All ICU patients should be fed soon after admission to the ICU. The only exceptions include:

The results of a number of RCT’s (and meta-analyses of these trials) have established the following principles on which to base nutritional support in critically ill patients [3–10]:

While no nutrition is bad (starvation) the unresolved question is how much is enough? Adequate nutrition is essential for the critically ill patient in order to ameliorate uncontrolled catabolism, maintain the integrity of the gastrointestinal tract, maintain a competent immune system, and ultimately improve patient outcome. Nutrition support attenuates the metabolic response to stress, limits oxidative cellular injury and favorably modulates the immune response [18–22]. However, the most important goal of nutrition is to promote anabolism and limit protein breakdown; as will be demonstrated further in this chapter none of the current methods of nutritional support have been demonstrated to promote protein synthesis. This would suggest that the amount of protein and calories delivered does not matter; however as reviewed further in this chapter, this is because the standard approach to nutritional support (continuous enteral feeding) does not support anabolism; wrong formula and wrong method of delivery!!

Aberda and colleagues conducted a large observational study to determine the association between the provision of energy and protein and patient outcomes [23]. Data were collected on 2,772 mechanically ventilated patients who received an average of 1,034 kcal/day and 47 g protein/day. An increase of 1,000 cal per day was associated with reduced mortality [OR for 60-day mortality 0.76; CI, 0.61–0.95, p = 0.014] and an increased number of ventilator free days. The benefit of increased calories was only seen in patients with a BMI < 25 and > 35 (moderate obesity may be protective against critical illness). Similar results were observed when comparing increasing protein intake and its effect on mortality. Furthermore a number of studies have demonstrated that the energy deficit accumulated by underfeeding patients during their ICU is an important factor in increasing the risk of adverse outcomes [24–26]. However, it is important to recognize that these are all observational studies that are likely confounded by severity of illness; less sick patients tolerate enteral nutrition better, are more adequately fed and have better outcomes. Four RCT’s support this contention; the first two studies randomized patients to trophic feeds or full nutrition, the third randomized patients to permissive underfeeding or full nutrition while the fourth randomized ICU’s to a “PepUP protocol” where additional nutrition was given to reach caloric targets (in the intervention ICU’s) compared to control ICU’s where patient received usual care. The EDEN study randomized patients (n = 1,000) with acute lung injury to receive either trophic feeding at 20 kcal/h (which is about 7 cal/kg/day) or full feeding at 25–30 kcal/kg/day for the first 6 days [8]. After day 6, all patients who were still receiving mechanical ventilation received the full feeding protocol. There was no difference in the number of ventilator free days (primary outcome), 60 day mortality and other secondary end-points between groups. Follow-up of these patients showed no difference in physical function, psychological and cognitive function as well as quality of life at 12 months [9, 27]. The results of the EDEN trial are similar to a smaller study (n = 200) conducted by Rice and colleagues with a similar study design and patient population to the EDEN trial [28]. Arabi and colleagues performed a two by two factorial design study in which 240 ICU patients were randomized to permissive underfeeding or target feeding (caloric goal: 60–70 % compared with 90–100 % of calculated requirement, respectively) and either intensive or conventional glycemic control [10]. Hospital mortality was lower in the permissive underfeeding group than in the target group (30.0 % compared with 42.5 %; RR 0.71; 95 % CI: 0.50, 0.99; P = 0.04). Heyland et al. randomized 18 ICU’s in the USA and Canada (1,059 patients mechanically ventilated >72 h) to a novel feeding protocol (PEP-uP protocol) which targeted a daily caloric goal or usual care [29]. In the intervention ICU’s enteral feeds started at a higher initial rate and they operationally shifted from an hourly rate to target a 24-h volume goal. This intervention allowed the nurses to increase the volume of feeds delivered if there was an interruption for non-gastrointestinal reasons. While patients in the intervention ICU’s received more calories and protein there was no difference in any outcome variable between the intervention and control ICU’s. These data suggest that achieving target caloric goals (by conventional feeding) does not improve patient outcomes.

The current SCCM/ASPEN guidelines recommend providing 1.2–2.0 g/kg/day of protein [30], with some authorities recommending up to 2.5 g/kg/day to “promote muscle anabolism” [31]. However, increased provision of protein by conventional feeding has not been demonstrated to limit muscle wasting, loss of lean body mass or improve clinical outcomes [29, 32, 33]. Paradoxically, the increased provision of protein either enterally or parentally has been demonstrated to accelerate loss of muscle mass (see below). In a post hoc analysis of the EPaNIC study, Casear et al. demonstrated a strong association between increasing cumulative protein intake with a lower likelihood of an earlier alive-discharge from the ICU [34]. These data suggest that the current guidelines for the provision of protein in critically ill patients are likely to be harmful.