The Associations Between Consumption of Coffee and Soy Food With Health Outcomes.

Obesity has become a global epidemic, and obesity prevalence rose from 4.8% to 9.8% in men and from 7.9% to 13.8% in women between 1980 and 2008 1. Preventing obesity and related chronic diseases, especially type 2 diabetes (T2D) and cardiovascular disease (CVD), is of crucial public health significance 2. Identification of dietary factors that are beneficial to health is of high priority. This dissertation focused on two kinds of foods, coffee and soy food, and their associations with obesity-related health outcomes. In Chapter 1, the association of coffee consumption with cardiovascular disease has been investigated in numerous epidemiological studies 3-5. However, a key issue that remains to be resolved is the dose-response relationship of long-term coffee consumption with CVD risk, including incidence of coronary heart disease, stroke, and heart failure, and CVD mortality. In the current study, I examined the association between coffee consumption and risk of CVD by meta-analyzing results from 36 prospective cohort studies with 1,279,804 study participants and 36,352 CVD cases. In Chapter 2, the study described in Chapter 1 showed that coffee consumption was non-linearly associated with risk of CVD: moderate coffee consumption was associated with lower risk of CVD, with the lowest CVD risk at 3 to 5 cups per day, and heavy coffee consumption was not associated with risk of CVD. However, whether the non-linear association was due to a true biological effect or confounding of smoking is not known. In this chapter, with 208,501 participants and 31,956 deaths in three large cohort studies, I prospectively examined the associations of coffee consumption with total mortality and cause-specific mortality among the overall population as well as never smokers. In Chapters 3 and 4, the association of isoflavone intake with risk of T2D has been assessed, and mixed results were observed 6-8. Clinical trials showed that isoflavone supplements did not improve glucose control 9-10, however, these clinical trials were limited by small sample size and short duration of follow-up. Therefore, I further examined the association of soy food with risk of type 2 diabetes. Two different approaches were used to assess soy food intake. First, soy food assessed by food frequency questionnaire (FFQ) was used as main exposure, and the association of soy food consumption with risk of type 2 diabetes was examined prospectively in three Harvard cohorts. Second, urinary isoflavones excretion was used as main exposure, and the association of urinary isoflavones concentration with risk of type 2 diabetes was assessed using a nested case-control design