Several studies have shown that long-chain triglycerides and PUFAs in total parenteral nutrition (TPN) increase TNF-alpha, which can stimulate the fever and hypotension of septic shock. TNF production has also been shown to increase with the duration of TPN. PUFA turnover depends on the activity of phospholipases, whose activities are enhanced in trauma and sepsis. The efficacy of eicosanoids is determined by the availabilty of different PUFAs in the cellular phospholipid pool. Omega-6 and omega 3 PUFAs give rise to eicosanoids of different series. The eicosanoids formed from omega-3 PUFAs, compared to those deerived from omega-6 PUFAs, develop similar quality but less intensity of action.
Other work has shown that PUFA and long chain triglycerides (LCT) increase PGE-2,which suppresses cell-mediated immunity and complement synthesis. It also increases the generation superoxide radicals and increases T-suppressions activity.
There are fewer septic complications with enteral feeding than with TPN. This is partly attributable to the central venous line. However, suspicion has been raised that fat emulsion might cause immune suppression. Previous work has noted contradictory findings of oxidative activity, neutrophil activity, chemotaxis, and lysozyme release in response to fat emulsion. Several conclusions can probably be drawn. First, interactions between PUFAs and the immune system are determined with omega-6 PUFAs. Second, lipids appear to have a deleterious effect on the lungs. Third, a combination of medium- and long-chain triglycerides (MCT/LCT) appears to be beneficial. Fourth, and probably most important, hyperglycemia from TPN suppresses phagocytic activity and may directly predispose to infection. Better control of blood sugar during TPN may greatly reduce the incidence of infection.
No discussion of the hazards of TPN or enteral nutrition would be complete without noting the benefits. There are certainly adverse immunologic effects of TPN. But the absence of nutritional support for prolonged periods puts the patient at greater risk for infection. Malnutrition, especially acute malnutrition, is a major immunosuppressant.