"The International Food Policy Research Institute’s (IFPRI’s) Hunger Index and FAO’s annual flagship publication, The State of Food Insecurity in the World (SOFI)—jointly prepared by the Food and Agriculture Organization of the United Nations (FAO), the International Fund for Agricultural Development (IFAD) and the World Food Program (WFP)—both publish prevalence of undernourishment for most countries in the world. ...

SOFI’s [ie the UN's] prevalence of undernourishment measure indicates the likelihood that a person in a given country finds herself food insecure, as measured against the minimum dietary energy requirement (MDER), or the amount of energy needed for light activity and minimum acceptable weight for attained height. The MDER differs by country as it depends on demographic, cultural, geographic, and climatic conditions within each country. This target is between 1,700 and 1,900 kcal per capita per day (the world average for 2014-16 is 1,844).

The USDA/ERS IFSA report uses 2,100 kcal per capita per day as a nutritional target, which is the average amount of energy needed to sustain a moderate level of activity."

USDA ERS - Questions & Answers
United States Department of Agriculture
Economic Research Service
Version of January 30, 2017
https://www.ers.usda.gov/topics/international-markets-trade/global-food-security/questions-answers.aspx#fao