Socioeconomic inequalities in the healthiness of food choices: Exploring the contributions of food expenditures

Investigations of the contribution of food costs to socioeconomic inequalities in diet quality may have been limited by the use of estimated (vs. actual) food expenditures, not accounting for where individuals shop, and possible reverse mediation between food expenditures and healthiness of food choices. This study aimed to explore the extent to which food expenditure mediates socioeconomic inequalities in the healthiness of household food choices. Observational panel data on take-home food and beverage purchases, including expenditure, throughout 2010 were obtained for 24,879 UK households stratified by occupational social class. Purchases of (1) fruit and vegetables and (2) less-healthy foods/beverages indicated healthiness of choices. Supermarket choice was determined by whether households ever visited market-defined high-price and/or low-price supermarkets. Results showed that higher occupational social class was significantly associated with greater food expenditure, which was in turn associated with healthier purchasing. In mediation analyses, 63% of the socioeconomic differences in choices of less-healthy foods/beverages were mediated by expenditure, and 36% for fruit and vegetables, but these figures were reduced to 53% and 31% respectively when controlling for supermarket choice. However, reverse mediation analyses were also significant, suggesting that 10% of socioeconomic inequalities in expenditure were mediated by healthiness of choices. Findings suggest that lower food expenditure is likely to be a key contributor to less-healthy food choices among lower socioeconomic groups. However, the potential influence of cost may have been overestimated previously if studies did not account for supermarket choice or explore possible reverse mediation between expenditure and healthiness of choices.


Introduction
A body of evidence shows that purchasing and consumption of unhealthy diets, in particular, eating fewer fruits and vegetables, is strongly patterned by socioeconomic status (SES) (Appelhans et al., 2012;Darmon and Drewnowski, 2008;Giskes et al., 2010;Pechey et al., 2013;UK Department for Environment, 2011). One likely contributor to the socioeconomic patterning in healthy diets is diet cost: less nutritious, energy-dense foods are often cheaper sources of calories (Drewnowski, 2010;Jones et al., 2014), and higher diet quality has been associated with higher diet cost (Bernstein et al., 2010;Lee et al., 2011;Rao et al., 2013;Rehm et al., 2015). Moreover, given most research to date has estimated diet cost by linking dietary intake data to prevailing food price data, the socioeconomic inequalities in expenditure may have been underestimated by assuming a constant price for particular foods (i.e. only accounting for differences between types of foods purchased and not variation between brands) (Monsivais et al., 2013). Even so, dietary cost explains some of the relationship between SES and nutrient density of consumed foods (Monsivais et al., 2010), and estimated diet cost has also been shown to mediate the pathway between socioeconomic status (income) and diet quality in a US sample (Aggarwal et al., 2011).
A potentially related avenue of research has suggested consumers who patronize low-priced supermarkets are more likely to have lower-quality diets (Aggarwal et al., 2014b) and higher BMI (Chaix et al., 2012;Drewnowski et al., 2012;Lear et al., 2013). Yet even within the same store, more educated households have been found to make healthier purchases (Handbury et al., 2015). One contributing factor may be prioritizing cost, which may lead to preferences for certain supermarkets and also limit food choices within store (Aggarwal et al., 2014a;. As such, concerns about cost may be driving some of the association between supermarket price tier and healthiness of diet. Conversely, if individuals choose to patronize a particular supermarket for reasons other than price (for example, believing a store to offer a wider range of healthy products), this choice may still contribute to subsequent diet cost, given expenditure is a consequence of customers' product choices. Both choice of products within store and choice of the store itself are likely to depend in part on individuals' motivations (e.g., price, health, convenience), which may vary by SES . As such, the relationship between diet cost and diet quality may be bi-directional. Although the extent of this bi-directionality cannot be determined in cross- sectional analyses, reverse mediation analyses offer an initial exploration of the potential contributions of each factor to socioeconomic inequalities.
Of particular interest is the extent to which financial motivations mediate socioeconomic differences in healthiness of choices, given that this would likely indicate the potential effects of changing food costs on healthiness of diet. However, financial motivations are often not possible to reliably measure and instead food expenditure has often been used as a proxy.
Food expenditure may be influenced by supermarket choice, which may in turn have been influenced by financial motivations, but may also have been determined by other motivations (e.g. convenience). As such, in the current study we will investigate whether food expenditure mediates the pathway between socioeconomic status and healthiness of choices, with and without controlling for supermarket choice. By holding supermarket choice constant, we potentially ignore self-selection to these supermarket tiers. On the other hand, without controlling for supermarket choice, we run the risk of ignoring that selection of supermarket is not necessarily a free choice (e.g. it may be limited by geographic access, which could be associated with SES). So the above mediation models may reflect upper and lower bounds. As such, this study aims to explore a range of values for the possible mediation of expenditure on the socioeconomic differences in healthiness of choices.
This study extends explorations of the role of food costs as a mediator of socioeconomic inequalities in healthiness of choices; firstly, by looking at actual expenditure (rather than estimated diet costs) in a large UK sample. Secondly, reverse mediation between food expenditure and healthiness of choices will be explored. Thirdly, it will explore the associations between SES, food expenditure and healthiness of choices alongside the contribution of supermarket choice.

