The duration of obesity and the risk of type-2 diabetes, cardiovascular disease, cancer and mortality

Recent meta-analysis studies have provided clear evidence for the association between obesity and the risk of chronic diseases and mortality. The quantification of this association, however, has primarily been accomplished by considering the severity of bodyweight at a single point in time. The potential impact of the duration of obesity as an independent risk factor has been neglected. As the onset of obesity is becoming earlier with their impact on prolonging the duration of obesity in the community suggest that an accurate estimate of these relationships is essential. The objective of this study was to investigate the association between the duration of obesity and the risk of mortality, type-2 diabetes, cardiovascular disease (CVD) and cancer. In addition, the study explored different methods for measuring the duration of obesity, including the first investigation of the effect of combining the severity and the duration of obesity into a single measure of obese-years. To answer these objectives, the study analysed a dataset from a long-term cohort study of the original Framingham Heart Study, where bodyweight and outcomes were measured reliably and regularly at two yearly intervals for up to 48 years. The association was analysed using time-dependent Cox models adjusted for a number of potential confounders. The key finding of this study is that the duration of obesity is directly associated with an increased risk of mortality (all-cause and cause-specific mortality) independent of the level of body mass index. This association was not fully mediated by the chronic diseases of CVD, type-2 diabetes and cancer. It is also shown that the duration of obesity is independently associated with the risk of type-2 diabetes and with CVD. The study detected no association between the duration of obesity and the risk of cancer, except with cancer of the cervix uteri and borderline association with intestine & colon cancer. In general, a dose-response relationship was clear; suggesting that the longer years lived with obesity, the higher the risk of mortality, type-2 diabetes, and CVD. The study quantified that for every additional twoyears lived with obesity, the risk of all-cause mortality increased by 6%, type-2 diabetes by 8%, and the risk of hard CVD by 3%. The increased risk of mortality did not differ between men and women, but the increased risk of type-2 diabetes and hard CVD appeared to be stronger for men than women. The study also found that a combined effect of the severity and the duration of obesity into a single term of “obese-years” provides better estimation of the risk of type-2 diabetes compared with simply using BMI or the duration of obesity alone. This study has implications for future public heath studies; it suggests that the duration of obesity variable should be included in future research estimating the burden of obesity related diseases. The findings in this thesis also indicate that without effective preventive strategies to delay the onset of obesity and to limit its duration, the risk of mortality and chronic diseases in the general population will increase in future.