College answer

Trophic feeding refers to enteral feeding below the minimum required caloric intake, with the aim of maintaining gut integrity rather than meeting patient’s nutritional requirements. Definition of volume feed/energy required varies. Between 10-30ml/hr or 15-25% of calculated caloric intake. Can’t be used as sole nutritional strategy long term. 

Permissive underfeeding is the provision of a reduced non-protein caloric target (around 40-60% of calculated total) hypothesing that lower non-protein calorie intake may be beneficial. May be used as sole nutritional strategy.

Trophic feeding
Advantages of trophic feeding
Include potential beneficial effects on the gut such as preserving intestinal epithelium, stimulating secretion of brush border enzymes, enhancing immune function, preserving epithelial tight cell junctions, and preventing bacterial translocation. Could be considered in patients unable to tolerate full enteral nutrition. May minimise complications associated with full enteral feeding such as feed intolerance, aspiration, high gastric volumes, and diarrhoea.

Available evidence for trophic feeding
2 RCT’s of patients with respiratory failure/ARDS (largest = EDEN trial JAMA 2012)

• Trophic feeding for up to 6 days does not improve ventilator free days, 60 day mortality or infectious complications.
• Less feed intolerance with trophic feeding (e.g. less prokinetic agents, vomiting, gastric residual volumes, lower GI symptoms), 
• lower blood glucose, less insulin requirement

Permissive underfeeding
Advantages of permissive underfeeding

• Based on the premise that ideal caloric targets for critically ill patients are unknown, calorie restriction is associated with increased longevity in animal models, and may have beneficial effects on critically ill patients via hormonal or metabolic pathways
• May be established with either enteral or parenteral routes
• Avoids delivery of large volumes that may predispose to fluid overload
• If tolerated may avoid other strategies such as placement of NJ tube, prokinetics

Available evidence for permissive underfeeding:

• Arabi et al, (PermiT trial) NEJM 2015
• Randomised >800 patients to permissive underfeeding vs. standard care. No difference in mortality, feeding intolerance or diarrhoea

Specific trials not needed for pass.
Additional Examiners Comments:
This question was answered poorly. The majority of candidates were unable to accurately describe or define the two feeding strategies. There was limited appreciation of the available evidence

Discussion

a) 

Trophic feeding: Sondheimer et al (2004); "The generally accepted definition of trophic feeding is a small volume of balanced enteral nutrition insufficient for the patient's nutritional needs but producing some positive gastrointestinal or systemic benefit." 

Permissive underfeeding: The systematic review by Owais et al (2010) reveals a massive variation of historical definitions, ranging though 13-14 kcal/kg/day, <20 kcal/kg/day, 1000 kcal/day,  or <33% of estimated requirement, or 5,000-10,000 kcal/week. The college used the 40-60% goal from Arabi et al (2015). The distinction is that this a nutritional strategy, rather than one focused on intestinal mucosal health.

b)

Advantages of permissive underfeeding:

• Avoids the disadvantages of full-volume enteral nutrition:
  • Gastric distension
  • Aspiration
  • Diarrhoea/constipation
  • Hyperglycaemia
  • Excess insulin use
  • Exposure to toxic prokinetics
  • Need for NJ tubes, etc
• Cheaper 
• Does not suppress the (possibly) constructive autophagy which may be required to recover from critical illness

Evidence for permissive underfeeding:

• Arabi et al.
• n=894 patients (mostly intubated)
• Randomised to either receive 70-100% of their calculated requirements, or 40-60%; for 14 days
• The groups ended up well separated (average 835 kcal vs. 1299)
• No difference in any of the primary outcome measures was found.
• A post-hoc analysis (Arabi et al, 2017) did not find any difference even among patients defined as being at a high nutritional risk.
• This has been viewed as evidence of safety.

Advantages of trophic feeding:

• Improved feed tolerance (reduced gastric residual volumes)
• Maintenance of gastric and intestinal mucosal integrity
• Prevention of bacterial overgrowth and bacterial translocation
• Prevention of excessive protein catabolism (prevention of starvation)

Evidence for trophic feeding:

• EDEN trial (Rice et al, 2012)
• 5 days of <25% of their estimated requirements
• No difference in any primary outcomes
• Again, can be viewed as a demonstration of safety
• Limitation: many patients were underfed with protein (0.6g/kg/day)