Fortified blended foods (FBF) are used for the prevention and treatment of moderate acute malnutrition (MAM) in nutritionally vulnerable individuals, particularly children. A recent review of FBF recommended the addition of animal source food (ASF) in the form of whey protein concentrate (WPC), especially to corn-soy blends. The justification for this recommendation includes the potential of ASF to increase length, weight, muscle mass accretion and recovery from wasting, as well as to improve protein quality and provide essential growth factors. Evidence was collected from the following four different types of studies: (1) epidemiological; (2) ASF versus no intervention or a low-calorie control; (3) ASF versus an isocaloric non-ASF; and (4) ASF versus an isocaloric, isonitrogenous non-ASF. Epidemiological studies consistently associated improved growth outcomes with ASF consumption; however, little evidence from isocaloric and isocaloric, isonitrogenous interventions was found to support the inclusion of meat or milk in FBF. Evidence suggests that whey may benefit muscle mass accretion, but not linear growth. Overall, little evidence supports the costly addition of WPC to FBFs. Further, randomized isocaloric, isonitrogenous ASF interventions with nutritionally vulnerable children are needed.