The affordability of a healthy and sustainable diet: an Australian case study

Food insecurity [6] is being exacerbated by climate change, with temperature changes, droughts and/or floods affecting food crops and consequently food accessibility in regions worldwide, including Australia [6‐10]. Increased carbon dioxide in the atmosphere also contributes to a reduction in the nutrient content of food [11‐13], which could have widespread health implications for the global population, in particular those who are already struggling to consume enough quality food to meet their nutritional needs [6, 13].

The extent to which climate change will affect future food security remains uncertain [7, 8], however, what is known is that while climate change affects food systems (e.g. in regard to the food able to be produced and the nutritional quality of this food), food systems also affect climate change (e.g. meat from ruminant animals contributing methane to greenhouse gas emissions) [3, 14], due to their mutually dependent relationship [8, 14‐17]. Indeed, global agriculture and food production accounts for 19–29% of greenhouse gas emissions [18], 70% of freshwater use [19], ≈40% of land use [20], 78% of eutrophication [21], and 94% of the biomass of non-human mammals is livestock [21]. Together, this makes agriculture and food production one of the largest causes of environmental damage [22] which has a great effect on human and planetary health, but is also an area that we have a degree of control over to bring about positive change [3, 23]. In the EAT–Lancet Commission report [3], Willett et al. describe a Great Food Transformation that is predicted to result in healthier diets from sustainable food systems, for the benefit of the entire population and the planet. The need to transition to a more healthy and sustainable diet is echoed by organisations such as the Food and Agriculture Organization of the United Nations [4, 6, 24] and the Food Climate Research Network [25, 26]. The EAT–Lancet Commission report provides some evidence that the most effective way to lessen the environmental impact of our food systems may be to change our diet to a more sustainable one, such as the Planetary Health Diet (PHD) discussed further below [3, 27].

A healthy and sustainable diet has been defined elsewhere but essentially is considered to be a diet that has low environmental impact while contributing to food security and meeting the health and nutritional needs of current and future generations [3, 4, 23, 25, 28‐36]. The Australian Dietary Guidelines (ADG), which have been criticised as having a reductionist approach to diet, consider nutrients first and foremost, not sustainability [37‐39]. Hence, this may not be the diet to propose as optimal, especially given the demands on the food system of the consumption of the amount of meat recommended in the ADG (ruminant meat in particular is a large contributor to greenhouse gasses due to the animals methane output) [3, 14]. Australians generally consume a diet that is neither healthy nor environmentally sustainable [40, 41], though to date few countries have adopted environmental sustainability as a focus in their dietary recommendations. In contrast, Sweden and Brazil are examples of countries who have already incorporated sustainability into their dietary guidelines by including recommendations such as a predominantly plant-based diet based on seasonal and local foods, reducing food waste, and reducing consumption of red and processed meat, ultra-processed foods, and sugar-sweetened beverages [24, 42, 43].

The EAT–Lancet Commission’s report [3] was the first to comprehensively integrate the nutritional needs of individuals with planetary sustainability principles into a single set of global dietary recommendations. The PHD reference diet [44] is an example of a diet that is both healthy and sustainable. This reference diet forms the framework of the PHD recommendations and can be customised to regional cultural preferences [3]. The PHD reference diet was analysed as being nutrient-sufficient, and modelling showed that the intake of most nutrients increased after adoption of this diet compared with current consumption patterns, with the exception of vitamin B12 which needs fortification or supplementation [3], consistent with the current general consensus on mostly plant-based diets [23, 28, 30, 45]. The EAT–Lancet Commission report stated that a global shift in dietary behaviours to align with the PHD could prevent around 19–23% of deaths per year (around 11 million deaths prevented) by way of improved human health [3], however under subsequent further analysis it appears that these prevented deaths may be purely the result of the changes in energy consumption associated with the PHD [46].

For the PHD to be widely adopted, it needs to be acceptable to consumers. While there are several factors that affect consumer food choices, such as accessibility, availability, health concerns and food preferences [2, 47], this review considered purely the role of affordability as a key factor that may influence the uptake of the PHD, while acknowledging there are many other factors that also influence food choices [48]. Cost is generally a major determinant of food choices [49‐57] and, although health and sustainability are desired outcomes of consumer choices, affordability often takes priority, particularly for lower-income consumers [49, 50, 58‐60]. Therefore, it is necessary to understand the cost and affordability of a healthy and sustainable diet, such as the PHD, for a range of socioeconomic groups.

