The debate of which route of nutrition to use is now fairly well settled. Most people will agree that the enteral route is preferred whenever it is available. So, the question "which is better" is probably the wrong question. The correct questions would be "when is it appropriate to resort to TPN", or "When NOT to use enteral nutrition". The indications for enteral nutrition are expanding (eg. whereas previously we would have given a severe pancreatitis patient on complete bowel rest, these days they are fed early, and without any adverse consequences). And there is still controversy as to when and how to start it (eg. how long one can safely wait before resorting to parenteral nutrition - some say it is safe to wait for anywhere up to 7 days, whereas others recommend to start supplemental PN as soon as it becomes clear that the enteral route is not effective).
In summary, enteral nutrition improves GI blood flow and improves GI mucosal health. It maintains villous height by causing the release of trophic agents e.g.. cholecystokinin, bombesin, bile salts, etc – which cause the GI mucosal cells to proliferate. Conversely, the GI mucosa atrophies if it is not regularly exposed to food. This gives rise to gastric ulceration and malabsorption due to villus height loss.
Gut permeability increases in critical illness. It happens within hours of the insult. This results in bacterial translocation. The more severe the illness, the more permeable the gut, and the more bacterial translocation occurs. But, more weirdly,
Adequate enteral nutrition impacts positively on the systemic immune response. A well-fed immune system is a more vigorous defence force. There also seems to be a link between a healthy GALT (Gastrointestinal-Associated Lymphoid Tissue) in the GI tract and a healthy MALT (Mucosa-Associated Lymphoid Tissue) in the respiratory tract, which may explain why enteral nutrition has been shown to decrease the risk of pneumonia in critically ill patients (specifically trauma patients).
The gut is an organ designed to extract useful nutrients from the diet. The efficiency of nutrient reclamation is therefore always going to be better from enteric formulae than from parenteral supplements because parenteral supplements are formulated with limited knowledge of what the patient's requirements actually are. The delivery of glucose may be all wrong, for example (the patient did not want so much glucose). The absorption of amino acids is never going to lead to an excess Thus, one can expecty
There are several non-clinical advantages to enteric feeding when compared to pareneteral nutrition. Specifically, it is cheap and easy to use. Enteric formulae are easier to prepare: it does not have to be super-sterile, and this is reflected in its bargain cost. The gut is an organ designed to receive dirty fungus-encrusted bacterially contaminated food from the Olduvai savannah, and to decontaminate it, making it safe for use - one may therefore tolerate a certain bug content in one's enteric feeds. Stomach acid will take care of those. Moreover, EN does not require a central line to administer, which drives down cost even more (a central line costs substantially more than a nasogastric tube, not to mention the cost of the consumables and sterile line access).
Not only is there no need for central line access (which reduces the risk of line-related complications), but the infecton risk associated with PN is also abolished. PN causes more infections because it is a very effective culture medium. Apart from this, there are many other complications associated with PN which (though non-life-threatening) are undesirable, such as fat overload, fatty liver, immune dysfunction due to intravenous lipids, hyperglycaemia due to hyperalimentation, and the normal anion gap metabolic acidosis which occurs when you metabolise all the lysine and leave the chloride behind.
It is not all roses. The complications of enteral nutrition are discussed in greater detail elsewhere. In brief:
Or, there may be actual hard contraindications:
In spite of the abovenoted theoretical benefits of enteral nutrition (and the much-spoke-op evils of PN), nobody has ever been able to demonstrate that enteral nutrition has any sort of mortality benefit when compared to PN. Not only the recent CALORIES trial (Harvey et al, 2014) had failed to find any mortality difference: five different meta analysis articles agree on this. Seems like nutrition in general improves mortality, rather than the route of nutrition specifically. Alternatively, there may be specific groups who benefit from a specific approach to nutrition, and this is not obvious from the undifferentiated populations of large trials.
If not mortality, then length of stay, severity of SIRS, or perhaps risk of infection are affected. Evidence in support of this exists in specific patient groups. Enteral nutrition is demonstrably better than TPN in the following ways and in the following settings:
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