World Health Organization (WHO), in Lyon, France - and Prof. Hoda Anton-Culver, of the University of California-Irvine.
The researchers used data from the United States Women's Health Initiative (WHI). They planned to identify dose-response relationships between duration of high BMI and the risk of developing several types of cancer.
The authors note that to date, most studies that have explored the relationship between excess weight and cancer risk have observed cross-sectional exposure information - such as height and weight measured at one point in time - on overweight and obesity.
Using multiple BMI measurements over time, the team examined data from a total of 73,913 postmenopausal women. Around two thirds of the included women were overweight or obese, and 6,301 obesity-related cancers were diagnosed during a mean follow-up of 12.6 years.
Arnold and colleagues explored how the timing, duration, and intensity of overweight and obesity impacts on cancer risk, taking into account other factors related to obesity, such as physical activity, diet, smoking, hormone use, and history of diabetes.
Out of all included study participants, 40 percent were never overweight, and 60 percent were overweight for some time during their adult life, almost half of whom were also obese at some point.
Long-term obesity may raise risk of other factors linked to cancer
The study found that being overweight for a longer duration as an adult significantly increased the incidence of all obesity-related cancers by 7 percent for every 10-year increase in overweight adulthood period. An increase in risk was also seen for postmenopausal breast cancer, by 5 percent, and endometrial cancer, by 17 percent.
After adjusting for the intensity of overweight - how overweight individuals were - these figures rose to 8 percent for postmenopausal breast cancer and 37 percent for endometrial cancer for every 10 years spent with BMI ten units above normal weight.
The findings highlight that the duration a woman spends overweight, and how overweight they are, play important roles in their risk of cancer, which emphasizes the importance of obesity prevention at all ages from early onset.
The authors write: "We found that longer durations of overweight and obesity were significantly associated with an increased incidence of obesity-related cancers, postmenopausal breast cancer, and colon, endometrial, and kidney cancer."
"Earlier and long-term exposure to overweight and obesity may also increase the risk and severity of chronic inflammation, oxidative DNA damage, and alterations in endogenous hormone metabolism, three key mechanisms that have been found to be associated with increased risk of cancer."
Arnold and team note that although the observational nature of the study prevents them reaching a conclusion about cause and effect, or making clinical recommendations, the findings suggest that reducing overweight duration in adulthood could reduce cancer risk.
"If this is true, healthcare teams should recognize the potential of obesity management in cancer prevention and that excess body weight in women is important to manage regardless of the age of the patient," they conclude.