Sample
Data were obtained from the Kantar WorldPanel (KWP) UK household survey from 2010 (as this involved analyzing de-identified existing data, ethical approval was not required). The sample consists of an ongoing panel, originally recruited via post or email to be representative of the UK in terms of age group, household size and region of residence.
Households must meet minimum volume and spending criteria based on household size for inclusion, based on 4-week purchasing blocks. Further details of sample recruitment and quality control have been described elsewhere .
Participating households (n=24,879) recorded all food and beverage purchases brought home (i.e. excluding purchases that were consumed away from home), including volume purchased, spend, nutritional content, and the retail chain from which products were purchased. Sociodemographic data including number of adults and children in each household, ages and genders of household members, and socioeconomic indicators were also collected.

Food expenditure
Expenditure was calculated from the households' total spend (£) on take-home food and beverages over the 52 week period, divided by the total number of calories those purchases for the same period, multiplied by 2000 to give an energy-adjusted food expenditure variable (£ per 2000 kcal).

Supermarket choice
We defined supermarket choice as in : firstly, supermarkets were categorized as high-, medium-or low-cost based on market definitions (Food and Drink Economics branch: DEFRA, 2006; USDA Foreign Agricultural Service: Global Agriculture Information Network, 2013). Households were then classified according whether or not they ever patronized high-or low-cost supermarkets (in addition to medium-cost supermarkets, which were almost universally patronized), giving four groups: Used low-cost supermarkets exclusively or low-and medium-cost supermarkets ('Low-cost'); Used medium-cost supermarkets only ('Medium-cost'); Used high-cost supermarkets exclusively or mediumand high-cost supermarkets ('High-cost'); Used all three tiers of supermarkets ('All-types').

Healthiness of food and beverage choices
Two outcome variables assessed healthiness of food and beverage choices, comprising lesshealthy and healthier indices: 1. Percentage of food energy purchased from less-healthy foods and non-alcoholic beverages, as classified by FSA Nutrient Profile (Rayner et al., 2005) scores for individual products (Scores are calculated from the energy, saturated fat, sugar, sodium, fiber, protein, and fruit, vegetable and nut content, per 100g; foods scoring 4 or more, and beverages 1 or more, are categorized as less-healthy).
2. Percentage of food energy purchased from fruit and vegetablesthis included fresh, canned, frozen and dried fruit, vegetables and legumes, but excluded juice, potatoes, and fruit and vegetables present in processed products.

Statistical analysis
Firstly, multiple regression analyses were conducted to explore the pathways linking

Descriptive analyses
The descriptive data presented in Table 1 indicated socioeconomic inequalities in expenditure (divided into quintiles for these descriptive analyses in order to explore patterning), with those in the lowest SES group tending to fall into lower quintiles of expenditure, and vice versa for those in the highest SES group. Expenditure for those in the middle SES group was fairly evenly distributed across the quintiles. The table also suggests that supermarket choice may be associated with expenditure, with 59% of those in the lowest quintile of expenditure shopping at low-/medium-cost stores compared to 16% in the highest expenditure quintile. Conversely, 3% of the lowest expenditure quintile vs. 29% of the highest quintile shopped at high-/medium-cost stores.  Table 1 also suggests a trend with those in the higher quintiles of expenditure purchasing higher percent energy from fruit and vegetables and lower percent energy from less-healthy foods and beverages than those in lower quintiles of expenditure.

