Our method is to measure food access as the cost and affordability of healthy diets

Food environments determine a household’s physical and economic access to sufficient safe and nutritious food for an active and healthy life. To improve measurement of food access, the Food Prices for Nutrition project has developed a suite of indicators known as the Cost and Affordability of a Healthy Diet (CoAHD). These metrics use the availability, price and nutritional composition of retail food items to identify the least expensive combination of items at each time and place that would meet a population’s nutritional requirements for lifelong health. For global monitoring we worked with FAO to develop a version of CoAHD as detailed in our methods brief, with further adaptation for within-country analyses using the approach summarized below. These methods are taught in our World Bank eLearning course (self-paced, approximately 6 hours) and implemented through our software toolkit.

THE COST OF A HEALTHY DIET

The cost and affordability of a healthy diet (CoAHD) is a new way of tracking food and nutrition security worldwide, tracking a population’s physical and economic access to the foods needed for an active and healthy life by identifying the least-cost items needed to meet national food-based dietary guidelines (FBDGs).

The CoAHD approach to measuring food security and access to healthy diets has been widely adopted for analysis across and within countries, bringing together four kinds of data from diverse sources:
(1) consumer prices for foods being sold in local markets, matched to the
(2) food composition of each item, accounting for edible portion and water weight, and
(3) daily requirements for health, using the least-cost items in each food groups to meet dietary guidelines, and
(4) available income, using household surveys or other data to measure affordability of the lowest-cost healthy diets.

International comparisons and within-country studies have used the CoAHD method to distinguish among causes of malnutrition and guide action to improve diet quality at each time and place. The key innovations brought by the Food Prices for Nutrition project are:

• using retail prices in local food environments for the many differentiated items whose cost to consumers reflects the entire food system, thanks to computational techniques using millions of price observations previously collected for other purposes such as inflation monitoring;
• matching items to their nutritional value, using standardized units of measurement to obtain the quantities needed each day for energy balance from a healthy diet, thanks to digitized food composition tables and food classifications from national and international sources;
• identifying the least expensive healthy items to meet dietary needs, based on nutrient requirements and national dietary guidelines, thanks to software tools and protocols in Excel, Stata, R and other computing environments used by analysts in diverse settings; and
• comparing healthy diet costs to actual food spending, based on survey data and national statistics used in previous work on food consumption and diet quality.

Taken together, the four steps of CoAHD analysis reveal whether people have access to healthy diets, and what can be done to improve food systems for health and to meet other development goals. Within-country analyses can be done using locally adapted national dietary guidelines, while international comparisons are facilitated by use of global targets for a Healthy Diet Basket.

THE HEALTHY DIET BASKET

The Healthy Diet Basket (HDB) is a globally-relevant dietary standard designed to reflect the common elements of most national food-based dietary guidelines (FBDGs). The HDB is designed to allow substitution among locally available items, while maintaining energy balance across and within food groups to meet dietary guidelines and also achieve nutrient adequacy. The specific number of items and food groups used in the HDB are based on median recommendations across the ten quantified national guidelines used for SOFI 2020 and 2021, and the global composite HDB was created as a unified standard reflecting the similarities among all food-based dietary guidelines for SOFI 2022 and other efforts to monitor the cost and affordability of healthy diets across countries. At each time and place, a Healthy Diet Basket is composed of eleven locally available items, each in sufficient quantities to maintain energy balance across six food groups. Substitution between items within energy balance allows for variation in the volume and water weight of locally available items, leading to a global average weight of items in each food group and energy balance across food groups as shown below:

OTHER METRICS

The CoAHD and its associated HDB method of monitoring access to a healthy diet is supported by other more granular indicators used for specific purposes. For example, many policies and programs are concerned with value chains for agricultural production and distribution of specific foods, aiming to improve access to the least-cost items in each food group. Other interventions provide safety nets and nutrition assistance aimed at improving access through transfer programs and nutrition education. Researchers also use cost of diet metrics to quantify the vulnerability or resilience of food markets and household access to nutritious foods. What the various metrics developed by the Food Prices for Nutrition project have in common is to match foods with nutritional characteristics needed for health, at different levels of diet quality. Beyond CoAHD, other commonly used indicators include:

Cost of Nutrient Adequacy (CoNA)

This cost level allows people to meet all essential nutrient requirements, using least-cost items to avoid deficiency or excess of total energy as well as protein, fats, carbohydrates, minerals and vitamins. Least-cost diets that achieve nutrient adequacy often do not have other attributes needed for long-term health, as specified in national food-based dietary guidelines.