Presently, information about the affordability of healthy and sustainable diets is scant. Only one study appeared to exist on the affordability of a healthy and sustainable food basket across various socio-economic groups in an Australian context (finding an increased cost to purchase the healthy and sustainable basket) [51], but this was not undertaken nationally and the basket did not include all of the sustainability principles incorporated in the PHD such as land use, nitrogen cycling, and phosphorous cycling (the EAT–Lancet Commission’s report was generally more comprehensive and developed specifically to help achieve the United Nations Sustainable Development Goals and Paris Agreement) [3]. Studies also exist that have been undertaken in small regions in Australia such as specific metropolitan areas [51, 61, 62], but not nationally, meaning results cannot be applied to all areas in all states, and national comparisons between different areas in different states is not possible. To our knowledge, a healthy and sustainable food basket based on the PHD has not been created and analysed for affordability nationally across various socio-economic groups in Australia. Country-specific studies are of importance due to the different cultures, customs and food availability in individual countries, as well as differing environmental factors in each country [63, 64]. This is essential for measuring the affordability, and therefore the feasibility, of a healthy and sustainable diet for all Australians. Globally, two studies from United Kingdom have determined the cost of a healthy and sustainable diet and compared it to the typical diet consumed in that country (both finding there was no cost increase to follow a healthy and sustainable diet) [65, 66]. Since the present study was completed, other research on the affordability of the PHD throughout the world has since been published, finding that the PHD was affordable for high-income countries such as Australia, but unaffordable for low-income countries [67].

The aims of this study were to: (a) Determine the affordability of the PHD food basket for low, middle and high socio-economic groups in metropolitan Australia; (b) Determine if the PHD food basket is more or less affordable than the Typical Australian Diet (TAD) food basket for low, middle and high socio-economic groups in metropolitan Australia.

This cross-sectional study developed food baskets for a reference family of four. Food basket surveys were then conducted at Coles supermarket [68] representing the PHD and the TAD baskets (Coles and Woolworths together account for around 80% of the total grocery spend in Australia) [69], to cost the baskets in metropolitan postcodes that vary in socio-economic status, for each Australian state/territory. Metropolitan areas were chosen due to the majority of Australians (71%) dwelling in these areas [70]. The baskets were then analysed using existing secondary data from the Australian Bureau of Statistics (ABS) [71] on area level (dis)advantage and median incomes of those areas to determine affordability.

The reference household represents a common Australian household structure to establish the quantity of food items needed in a food basket [72]. In this study, a family of two adults (19–60 years) and two children (boy 15 years, girl 4 years) was chosen to allow for comparison to other food basket studies using the same reference household [40, 51, 61]. Additionally, the 2016 census reported that the ‘typical Australian’ (i.e. 38 years old, born in Australia of English ancestry) lives as a married couple with two children, making this household structure a sensible and representative choice [73].

To compare across various socio-economic groups, data from the ABS Socio-Economic Indexes for Areas – Relative Socio-economic Advantage and Disadvantage (SEIFA-IRSAD) was used [74]. To cover a wide range of socio-economic groups, one survey area from SEIFA-IRSAD quintile 1 (most disadvantaged), quintile 3 (no real (dis)advantage) and quintile 5 (most advantaged) from each state/territory was selected. Within each quintile in each state/territory, survey areas were defined by postcodes. Postcodes chosen were the median-ranked postcode in each state/territory (Australian Capital Territory was included in New South Wales), and non-metropolitan postcodes were excluded.

The resulting list of survey areas was composed of one postcode in each of three SEIFA-IRSAD quintiles in each of the state/territory capital city metropolitan areas in Australia (Darwin, Sydney, Melbourne, Brisbane, Adelaide, Perth and Hobart).

An assortment of food items from each category listed in the PHD reference diet [44] were selected, informed by the options proposed by Friel, Barosh and Lawrence as being both healthy and sustainable [75]. The items selected enabled sufficient consumption for the reference household for 7 days, were commonly known brands/varieties (as decided at the discretion of lead author), widely available in Australian supermarkets, and allowed for dietary variation over 7 days (the nutritional requirement of the PHD has already been established in the EAT Lancet report) [3]. The basket contents were analysed using FoodWorks v9 [76] software to ensure that the amount and energy intake in each category matched the PHD reference diet [44] as closely as possible.

The PHD reference diet [44] recommended an intake of 1323.8 g of food per adult per day, providing energy of 10,472 kJ. As the reference household used in this study comprises two adults and two children, the basket contents were increased to reflect this. The estimated energy requirements of the 15 year old boy is 12,600 kJ and of the 4 year old girl is 6100 kJ, determined using Nutrient Reference Values [77] using a physical activity level of 1.8 (moderate). Therefore, the total estimated energy requirements of the two children is 18,700 kJ, which is 89% of the combined intake of the two adults (20,944 kJ). Hence, the PHD basket was developed using the daily per-adult quantities in the reference diet [44], then multiplying by two to arrive at the basket contents for both adults, then multiplying by 1.89 to increase the basket contents by 89% to include the children’s needs, and then multiplying by seven to arrive at the final weekly basket amount.