Multiple regression analyses
Expenditure Figure 2 shows the results of a regression analysis estimating expenditure among occupational social class and supermarket choice groups, with both higher SES and highercost supermarkets being significantly associated with greater expenditure. The variation in expenditure between supermarket choice groups (approximately £0.90 -£1 difference between groups) was greater than the variation between SES groups (around £0.50-£0.60).
This variation by supermarket choice groups was consistent across SES groups, with heterogeneity in food expenditure even in the lower SES group.

FIGURE 2 ABOUT HERE
Healthiness of choices Table 2 shows the results of the three models of expenditure as a predictor of the healthiness of choices. In terms of expenditure, in the basic model (Model 1: expenditure and demographic variables only) a 20% increase (equating to a household at the median value within each expenditure quintile moving into the quintile above) was associated with a 0.7 percentage point decrease in the percent energy from less-healthy foods and beverages, whereas a 20% increase in expenditure was associated with a 7.2% increase in percent energy from fruit and vegetables. There was little change to the coefficients for expenditure between the different models (when adding occupational social class (Model 2) and then supermarket choice (Model 3)), although the increase in percent energy from fruit and vegetables was slightly reduced, to 6.4%. Figure 3 shows socioeconomic inequalities in healthiness of choices, with and without controlling for expenditure, suggesting that socioeconomic inequalities in healthiness of choices was reduced when expenditure was included in the models for both outcomes.   Analyses from hierarchical regression models that clustered households within regions (rather than adjusting for region) showed similar results. Supplementary Table S1 shows percent energy purchased from less-healthy foods and from fruits and vegetables estimated from multivariable regression models (like Table 2) and again from hierarchical models.

Mediation analyses
Additionally, estimated food expenditure across socioeconomic groups was similar whether based on hierarchical models or models that adjusted for region (Supplementary Table S2).

Discussion
This analysis of a large UK dataset, employing detailed scanner data, suggests that food costs may be an important contributor to socioeconomic inequalities in healthiness of food and beverage choices. Our analyses (using actual rather than estimated food expenditure) supported the findings of previous studies in that higher SES households were found to have significantly higher food spending and also had somewhat healthier patterns of food A C C E P T E D M A N U S C R I P T

ACCEPTED MANUSCRIPT
10 purchasing (2-6). Independent of SES, food expenditure had a small positive association with healthier patterns of food and beverage purchasing (Andrieu et al., 2005;Bernstein et al., 2010;Darmon et al., 2005;Maillot et al., 2007;Monsivais et al., 2013). In order to explore these relationships in more detail, we investigated the role of expenditure as a mediator of socioeconomic inequalities in healthiness of choices, going beyond previous research by analysing: (1) reverse mediation and (2) the likely range of values for these mediation effects in this dataset by comparing analyses with and without controlling for supermarket choice.

Expenditure as a mediator
Further exploration of the role of expenditure suggested this may be a significant mediator of socioeconomic inequalities in healthiness of food and beverage choices, as has been found in US studies, using different indicators of SES and diet quality (Aggarwal et al., 2011). In addition, while the association between expenditure and healthiness of choices was larger for fruit and vegetables than for less-healthy items, the extent to which expenditure mediated socioeconomic inequalities was greater for less-healthy foods and beverages (63%) than for fruit and vegetables (35%). This may tie in with previous findings that less mediation was seen when looking at the mean adequacy ratio (representing micronutrients in the diet, and perhaps healthier purchases) than when using energy density, which is likely to reflect the ratio of healthier to less-healthy foods and beverages (Aggarwal et al., 2011).

Healthiness of choices as a mediator
However, the reverse mediation pathway (healthiness of choices as a mediator of socioeconomic inequalities in expenditure) was also significant, albeit appearing smaller.
This highlights the need for caution when interpreting mediation results from cross-sectional analyses. The prioritisation of either price or health could lead to socioeconomic patterning of both food expenditure and healthiness of food choices: prioritising price may constrain healthiness of choices, while prioritising health may necessitate higher expenditure.
Moreover, prioritisation of price vs. health may vary by socioeconomic group, with previous studies suggesting that lower SES groups are more likely than higher SES groups to prioritise price, and vice versa for prioritising health (Bowman, 2006;Konttinen et al., 2013;. One question of interest to public health researchers is whether changing food costs could influence healthiness of choices and/or on socioeconomic inequalities in choices.
If the reverse mediation does indicate a bi-directional relationship, one consequence may be that the possible range of effects of price changes on socioeconomic differences in diet healthiness are prone to over-estimation in analyses where causality is not determined.