 Cost of Caloric Adequacy (CoCA)

This cost level provides for just enough energy to meet day-to-day subsistence needs, using the least expensive starchy staple food available on local markets. Such a diet allows for short term survival but the resulting nutrient deficiencies would lead to high rates of disease, disability, and premature mortality.

Least-cost diets as a measure of food access

The innovations associated with Food Prices for Nutrition involve computing least-cost diets at each time and place, to measure a person’s access to locally available foods in sufficient quantities for lifelong health. This approach allows for substitution among food items while maintaining energy balance and criteria based on international standards of diet quality.  

By definition, the purpose of least-cost diets is not to reflect actual consumption. The longstanding use of least-cost diets has been to make recommendations for nutrition assistance in low-income settings, but improved data and modern computing allows us to automate least-cost diet calculations, creating a new kind of price index that measures whether a population already has access to adequate nutrients and a healthy diet. The first major publication comparing countries over time used data from Ghana and Tanzania, and our project has substantially improved and extended the method since then. The purpose of these innovations is to guide systemic interventions: where and when healthy diets are unaffordable, reaching that goal would require some combination of lower prices, higher income and safety nets. Once a healthy diet is affordable, other factors intervene such as nutrition knowledge and decision-making in the household, as well as the difficulty of meal preparation, taste and preferences or aspirations that are shaped by food formulation and marketing.

The cost of a healthy diet is based on global average quantities of food groups recommended in national food-based dietary guidelines (FBDGs). National FBDGs are government policy documents showing how individuals and institutions can meet nutrient needs and protect health, by choosing at least the recommended number and quantity of food items in specific categories. Items are classified into food groups based on nutritional composition, so that choosing one, two or three items within each group in the amounts recommended will, on average, also meet nutrient requirements. Empirically, we find that meeting FBDG targets with least-cost items meets almost all essential nutrient requirements, in almost all countries. Choosing items that meet FBDG recommendations also delivers other bioactive compounds needed for healthy diets such as fiber and phytochemicals. Using items that are being sold at each time and place ensures each food is acceptable in that country, and combining them in proportions specified in FBDGs ensures that the resulting diet meets at least a minimum standard for palatability and cultural norms.

In summary, we use the availability and price of retail consumer items to compute least-cost diets that meet national FBDGs, as a way of quantifying nutrition security and access to a healthy diet around the world. Least-cost diets that meet FBDGs measures whether or not a population can acquire diets that: (1) have adequate levels of all essential nutrients, (2) protect against diet-related diseases such as diabetes or hypertension, and (3) are dignified and culturally appropriate, based on (4) official food policy documents that are regularly updated by a large and growing range of governments in all regions of the world. The dietary standards used for FBDGs are intended for the general population, not just for a privileged few, and many countries use them to guide their nutrition education and assistance programs. For all these reasons, least-cost diets that meet FBDGs are an appropriate nutritional standard against which to measure whether all people, at all times, have physical and economic access to the safe and affordable foods needed for a healthy diet.

Recommended citations

The first use of least-cost items by food group to monitor food environments across countries was:
Masters, W.A., Y. Bai, A. Herforth, D. Sarpong, F. Mishili, J. Kinabo, and J.C. Coates (2018).  Measuring the affordability of nutritious diets in Africa: price indexes for diet diversity and the cost of nutrient adequacy.  American Journal of Agricultural Economics 100(5): 1285-1301. [replication file]

The recommended citation for details on the CoAHD methodology is:
Herforth, A., Venkat, A., Bai, Y., Costlow, L., Holleman, C. & Masters, W.A. (2022). Methods and options to monitor the cost and affordability of a healthy diet globally. Background paper for The State of Food Security and Nutrition in the World 2022. FAO Agricultural Development Economics Working Paper 22-03. Rome, FAO.

For the specific version of CoAHD available on FAOSTAT, the recommended citation is:
Herforth, A., Holleman, C., Bai, Y. & Masters, W.A. (2023). The cost and affordability of a healthy diet (CoAHD) indicators: methods and data sources. Metadata for FAOSTAT domain on Cost and Affordability of a Healthy Diet. Rome, FAO.

For a more general review of this work in context, we recommend:
Masters, W.A., A.B. Finaret and S.A. Block, 2022. The economics of malnutrition: Dietary transition and food system transformation. Handbook of Agricultural Economics, vol. 6: 4997-5083.  Amsterdam: Elsevier. (preprint)

For data and software tools for calculating the Cost of Nutrient Adequacy, see the data & code page of this website.

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