Nutritional support in the intensive care unit (ICU) is highly debated as critically ill patients are frequently hypermetabolic, catabolic and at risk of both underfeeding and overfeeding.

Enteral nutrition (EN) is frequently recommended over parenteral nutrition (PN) as it may preserve gut mucosal barrier function [1,2] and has been shown to demonstrate beneficial effects on (gut) immunity

The current literature shows evidence in favour of early enteral nutrition (EEN) commenced within 24 to 48 h after ICU admission [3]. EEN is associated with decreased morbidity, healing, decreased mechanical ventilation duration, ICU and hospital length of stay and duration of recovery [4] and even reduced mortality [5].

Therefore, EN is the preferred route over parenteral nutrition whenever EN is possible. Achieving caloric targets with EN may be challenging in the critically ill, a caloric deficit frequently occurs due to slow intake progression, unnecessary stoppages, Delayed gastric emptying, enteral feed intolerance and delays in post-pyloric feeding tube placement [6].

The cumulative deficit or caloric debt has been reported to be associated with adverse clinical outcomes.

Villet and co-workers showed that delayed initiation of feeding resulted in a marked cumulative energy debt during the first week after ICU admission associated with an increase in infectious complications, days of mechanical ventilation and length of ICU stay. However, possibly not only energy deficit but also deficient protein intake may be relevant and is suggested to play a role in outcome [7, 8].

Does optimised nutrition support and monitoring in the intensive care unit improve clinical outcome?

Enteral nutrition (EN) alone is often associated with an insufficient energy provision, leading to energy deficit; the latter is associated with fat-free mass loss, increased risk of infections and complications, and increased mortality. Systematic parenteral nutrition (PN) without appropriate indication is associated with increased risks of overfeeding, hyperglycaemia, and promotes infections, hepatic steatosis, liver disease, hypoglycaemia (as a result of high insulin doses), and mortality.

Optimal nutrition support improves the clinical outcome. It includes the adequate choice of nutritional support:

1. EN in first line, then together with supplemental parenteral nutrition in case of failure or contraindication to optimised EN;
2. Parenteral nutrition alone when appropriate with respect to the indications, the preferred use of ‘all-in-one’ solutions, and the glycaemic control;
3. The nutritional and metabolic monitoring: adequation of protein/energy provision towards target, blood glucose and insulin doses According to on-going protocols. The assessment of fat-free mass may become the key part of the nutritional management of ICU patients, but validation studies are needed. The following Figure 1 show us how to optimize nutritional support;

Figure 1

Does Supplemental parenteral nutrition improve energy provision and nutritional status?

The combination of parenteral nutrition with enteral nutrition increases calorie delivery in comparison with enteral nutrition alone (28 ± 5 vs 20 ± 5 kcal/kg/day, p <0.0001) [10]. Suggesting that the combination of enteral nutrition and parenteral nutrition allows the achievement of the energy target sooner during critical illness.

A randomized controlled trial (Heidegger, Lancet 2012) was undertaken in two centers in Switzerland concluded that Individually optimized energy supplementation with SPN starting 4 days after ICU admission should be considered as a strategy to improve clinical outcome in patients in the ICU for whom EN is insufficient [11].

Does supplemental parenteral nutrition reduced nosocomial Infections?

Heidegger and colleagues found that short-term (ICU days 4 to 8) Supplementation of EN with PN reduced nosocomial infection, the primary endpoint, but did not affect several secondary endpoints, Including mortality, duration of mechanical ventilation, and ICU length of stay [12]. These findings suggest that the treatment may be of benefit in a select group of patients.

When should you start enteral and parenteral nutrition? Early versus late;

Initially according to EPaNIC (Casaer, NEJM 2011) Trial [12], they found that early initiation of TPN increased ICU and hospital stay, the incidence of infection, and total healthcare costs. Delaying parenteral nutrition up- to 7 days had no effect on mortality.

This study has certain limitations:

1. First, the parenteral nutrition that used contained neither glutamine nor specific immune-modulating compounds, but rather reflected the parenteral nutrition given in common daily practice. The data favouring the administration of glutamine remain controversial. 
2. Second, the use of standardized, premixed parenteral-nutrition products resulted in a relatively low protein-to-energy ratio However, high-level evidence of an improved outcome with increased protein doses is currently lacking.
3. Third, the amount of nutrition was calculated without measurement of energy expenditure with the use of indirect calorimetry, a technique that is not recommended by evidence-based guidelines.
4. Finally, because of the nature of the study, patients or their designated representatives and their ICU providers were aware of study-group assignments.

In contrast to the EPaNIC study, (Doig, ANZICS, JAMA, May 2013) a Multicentre, randomized, single-blind clinical trial[14], conclude that The provision of early PN to critically ill adults with relative contraindications to early EN, compared with standard care, did not result in a difference in day-60 mortality. The early PN strategy resulted in significantly fewer days of invasive ventilation but not significantly shorter ICU or hospital stays.

In recent guidelines, controversy regarding the timing of supplemental PN (SPN) in ICU patients was found [7-9]. The European Society for Clinical Nutrition and Metabolism (ESPEN) guidelines recommend the addition of SPN within 24 to 48 h in patients who are expected to be intolerant to EN within 72 h of admission [7]. The American Society for Parenteral and Enteral Nutrition (ASPEN, 2016) recommends postponing the initiation of PN until day 7-10 after ICU admission [9].

Whereas Canadian Clinical Practice Guidelines 2015, Recommend that parenteral nutrition not be used routinely, but early PN should be considered in nutritionally high-risk patients with a relative contraindication to early EN [12].

What if you can’t provide adequate nutrition enterally? …

To TPN or not to TPN…

1. Case by case decision
2. Maximize EN delivery prior to initiating PN
3. Use early in high risk cases

The following diagram may simplify the answer of that question (Figure 2).

Figure 2

SPN is used in a step-up approach when full enteral support is not possible or fails to reach caloric targets. This review based on a systematic literature review summarizes results of trials reported in ICU patients. Addressing the optimal timing of (S) PN.