For comparison to the usual diet consumed by Australians, the TAD basket previously developed by Friel, Barosh and Lawrence [41] was used. This pre-established food basket was based on actual consumption habits over 7 days for a reference household of two adults (19–60 years) and two children (boy 15 years, girl 4 years) as determined by national nutrition survey data [40, 41]. The household structure used for the TAD basket was the same as for the PHD basket, allowing for clear comparison. Following construction of the two baskets, each was costed to determine affordability. Additional file 1 shows both the newly-developed PHD basket and the existing TAD basket. The PHD basket matched the PHD reference diet [44] in regards to the quantity of food and energy intake.

Costing was determined using online shopping pricing data from Coles supermarkets [68] to build a hypothetical order of the basket contents to determine the cost of the food items. As Coles Online uses the same pricing for online sales as the store from which the order will be delivered from or collected [68] (confirmed via Coles Customer Care phone call, 21 May 2019), using this online pricing gives an accurate representation of prices as if the basket was purchased in store at one of the 21 postcodes selected. The survey was conducted 14th–15th August 2019.

The cheapest item available for each food item in the food basket was selected, including generic brands and temporarily out of stock items (which were assumed to be otherwise available). The item of the same size/quantity as the food basket item was selected. If there was no item of the same size, a larger size was selected and only the cost of the food basket portion was calculated and included in the basket cost on a unit-cost basis. Only non-sale prices were used.

The collection store entered into the Coles Online website was the same postcode as each survey area to capture the prices from the Coles store that residents of that postcode would likely frequent. In the event there was no Coles store in the survey area postcode, the closest store in a nearby postcode was used. The same food basket contents were used for each survey area and only the collection store changed, to determine the price of the same food basket items in each survey area. If the same item was not available in a particular store, the closest matching item was chosen. If there was no closest matching item available, the price of the item in the nearest survey area was used.

To determine the affordability of the PHD and TAD baskets, income data was required. The median family income in the postcode survey areas was determined using ABS Census data [78]. Family income data was used, as only family members are included and this calculation does not include non-family households such as group or lone households [79]. As the Census median family income data is the total income before tax, an estimate of tax paid and therefore resulting disposable income was performed using an online calculator from the Australian Taxation Office [80].

Affordability of both baskets was calculated and compared for each socio-economic group in the survey areas using the formula Affordability = Cost÷Incomex100, rounded to the nearest whole percent.

Data were analysed using SPSS v23.0 [81], checked for errors, and outliers were included as the 5% trimmed mean values were very similar to the mean values. Tests of normality showed the data was non-parametric, therefore a Mann-Whitney U test was used to determine if there was a significant difference between the median costs of both baskets, and the median affordability of both baskets, using p < 0.05 for statistical significance. Assumptions for the Mann-Whitney U test were met for both tests.

Further, mean cost and affordability were determined for each food basket in each SEIFA-IRSAD quintile in each metropolitan area.

Table 1 details PHD and TAD weekly basket costs and affordability. While the weekly cost of each basket was the same across all SEIFA-IRSAD quintiles for each metropolitan area (i.e. Coles did not vary their prices as postcodes were changed, within areas), the weekly family disposable income varied, meaning the proportion needed to purchase each basket was greater in the lower socio-economic areas (SEIFA-IRSAD quintile 1), and lesser in the higher socio-economic areas (SEIFA-IRSAD quintile 5). At the time of the survey, the weekly cost of the PHD basket was highest in Brisbane metropolitan area ($196.60) and lowest in Sydney metropolitan area ($182.31), with affordability highest in SEIFA-IRSAD quintile 5 of Darwin and Sydney metropolitan areas (9% of weekly disposable income) and lowest in SEIFA-IRSAD quintile 1 of Darwin metropolitan area (24% of weekly disposable income). The PHD basket required a much higher proportion of income in Darwin metropolitan area SEIFA-IRSAD quintile 1 than in other survey areas, and was a statistical outlier in the data set. A comparison of the cost and affordability of PHD and TAD baskets is discussed further below. As a national average, the lowest SEIFA-IRSAD quintile required 17% of the weekly family disposable income to purchase the PHD basket compared to the highest SEIFA-IRSAD quintile at 11%. Affordability for the PHD basket was highest in Darwin and Hobart metropolitan areas (15%), and lowest in Sydney, Melbourne, Adelaide and Perth metropolitan areas (13%).

