Lipids are structural components of cellular membrane and functional components exerting an important role in different metabolic pathways through regulation of cell signaling and metabolic homeostasis. Although the human body can synthesize some fatty acids, essential fatty acids (EFA) should be exogenously provided. Parenteral nutrition (PN) can be utilized to provide EFA for patients without normal gastrointestinal functions and those at risk of malnutrition. Parenteral lipids provide more energy-dense nutrition in a reduced volume and prevent EFA deficiency. However, prolonged PN with lipid emulsions is associated with metabolic complications including PN-associated liver disease. Currently available parenteral lipid emulsions provide various combinations of soybean oil, fish oil, olive oil and medium-chain triglycerides. The effects of parenteral lipid emulsions on the lipid nutrition status and PN-associated complications vary depending on the type and dosages of main oil sources used in parenteral lipid solutions. Therefore, understanding the composition, metabolism, and functional effects of lipids can be helpful in considering their clinical applications in PN.

Paradoxically, the obese are more susceptible to malnutrition. The more obese they are, the greater the risk for micronutrient deficiencies particularly in fat-soluble vitamins and anti-oxidants. Several micronutrient deficiencies could be worsened because of poor intake and absorption difficulty after bariatric surgery. Micronutrient deficiencies could not only decrease the effect of bariatric surgery but also cause various diseases such as anemia, neurologic impairment, or osteoporosis. Micronutrient deficiencies should be monitored, recognized, and corrected properly in bariatric patients in order to maximize the effect of surgery and improve the health-related quality of life.

Loss of body protein?mostly from skeletal muscles?is the most characteristic sign of critical illness. The most common immune-enhancing nutrients for favorable Compensatory Anti-inflammatory Response Syndrome (CARS) are glutamine, arginine, poly-unsaturated fatty acid, some trace elements and probiotics. Glutamine is an essential amino acid with an important role it fuels the proliferation of cells and acts as a precursor to antioxidant glutathione. The conflicting results of glutamine trials are largely related to its dosage and duration of treatment. However, its overall effects, when parenterally or enterally supplied, are thought to be helpful in immune-enhancing and decreasing infectious complications. Arginine is also conditionally essential and has an important role in the synthesis of anabolic hormones and in the activation of T lymphocytes. It also is converted to citrulline and nitric oxide, the latter is a potent intracellular signaling molecule. Leucine and citrulline are common in the mechanism of action and are mediated by the mTOR signaling pathway. Both leucine- and citrulline-enriched diets have been proven to increase nutritional status in various experimental models of injury. However, there are conflicting data about when they were supplied to the critically ill patients. The role of the most immune-modulating nutrients have not been fully discovered thus far. For critically ill patients, basic support with macro-nutrients should come first, followed by other specially provided nutrients, such as immunonutrients.

Nutrition therapy is a critical component of treatment for patients with acute respiratory distress syndrome/acute lung injury. Underfeeding can cause cumulative caloric deficits, loss of lean body mass, and deteriorationof respiratory muscle strength, which can all lead to poor outcomes. However, overfeeding, on the other hand, is also harmful due to hypercapnia and hyperglycemia, which can increase the risk of infection. Therefore, an optimal nutrition therapy is very important. Although the importance of nutrition in this patient population is widely accepted, trophic enteral feeding during the acute phase is suggested in immunologic aspect to prevent nutrition-related complications. High-fat and low-carbohydrate enteral nutrition can decrease CO₂ production and help shorten the duration of mechanical ventilation in patients with acute respiratory distress syndrome. There are still areas of controversy regarding the role of omega-3 fatty acids and antioxidants, which are known to prevent oxidative cellular injury, modify the metabolic response, and modulate immunity and inflammation.