The metrics we use

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.

THE COST OF A HEALTHY DIET

The cost of a healthy diet is defined as the least expensive combination of locally available items that would meet food-based dietary guidelines, based on the number and quantity of items in each food group. Least cost diets that meet dietary guidelines also achieve adequacy for almost all essential nutrients, with additional benefits from energy balance between food groups.

The cost and affordability of a healthy diet (CoAHD) is one of several possible ways of measuring food access. The CoAHD approach measures a person’s physical and economic access to locally available foods, in quantities sufficient for an active and healthy life, using the least-cost items needed to meet national food-based dietary guidelines (FBDGs) . This approach is particularly useful for global monitoring because it measures food access using existing data collected for other purposes, and quantifies the affordability of meeting diet quality standards defined by national governments in their official policy statements on nutrition and health.

We measure the cost of access to a healthy diet using three kinds of data:
(1) food prices at local markets, revealing the availability and cost of items commonly consumed,
(2) nutritional composition of each item, accounting for edible portion and water weight,
(3) daily requirements for a healthy diet, using the least-cost items in each food groups.
Then for affordability, we compare the total cost per day at each time and place to the population’s available income, based on household surveys or other data.

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:

by volume (% grams)

by energy share (% kilocalorie)

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, the difficulty of meal preparation, taste and preferences or aspirations and differences among foods in their 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.

For more information on the methodology, see: Herforth, A., Y. Bai, A. Venkat, K. Mahrt, A. Ebel, & W. A. Masters. Cost and affordability of healthy diets across and within countries: Background paper for the State of Food Security and Nutrition in the World 2020. FAO Agricultural Development Economics Technical Study, No. 9. Rome, Italy: Food & Agriculture Org., 2020.

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

For more information on considerations related to the Cost of Nutrient Adequacy calculation, see also: Schneider, K. & A. Herforth, A. (2020). Software tools for practical application of human nutrient requirements in food-based social science research. Gates Open Research 4:179.

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