Total parenteral nutrition (TPN) frequently is used to provide adequate nutrition intravenously in small animals and humans, and it is becoming more commonly used in horses and foals when feeding via the gastrointestinal tract is impractical, inadequate, ill-advised, or impossible. The term parenteral means administration by means other than orally. The term "Total" parenteral nutrition is misleading at times because "partial" parenteral nutrition also is used (giving only part of the total daily calories necessary by the intravenous route).

There are three basic nutritional elements that the body uses as building blocks for tissue and the generation of energy: carbohydrates (sugars, of both a simple and complex nature), proteins (and when in their most basic form, amino acids), and lipids (fat). Normally these nutritional components are ingested orally, digested within the gastrointestinal system, and the vital carbohydrates, proteins, and fats are absorbed into the bloodstream from the gastrointestinal system. If the gastrointestinal system is "off-line" for whatever reason, these nutritional factors can be administered directly into the bloodstream.

This treatment can be vital because the daily caloric need exists regardless of the inability to ingest/digest food. In fact, in the case of illness, this becomes an even more important factor, as with many disease states, the daily caloric need can be greatly increased (almost doubled during some conditions).

The normal function of the immune system as well as any healing that might need to take place is highly dependent on adequate nutrition. For example, the caloric needs of a horse recovering from colic surgery might be nearly double that of a normal horse. Without those calories and tissue building blocks, the healing of the surgical site could be delayed. In addition, as the body uses vast amounts of energy and protein stores, a significant amount of weight could be lost if the daily caloric and protein intake is not sufficient