Acute critical illness requiring mechanical ventilation carries a risk of severe malnutrition with substantial adverse effects including infections, muscle wasting, delayed recovery and even increased mortality. It is already agreed that nutritional support is a vital component of care process in clinical management of these patients, however, it is a complex combination of timing, dose, duration, delivery among which type and appropriate route of administration is still matter of debates and uncertainties. We have intensively reviewed the current literature and re-visited advantage and disadvantage of enteral or parenteral nutrition (EN, PN, respectively) in patients requiring mechanical ventilation with regard to clinical implications and outcomes. It is speculated that outcomes and costs should be considered on an individual base of patient in intensive care units (ICUs) for admission duration more than 48 h. In the case of contraindications to EN, PN should be normally implemented within 3-7 days. Further, there are accumulative evidence that EN is associated with improvements in gastrointestinal mucosa integrity, immune function, and tissue repair responses which results in lower nosocomial infections, shorter hospital and ICU stay lengths and more affordable healthcare costs. [ABSTRACT FROM AUTHOR]