When assessing swallowing in infants, it is critical to have confidence that the liquids presented during the swallow study closely replicate the viscosity of liquids in the infant's typical diet. However, we lack research on rheological properties of frequently used infant formulas or breastmilk, and various forms of barium contrast media used in swallow studies. The aim of the current study was to provide objective viscosity measurements for typical infant liquid diet options and barium contrast media. A TA-Instruments AR2000 Advanced Rheometer was used to measure the viscosity, five standard infant formulas, three barium products and two breastmilk samples. Additionally, this study measured the viscosity of infant formulas and breastmilk when mixed with powdered barium contrast in a 20% weight-to-volume (w/v) concentration. Study findings determined that standard infant formulas and the two breastmilk samples had low viscosities, at the lower end of the National Dysphagia Diet (NDD) thin liquid range. Two specialty formulas tested had much thicker viscosities, close to the NDD nectar-thick liquid range lower boundary. The study showed differences in viscosity between two 60% w/v barium products (Liquid E-ZPaque® and E-Z-Paque® powder); the powdered product had a much lower viscosity, despite identical barium concentration. When E-Z-Paque® powdered barium was mixed in a 20% w/v concentration using water, standard infant formulas or breastmilk, the resulting viscosities were at the lower end of the NDD thin range, and only slightly thicker than the non-barium comparator liquids. When E-Z-Paque® powdered barium was mixed in a 20% w/v concentration with the two thicker specialty formulas (Enfamil AR 20kcal and 24 kcal), unexpected alterations in their original viscosity occurred. These findings highlight the clinical importance of objective measures of viscosity as well as objective data on how infant formulas or breastmilk may change in consistency when mixed with barium.A modified barium swallow (MBS) is considered the gold standard for the evaluation of deglutition disorders. During a pediatric MBS, a commonly used approach is to mix barium Corresponding Author: Jackie Frazier, M.A, 13123 East 16th Avenue, Box 030, Aurora, CO 80045, USA, Phone: (720) 777-6810, Jacqueline.Frazier@childrenscolorado.org. HHS Public Access Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript contrast with the child's typical diet to facilitate optimal cooperation. In addition, to maximize confidence that the MBS represents the child's typical swallow function, it is critical that the viscosity of the food or liquid presented be as close as possible to the viscosity of the child's typical diet. While accurate knowledge of the influence of liquid viscosity on deglutition is important for all children with dysphagia, it is particularly critical in young infants who must meet their nutrition and hydration needs exclusively through liquid intake. Yet, the field currently lacks information regarding the viscosi...