One of the leading risk factors for disease in high-income countries is excess weight. While many studies have linked excess weight to cancer and other diseases, little research has examined its impact over the course of time.
Melina Arnold, a scientist at the International Agency for Research on Cancer (IARC), the specialized cancer agency of the World Health Organization (WHO), monitors, describes and studies risk factors for cancer worldwide. She and her colleagues wondered how the length of time a woman carries excess weight throughout adulthood affects her risk of cancer. Using body mass index (BMI) measurements from over 70,000 U.S. women, the team found that every ten-year increase in adult overweight duration was associated with a 7 percent increase in the risk of all obesity-related cancers. This observational study highlights the importance of weight management and obesity prevention for women at all ages.
Arnold was gracious enough to answer a few questions about her recent study via email.
Your work at the IARC focuses on cancer epidemiology. How did you become interested in obesity and cancer?
MA: Although the association between obesity and cancer is well-established in the scientific literature, we actually know relatively little about this relationship and how other factors such as physical activity and diet interact with it. And even more importantly from an international perspective, obesity (having a BMI above 30, according to the WHO classification) has become the leading risk factor for disease burden in high-income countries and has been even offsetting or surpassing the decreasing disease burden caused by tobacco smoking. Given that in a country like the US, currently about 70% of all adults are considered overweight (BMI ≥ 25) and 36% obese (BMI>30), this development and continuing increases in obesity prevalence seen over the past decades have become a great concern to public health. For those reasons and because I believe that obesity will play an even more important role in terms of disease burden in the future, it became one of my research interests.
This study comprised over 70,000 post-menopausal women. Why did you decide to focus on this age group?
MA: We used data from a large cohort of postmenopausal women from the United States, the Women’s Health Initiative. All women were recruited at postmenopausal ages (50–79y) at 40 clinical centers nationwide between 1993 and 1998. Most of the study participants are still being followed-up for the development of common diseases, including cancer. As all women were asked for their height and weight at different times during their lives, this cohort was perfect for the purpose of studying lifetime overweight duration in relation to cancer risk.
“The etiology of cancer is always multifactorial, meaning that both genetic and multiple environmental components might play a role.”
Could you tell me a bit about the duration of obesity during adulthood for these patients and the likelihood they would develop cancer? For example, the study mentions that the longer a woman remained overweight in adulthood, the greater her chance of getting an obesity-related cancer. For how long did the typical person in this study have to be overweight in order to develop an obesity-related cancer, and what percentages of women ended up with cancer?
MA: In this study, we found that a longer duration of overweight during adulthood increases the risk of several cancers in women after menopause. This is biologically plausible as earlier and longer exposure to overweight have been found to increase the risk and severity of hypertension, insulin resistance, chronic inflammation, DNA damage and changes in hormone metabolism –key mechanisms increasing also the risk of cancer. Among the more than 70,000 women in our study, about 15% developed a cancer; half of those occurred at cancer sites that are known to be associated with obesity. Because the time between being obese and developing cancer is difficult to determine and will depend on the underlying biological mechanisms as well as the cancer type, this aspect warrants further research.
I also have to note that the overall conclusions from this study cannot be directly translated to the individual level. This means that not everyone who has been overweight for some time in their life will develop cancer at some point. The etiology of cancer is always multifactorial, meaning that both genetic and multiple environmental components might play a role here.
BMI has often been considered not ideal for measuring body fat. Why did you base the study on this measurement?
MA: It is true that BMI is an imperfect measure of body composition and that for several reasons. For example, BMI does not differentiate tissue type (whether it is fatty, lean or bone) and is therefore not useful to determine body fat. Also, body fat distribution varies across ethnic groups with similar BMI levels, making comparisons more difficult. Yet, BMI continues to be the most commonly used proxy for overall body fat in epidemiologic studies and in clinical settings. This is mainly because it is fairly easy to calculate, data collection costs are low, and standardized cut-off points set for example by WHO, are available.
In this study, we needed information on body composition from several time points for every study participant in order to estimate duration of overweight and obesity during adulthood. As height and weight were available at several occasions, BMI was the only possible choice.
“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.”
Were you surprised by any of your findings?
MA: We know already from previous studies that there is a link between obesity and several forms of cancer. In our study, we for the first time assessed how the duration of overweight and obesity during adulthood impacts on the risk to develop cancer. We wanted to answer questions like ‘Does it matter how many years you have been obese during your life?’ and also ‘Does the degree of overweight and obesity play a role in this?’ In epidemiology we call this dose-response relationship or cumulative exposure. We found that a longer overweight increases the risk of several cancers, and that for some cancer sites the increase in risk with increasing overweight duration is ‘steeper’ than for others. 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.
“The results of this study indicate that overweight and obesity prevention is essential, at any age.”
How do you hope this study will be used, and where would you like to go next with your research?
MA: The results of this study indicate that overweight and obesity prevention is essential, at any age. If we manage to raise awareness among people and health practitioners that obesity is a risk factor for several diseases, not only cancer, this would be a great success.
Given the interesting findings of the present study, we will continue this research and try to answer some more specific questions, for example ‘does the age at obesity onset matter (whether you become obese during young adulthood or only later in life)?’ or ‘does lifelong normal weight actually reduce the risk to develop cancer?’. There is also more research needed to explore the role of obesity beyond cancer diagnosis, i.e. how it affects prognosis.
Research Article: Arnold M, Jiang L, Stefanick ML, Johnson KC, Lane DS, LeBlanc ES, et al. (2016) Duration of Adulthood Overweight, Obesity, and Cancer Risk in the Women’s Health Initiative: A Longitudinal Study from the United States. PLoS Med 13(8): e1002081. doi:10.1371/journal.pmed.1002081
Image Credit: Weight of the weightless by mrd00dman via Flickr