Your understanding of the FAO method is faulty.
You understood that it "(i) determines the minimum caloric need of adults in a population, (ii) estimates the number of adults whose long-term caloric intake falls below this level and (iii) concludes that this is the number of chronically undernourished adults."
All three points are ill informed, and the remainder of your argument is therefore irrelevant as a criticism of FAO estimates.
(i) we determine the range of caloric needs in the population taking into consideration the distribution of people by gender, age, body masses (proxied by attained height) and physical activity levels; we therefore do not focus just on adults, and we recognize that there are various types of pshysical activity levels ranging from sedentary to very active;
(ii) we estimate the proportion of individuals (not only adults) whose average caloric intake over the year falls below their specific caloric requirement;
(iii) multiply the estimated proportion by the total population size to obtain an estimate of the total number of individuals (not only adults) who are "undernourished" in the specific sense of likely having insufficient caloric intake to fulfill the needs for a normal and healthy life.
Carlo Cafiero, PhD
Project Manager of “Voices of the Hungry”
Statistics Division (ESS)
Food and Agriculture Organization of the United Nations (FAO)
Viale delle Terme di Caracalla, 00153 Rome, Italy
Tel: +39 06 570 53895