How well do micro-economic factors explain obesity rates? Assessing the influence of income and cost of diet on dietary intake and body mass index in a representative UK sample

Abstract

Rates of obesity are predicted to increase, which is worrying given the association with adverse health outcomes. Cost of food or diet is one proposed contributor to an ‘obesogenic environment’. The “food price-obesity hypothesis” supposes that, with limited purchasing power, consumers may purchase energy-dense foods to obtain the maximum calories, resulting in excess energy intake. This thesis attempts to gauge whether obesity may be attributed to food prices. Firstly, the published literature was synthesised. Secondly, the study examined how income and cost of diet are implicated in excess energy intake, as implied by the body mass index (BMI) and dietary energy density (DED), of adults in the National Diet and Nutrition Survey (NDNS). The literature review revealed a heterogeneous body of studies that was generally supportive of the food price-obesity theory, but not conclusive. Studies of diet costs and DED overwhelmingly report a negative association. A limited number of studies investigating diet costs and BMI reported contradictory findings. The evidence linking income and DED was not strong. In the NDNS sample, income was found to be negatively associated with DED, BMI, and overweight/obesity. In addition, a negative association was observed between diet costs and DED. There was no association between whole diet costs and BMI. In contrast, using proportional food group costs revealed some significant associations. This suggests that measuring how people apportion their food budget, rather than how much the whole diet is worth, may be insightful. The thesis also addresses some methodological issues. Firstly, analyses demonstrate how equivalizing household income to take into account household composition can impact on findings. Secondly, a comparison of diet costing methods is presented. Despite methodological challenges, the findings presented in the thesis suggest there is merit in pursuing research into diet costs, with many unexplored opportunities in this emerging field