Could it be anything to do with nutrition or malnutrition?
• Mid — Upper Arm Circumference
(MUAC) MUAC is relatively easy to measure and a good predictor of immediate risk of death. It is used for rapid screening of acute malnutrition from the 6-59 month age range (MUAC overestimates rates of malnutrition in the 6-12 month age group).
MUAC can be used for screening in emergency situations but is not typically used for evaluation purposes. (MSF, 1995)
MUAC is recommended for assessing acute adult undernutrition and for estimating prevalence of undernutrition at the population level.
http://www.pronutrition.org/files/Anthropometric.pdf
See page 13 of the document.