As shown in Table 1 above, the weekly cost of the TAD was highest in Perth metropolitan area ($234.01) and lowest in Melbourne metropolitan area ($220.43), with affordability highest in SEIFA-IRSAD quintile 5 of Darwin and Sydney metropolitan areas (11% of disposable income) and lowest in SEIFA-IRSAD quintile 1 of Darwin metropolitan area (28% of disposable income). The TAD basket weekly cost of $234.01 was a statistical outlier for all areas in WA. Additionally, the TAD basket required a higher proportion of income (28%) in Darwin metropolitan area SEIFA-IRSAD quintile 1 than in other areas, and was also a statistical outlier.

Figure 1 shows mean and median weekly costs of the PHD and TAD baskets nationally, demonstrating that the PHD costs less than the TAD (median is provided due to statistical test used). A Mann-Whitney U Test [81] on the median weekly costs of both baskets revealed that the PHD basket was significantly less expensive (Md = 188.21, n = 21) than the TAD basket (Md = 224.36, n = 21), U = 0.000, z = − 5.559, p < 0.001, r = − 0.86. The TAD basket also had a larger mean rank (32.00) than the PHD basket (11.00).

Figure 2 demonstrates the PHD is more affordable than the TAD (median is also provided due to statistical test used). A Mann-Whitney U Test [81] on the median affordability of both baskets revealed that the PHD basket (Md = 13, n = 21) was significantly more affordable than the TAD basket (Md = 16, n = 21), U = 128.00, z = − 2.340, p = 0.019, r = − 0.36. The TAD basket also had a larger mean rank (25.90) than the PHD basket (17.10).

Figure 3 shows the comparison between affordability of PHD and TAD baskets across SEIFA-IRSAD quintiles, showing that the PHD basket was more affordable (i.e. required less weekly disposable income) than the TAD basket in all quintiles nationally.

The PHD basket was less expensive than the TAD basket in all metropolitan areas across Australia, with the biggest gap in the Sydney metropolitan area supermarkets where the PHD basket was over $43 per week less expensive than the TAD basket. Consequently, the PHD basket was more affordable than the TAD basket in all state metropolitan areas.

To our knowledge, this study is the first time that a food basket has been developed that is modelled on the PHD, contributing to current literature on both development and use of food baskets, and healthy and sustainable diets for different population groups. Additionally, to our knowledge, this study is the first time that a PHD basket has been costed for affordability in Australia, and that food baskets in general have been costed for affordability nationally across various socio-economic groups. This cost and affordability analysis of the PHD basket fills a gap left by the EAT–Lancet Commission, which did not address the diet’s economic viability for consumers in Australia [3] (global affordability has been analysed in other studies) [67]. Further research is needed to ascertain the financial feasibility of the PHD diet in rural and remote areas of Australia where food baskets cost more than in metropolitan areas [72, 90, 92, 96], as well as constructing and costing PHD food baskets for particular dietary habits such as gluten free and vegetarian and perhaps sub-population groups such as varying employment status, family structure and cultural ancestry. This study suggests that price may not be a major hurdle to shift towards sustainable and nutritious diets, and therefore attention on other consumer behaviours and levers for change is required. Further research is needed to ascertain consumer acceptance of the types and quantities of various foods that should be included in a healthy and sustainable diet, in particular the Planetary Health Diet or similar, and to determine the best method to facilitate this dietary change for all Australians.

This study has shown that the PHD is a potentially affordable, and therefore financially feasible, diet for metropolitan-dwelling Australians regardless of socio-economic status or location. These results can help to inform public health and food policy aimed at achieving a healthy and sustainable future for all Australians. This can lead to a reduction in overweight/obesity rates and subsequent non-communicable diseases, and increased food security in the face of predicted population increases and environmental uncertainty due to global climate change effects. These results add to the available evidence used to promote food and nutrition literacy for Australians, and consumers may transition their dietary behaviour towards the PHD if not to be healthier and more environmentally friendly, then perhaps for financial benefits. For example, the existing Health Star Rating System [97] which currently rates the nutritional profile of food items, could potentially include a comparison of sustainability profiles to help consumers choose food items which fit into the framework of the PHD; or perhaps a new “Planetary Health Diet” or sustainability logo could be developed and used on PHD-compliant food items to encourage consumers to choose wisely. Given the likely increased time and cooking skills required by consumers to adopt the PHD, creating and supporting education campaigns based around food preparation and cooking skills would also be needed.