Fiber decisions in the ICU

Historically, fiber-containing tube feeding formulations were not used in the intensive care unit (ICU) due to the volatile hemodynamics of this unique patient population. The 2016 “Guidelines for the Provision and Assessment of Nutrition Support Therapy in the Adult Critically Ill Patient: Society of Critical Care Medicine (SCCM) and American Society for Parenteral and Enteral Nutrition (ASPEN)” addressed fiber in various recommendations.

First, they recommended avoidance of both soluble and insoluble fiber in patients at high risk for bowel ischemia or severe dysmotility.  A fermentable soluble fiber additive was recommended for consideration in hemodynamically stable patients. Mixed fiber-containing formulas were recommended for consideration in the case of persistent diarrhea when other sources of diarrhea, such as medication or C difficile, have been ruled out.

With emerging research on fiber in the ICU and given that mixed fiber is beneficial for maintaining gut barrier function, modulating immune responses, and supporting the intestinal microbiome, it may be cautiously considered for the critical care population. Careful patient selection and specific indications need to be utilized to avoid the risks of serious complications.  

What are the risks? 

The heightened concern of using fiber in the critical care population is based on the risk of inspissation of formula resulting in nonocclusive mesenteric ischemia (NOMI), nonocclusive bowel necrosis (NOBN), or an obstruction, which are rare yet very serious conditions. Other conditions predisposed to inspissation of formula from insoluble fiber include dehydration, hypotension, severe dysmotility, and vasopressor therapy. The incidence of NOMI is 0.2%-0.3% of ICU patients.  

Enteral nutrition and hemodynamic instability 

Based on guideline recommendations and expert opinion, it is appropriate to withhold enteral nutrition (EN) for ICU patients until resuscitation is complete and they are hemodynamically stable. The gastrointestinal (GI) tract receives blood from three main arteries that branch off the aorta: the celiac, superior mesenteric, and inferior mesenteric arteries.  

Vasopressors, such as norepinephrine, epinephrine, and vasopressin, can significantly decrease blood flow to the GI tract due to their vasoconstrictive effects on the mesenteric arteries. Decreased GI blood flow can lead to complications such as intestinal ischemia, mucosal damage, and impaired gut barrier function. Luminal nutrients via EN promote splanchnic blood flow and increase epithelial cell oxygen levels. Even trickle feeds, also referred to as trophic feeds (10-20 mL/hr), when hemodynamics are unstable, can stabilize hypoxia-induced factor (HIF) and improve gut barrier function.  

A 2022 consensus statement from ASPEN recommends EN initiation once the mean arterial pressure (MAP) is greater than 60 mmHg. Additional considerations for initiating EN include the vasopressor agent and its dose, the timing of EN, and the feeding location. Recommendations include gastric feeding and consideration of holding EN (or limiting to trophic feeds) if the vasopressor dose equivalent is greater than 12. Lastly, the consensus statement recommended the use of a 1-1.2 kcal/mL higher-protein, low-fiber formula when administering EN with vasopressors.   

Different types of fiber 

Fiber has been categorized as soluble or insoluble based on its solubility in water. It is now thought to be an imprecise distinction, as many fiber sources include both soluble and insoluble components, and solubility is influenced by external factors. Tolerance to fiber relates to gas formation from the fermentation process, which can be seen as bloat, flatus, abdominal discomfort, or distention.  

Four fibers typically used in EN formulations include inulin, oligofructose, starch, and pectin. Inulin and oligofructose qualify as fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) carbohydrates. Tolerance to dietary fiber is related to the type of fiber, its fermentation rate, the dose, and each individual patient’s response. The fiber doses and types used typically in tube feeding formulations are often well tolerated.  

How is the GI tract affected by fiber?  

Short-chain fatty acids (SCFAs) are mainly produced by gut microbiome fermentation of dietary fiber. A reduction in SCFAs (acetate, propionate, and butyrate) contributes to intestinal dysbiosis. Levels of SCFA decrease within 6 hours of ICU admission and remain low upon discharge.  

These changes in the intestinal microbiome and a rise in luminal pH reduce barrier defenses, decrease mucus production, and increase permeability and proinflammatory immune response. A thinner mucus layer with reduced tight junctions and decreased beneficial bacteria yields an increased risk for opportunistic pathogens to take over.

The “butyrate effect” refers to protective changes, such as maintaining gut barrier function, promoting mucus production, and exerting anti-inflammatory effects that are seen with SCFA provisions. Reduced SCFAs are linked to increased mortality.  

Inflammatory markers have been shown to be significantly reduced in groups receiving fiber provisions. The use of fiber with the production of SCFAs may help reduce cytokine storm, which is more likely to escalate in the setting of gut dysbiosis related to a less healthy Western diet combined with older age, polypharmacy, comorbidities, and/or antibiotics. Critical illness, even in the absence of antibiotics, tends to raise the pH within the intestinal lumen. SCFA production lowers the pH and promotes the growth of butyrate-producing bacteria while inhibiting or preventing the growth of C difficile.  

Fiber promotes GI tolerance to EN by assisting in gut motility and is seen as an important component of a prophylactic bowel regimen. Fiber use has been shown to reduce hospital length of stay in patients with pancreatitis, post-surgical gastric cancer, and in ICU patients with sepsis.   

Improved glycemic control has been seen with the use of soluble fiber provisions. Studies evaluating fiber in preventing or treating diarrhea in the ICU have been inconsistent and have not shown a reliable, consistent benefit. Etiologies for diarrhea in the ICU are most likely related to medications (30%-50%) or C difficile infections (10%-17%).  

Critical illness fosters GI dysbiosis with a loss of gut barrier function, which may escalate to pathogen invasion. The provision of luminal nutrients is vital for the maintenance of gut integrity. Both insoluble and soluble fibers play a crucial role in the production of SCFAs and should be taken into consideration when determining the nutrition plan of care for patients in the ICU.  

Careful patient selection and timing are paramount in determining who may safely benefit from fiber provision. Consider patient hemodynamics and closely monitor tube feeding tolerance, GI status, and hydration, as these can all be volatile in the ICU.  

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References

McClave, Stephen A., et al. The Importance of Providing Dietary Fiber in Medical and Surgical Critical Care. Nutr in Clin Pract. 2023(39)3:546–556. doi:10.1002/ncp.11092. 

McClave, S.A., Taylor, B.E., Martindale, R.G., et al. Guidelines for the Provision and Assessment of Nutrition Support Therapy in the Adult Critically Ill Patient. J Parenter Enteral Nutr. 2016(40)2:159-211. https://doi.org/10.1177/0148607115621863 

Bechtold ML, Brown, PM, Escuro A, et al. When is enteral nutrition indicated? J Parenter Enteral Nutr. 2022(46)7:1470-1496. https://aspenjournals.onlinelibrary.wiley.com/doi/full/10.1002/jpen.2364