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A parenteral nutrition order is an arithmetic reconciliation of three macronutrient sources against a caloric goal, a protein goal, and a volume limit. Dextrose in intravenous solutions is the monohydrate form and yields 3.4 kcal per gram rather than the 4 kcal of anhydrous glucose; amino acids yield 4 kcal per gram; lipid emulsions yield 1.1, 2.0, or 3.0 kcal per milliliter at 10, 20, and 30 percent concentrations, counting the glycerol and egg phospholipid as well as the triglyceride itself. Protein is conventionally excluded from the non-protein calorie pool because it is prescribed for anabolism rather than as fuel, so lipid is usually set at 20 to 30 percent of non-protein calories and dextrose supplies the remainder. Two safety ceilings govern the result. The glucose infusion rate, computed as dextrose grams times 1000 divided by weight in kilograms times 1440 minutes, should stay below roughly 4 to 5 mg/kg/min in adults, above which glucose is diverted toward lipogenesis and hyperglycemia and hepatic steatosis follow. Lipid should not exceed about 2.5 g/kg/day, with serum triglycerides kept under 400 mg/dL. Osmolarity decides vascular access: each gram of dextrose contributes about 5 mOsm and each gram of amino acid about 10 mOsm, and anything above roughly 900 mOsm/L requires a central catheter because peripheral veins develop thrombophlebitis. In severely malnourished patients the prescription must be deliberately underfilled at first to avoid refeeding syndrome.

Feeding a person entirely by vein was considered impractical until the late 1960s; earlier attempts failed on vein sclerosis from hypertonic sugar and on the absence of a safe intravenous fat source. Two developments broke the deadlock. In Sweden, Arvid Wretlind produced Intralipid, a soybean oil emulsion stabilized with egg yolk phospholipid, in 1961, after American cottonseed oil emulsions such as Lipomul were withdrawn for toxic reactions. In Philadelphia, Stanley Dudrick, working under Jonathan Rhoads with Harry Vars and Douglas Wilmore at the University of Pennsylvania, showed that beagle puppies could grow normally on intravenous feeding alone, then applied the method through a superior vena cava catheter to an infant with intestinal atresia in 1967. Dudrick and colleagues reported long-term intravenous feeding with growth, development, and positive nitrogen balance in 1968, and the technique spread rapidly through surgical services. Standardization followed the enthusiasm: the American Society for Parenteral and Enteral Nutrition was founded in 1976, and its safe practice documents, including the guidelines led by Jay Mirtallo in 2004 and later revisions, imposed order on compounding, labeling, and compatibility after a series of precipitation and infection incidents. Intralipid was approved in the United States in 1975, while fish oil based emulsions arrived far later, with Omegaven approved by the FDA in 2018 for parenteral nutrition associated cholestasis in children.