Supermarket choice and expenditure
Supermarket choice was systematically associated with food expenditure (see Table 1). To explore the role of supermarket choice in these associations between SES, food expenditures and healthiness of choices, we additionally controlled for this variable in the analyses, finding this had no or limited effects on the coefficients for expenditure on healthiness of choices.
When controlling for supermarket choice in mediation analyses, the proportion of socioeconomic inequalities in healthiness of choices that was mediated by expenditure was reduced for both of the outcome variables, though even controlling for supermarket tier, we observed substantial mediation of socioeconomic inequalities by expenditure. Individuals' choice of supermarket may be determined in part by factors directly linked to expenditure, e.g. financial constraints, but are also likely to take account of other motivations (such as convenience, perceptions of quality or preferences for particular product ranges).
Supermarket choice may influence expenditure for patrons to some extent regardless of their initial motivations. By comparing the mediation effects with and without supermarket choice, this study allows us to examine a likely range of values for the mediation effects of food expenditure in this dataset. As such, the influence of supermarket choice in these mediation analyses may in part represent other motivations that may influence healthiness of food choices and vary by SES, for example, health attitudes or knowledge (Aggarwal et al., 2014b;McKinnon et al., 2014;Turrell and Kavanagh, 2006).

Implications for research
By exploring the role of expenditure across these different analyses, this study considers the potential contribution of food costs to socioeconomic differences in the healthiness of food choices. While each of the analyses in this study suggested that food costs were likely to be playing a role in socioeconomic differences in food purchasing choices, this paper highlights uncertainties in determining the size of this contribution. In order to explore this more fully, future studies investigating the pathways illustrated in Figure 1, and in particular, integrating households' motivations in choosing stores and products, would be beneficial. Such analyses might help establish the extent to which these different pathways may reflect different households' behaviour (including the extent to which supermarket choice may result from financial constraints or other motivations), and to what extent this varies by SES. In particular, integrating these results alongside those of experimental or intervention studies could help to disentangle possible bi-directionality in the pathways between expenditure and healthiness of choices. Indeed, the findings are consistent with intervention studies that have provided financial incentives for improving diet; for example, cash-back and subsidy schemes are providing evidence of positive effects of changing food prices on healthiness of choices in low-and middle-income households Klerman et al., 2014;Ni Mhurchu et al., 2010;Waterlander et al., 2012). Unpicking the pathway between socioeconomic status, expenditure and healthiness of food choices in detail could help inform more effective programs and policies to promote healthier food choices.

Methodological considerations and limitations
Several limitations need to be borne in mind, however; not least that the data were crosssectional. Moreover, these results reflect purchasing, and as such may not translate directly to diet. It should be noted that the overall low volumes of food and beverages recorded in this ). In addition, the mean expenditure per calorie is very similar between these two datasets (Living Costs and Food Survey: 0.172 pence/kcal; study dataset: 0.171 pence/kcal).

Conclusions
In summary, this study involved actual food expenditure data and allowed a more nuanced exploration of the potential role of this variable in the socioeconomic inequalities of healthiness of food choices than has previously been reported. These findings suggest cost is still likely to be a significant contributor to healthiness of food choices, and that a sizeable proportion of the socioeconomic inequalities in healthiness of choices may be mediated by expenditure in a large UK sample. This suggests actual and/or perceived cost of healthy diets may be key factors in tackling socioeconomic disparities in food purchasing choices.

ACCEPTED MANUSCRIPT
23 Socioeconomic inequalities in the healthiness of food choices:

Exploring the contributions of food expenditures
Highlights -Large dataset linking detailed food expenditure data with healthiness of choices -Lower food expenditure mediates less-healthy choices for lower SES groups -Reverse mediation was also observed, however; albeit to a smaller degree -The potential influence of cost may have been overestimated previously