(A) Over a 62-day NICU course, a Mennonite infant with Down syndrome suffered from duodenal atresia, large ventricular septal defect, and two episodes of sepsis. Serial monitoring shows the affect of catabolic stress on plasma leucine levels, in each case treated with parenteral nutritional therapy to restore net protein anabolism. (B) The figure depicts once weekly plasma leucine values over a 34-week period from two infants, one shown with a solid line and the other with a dashed line. Leucine levels are erratic relative to the normal homeostatic range (blue shaded area), and metabolic control varies considerably between individuals. Episodic leucine elevations (white circles) reflect transient catabolic states triggered by common infections (e.g., gastroenteritis, otitis, bronchiolitis, etc.).