Global variation in the cost of a nutrient-adequate diet by population group: an observational study

Abstract

Background: Nutrient deficiencies limit human development and could be caused by the high cost of locally available foods needed to meet nutrient requirements. We aimed to identify the populations whose nutrient needs are most difficult to meet with existing global food systems.

Methods: In this observational study, we used the International Comparison Program 2017 collection of global food prices to measure cost per day and cost per calorie of meeting nutrient needs, based on least-cost diets within upper and lower bounds for energy and 20 nutrients for healthy populations across 20 demographic groups in 172 countries. We then analysed the composition of these least-cost diets by food groups to estimate how the affordability of foods for meeting nutrient needs varied by age, sex, and reproductive status.

Findings: In 2017, the global median of diet costs per day was US$2·32 (IQR 1·95-2·76), with cost highest for adolescent boys aged 14-18 years at $2·72 (2·31-3·15). For females, median cost was highest for adolescents aged 14-18 years during pregnancy and lactation at $2·64 (2·29-3·15), exceeding the cost for adult men aged 19-30 years. The global median of diet cost per 1000 kcal was $0·94 (IQR 0·80-1·12), and was higher for females throughout the life course than for males, peaking for adolescent girls aged 9-13 years ($1·17 [95% CI 1·15-1·19]) and women older than 70 years ($1·18 [1·17-1·19]). Diet costs were most sensitive to requirements for calcium, iron, zinc, and vitamins C and E, as well as the upper bounds on carbohydrates and sodium. Total diet costs per day did not vary significantly with national income; however, in high-income countries, the composition of least-cost diets included more animal-source foods, whereas in low-income countries, diets with more pulses, nuts and seeds, and fruits and vegetables provided the most affordable way to meet nutrient requirements.

Interpretation: Diets with adequate nutrients were unaffordable for many demographic groups, especially women and girls. These results could help to guide agriculture and food policy or transfer programmes to support populations at risk of inadequate intake.

Funding: The Bill & Melinda Gates Foundation and UKAid.

Figure 1

CoNA for 20 demographic groups across 172 countries in 2017 by national income category (A) The CoNA in US$ per day reflects the range of cost per day in 2017 for sufficient quantities of the most affordable foods available in each country to meet daily nutrient requirements for a healthy population in each demographic group. The horizontal dashed line facilitates comparison to a global poverty line with food expenditure levels of $1·32 per day, corresponding to 63% of $2·10 per day in expenditure on all goods and services in 2017. (B) The ratio of CoNA to national food expenditure per capita reflects the range of ratios between the least-cost diet and the observed quantity of food expenditure per capita per day in 2017, using the total annual food expenditure in the International Comparison Program 2017 divided by the 2017 population and 365 days. The horizontal line at 1 refers to the country's actual mean food expenditure per capita. (C) The CoNA per 1000 kcal reflects the range of diet costs per 1000 kcal for sufficient quantities of the most affordable foods available in each country to meet median daily nutrient requirements for a healthy population in each demographic group. Vertical lines from the box plots show the range of median (1·58 × IQR/N^1/2). CoNA=cost of nutrient adequacy.

Figure 2

Variation in mean diet cost among demographic groups CoNA per day (A) and per 1000 kcal (B). All data shown refer to the least-cost diet meeting all nutrient requirements for the median person in a healthy population from each group. Error bars show 95% CI around the mean for each group, from a regression with the fixed effects of 172 countries that controlled for each country's differences in mean costs for all demographic groups. CoNA=cost of nutrient adequacy.

Figure 3

Composition of least-cost diets by food category and demographic group Quantities per day of starchy staples (A); pulses, nuts, and seeds (B); animal-source foods (C); fruits and vegetables (D); oils and fats (E); and sweets and beverages (F) that meet all nutrient requirements in a least-cost diet for the median person in a healthy population from each group. Error bars show 95% CI around the mean for each group, from a regression with the fixed effects of 172 countries that controlled for each country's differences in mean costs for all demographic groups.

Figure 4

Sensitivity of diet costs to selected nutrient requirements by demographic group Shadow price elasticities, defined as the percentage change in diet cost for each 1% change in the nutrient requirement shown, for the eight most important constraints: energy (A); the EARs for calcium (B), iron (C), zinc (D), vitamin C (E), and vitamin E (F); the upper bound limit (ADMR) of carbohydrates (G); and the upper bound limit of sodium for chronic disease risk reduction (H). For comparability purposes, we used the magnitude (absolute value) of shadow price elasticities for the upper bound limits of carbohydrate and sodium. Error bars show 95% CI around the mean for each group, from a regression with the fixed effects of 172 countries that controlled for each country's differences in mean costs for all demographic groups. EAR=estimated average requirement. AMDR=acceptable macronutrient distribution range.