A quarter of adult deaths are attributable to suboptimal diets globally, with cardiovascular (CVD) and metabolic diseases being the leading cause of diet-related deaths. The aim of this thesis was to investigate the relationship of sex and gender with diet and cardiometabolic diseases via a range of geographically diverse studies, through a mixed methods approach. Three quantitative studies focusing on sex differences were conducted: one looking at biases in relation to self-reported energy intake by a systematic review and meta-analysis; one cross-sectional analysis of dietary behaviours and associations with cardiometabolic risk factors in seven low- and middle-income countries (LMICs); and a prospective analysis of cohort data from the UK looking at dietary intake and associated risks of CVD and premature mortality. Questions arising from these studies were explored through qualitative studies in Fiji: a policy landscape analysis and focus group discussions to understand gender differences in diet knowledge, attitudes and behaviours and gender considerations in policies. No sex bias in the accuracy of dietary assessment was identified, with similar levels of energy underestimation by women and men. Across the seven LMICs in the cross-sectional analysis, both women and men had poor dietary behaviours, however, women who reported positive (good) salt use behaviour were less likely to have undiagnosed hypertension (not evident for men). Diets of women and men were also poor in the UK cohort, yet an association between specific combinations of macronutrients and a reduced risk of mortality was identified for women and men, and a reduced risk of CVD for men. The policy analysis conducted in Fiji revealed a conflation between “gender” and “reproductive health”, and that gender differences in diet-related diseases were not viewed as policy issues. Finally, the focus group discussions identified gender constructs around food, however, upstream determinants of poor diets such as climate change and socioeconomic factors were identified as crucial influences on diet, by women and men. Collectively, findings identified poor diets for both sexes, with some modest sex differences in associations between diet and disease, which are unlikely due to differences in reporting. Results from the qualitative studies highlight the importance of considering gender in view of other equity factors. These findings will be important in the development of equitable food policy globally.