The association of fast food, fruit and vegetable prices with dietary intakes among US adults: is there modification by family income?

We examined the effects of prices of fast foods and fruits and vegetables on dietary intake, body mass index (BMI) and obesity risks and whether the associations varied across groups according to their family income. Data from the US Department of Agriculture Continuing Survey of Food Intakes by Individuals (CSFII 1994-96) for 7331 individuals aged 20-65 years with complete data on two 24-h recalls were used. We computed two food price indices (FFPI and FVPI) which were linked to individuals through geocoded identifiers. Main outcomes included dietary intakes of energy, selected nutrients and food groups, fast food consumption, and diet quality measured using two indices (HEI and aMED), BMI and obesity. Interaction terms between key variables were tested in regression analyses and in further stratified analysis by family income. Higher fast food price indices (FFPIs) were associated with higher fiber intake, lower saturated fat, and better overall diet quality as measured by aMED. FVPI was positively associated with improved dietary quality as well as in terms of lower cholesterol and sodium intakes, improved HEI and lower BMI. Most of these associations showed homogeneous strengths across income groups as evidenced by a non-significant FFPIxPIR or FVPIxPIR interaction term (p>0.10). While increasing FFPI by 1 standard deviation was only borderline protective against fast food consumption, its association with other binary outcomes that were considered was non-significant. In contrast, FVPI was protective against obesity, particularly among the near poor. It was also associated with improved aMED score. Analyses of these national data suggest that changing fast food and fruit and vegetable prices may affect people's dietary quality and to some extent their adiposity, although the present study is limited by the available food price data.