Hello, my name is Gail Cresci and I'm Associate Professor in the Cleveland Clinic Lerner College of Medicine of the Case Western Reserve University. And I'm also staff in the departments of Gastroenterology, Hepatology and Nutrition, and the Department of Inflammation and Immunity at the Cleveland Clinic in Cleveland, Ohio. I'm going to present to you today a talk on targeting the gut microbiome as part of nutritional planning in critically ill patients. Here are my disclosures. My learning objectives for this talk are to review the gut microbiome in critical illness, discuss the effects of enteral feeding formulas and parenteral nutrition on the gut microbiome, and discuss nutritional strategies to optimize gut microbiome health in patients with critical illness. We've learned a lot over the past several decades about the human gut microbiota, that there's trillions of microbes comprised of bacteria, fungi, yeast, viruses, archaea, residing within our intestinal tract, and that the majority of these microbes reside within our distal gut, the colon. We know that there's seven major phyla, of which bacteroidetes and firmicutes comprise about 90% of our gut microbiota. And these result in more than 800 species and 7,000 strains. We also know that the gut microbiota does many good things for us, the host. It helps us digest foods that we can't digest, such as dietary fibers. It helps produce beneficial metabolites, such as vitamins, hormones, short-chain fatty acids. It also helps keep us safe against pathogenic bacteria that we may ingest through our food and water supply. And it helps protect our gut barrier, keeping it strong to prevent bacterial translocation. We are born sterile, technically, within our intestinal tract. And the first colonization of our intestinal tract occurs with the birthing process, and then continues with the infant feeding method. But by the age of three, the human gut microbiota is pretty well established to that of which it'll be as an adult. And a lot of this is driven by the diet. However, even though the gut microbiota resembles that of an adult at the age of three, there's many things that happen throughout the life cycle that can impact the gut microbiota, listed here on this slide. You can see many factors impacting negatively how the gut microbiota may be shaped over the course of a lifetime. Diet is definitely one of the main driving factors that influence the gut microbiota. In fact, studies have shown that a change in diet can shift the gut microbiota within 24 hours. We also know that medications can greatly impact the gut microbiota, such as antibiotics can destroy not only the pathogenic bacteria, but also disrupts the good commensal microbes as well. Histamine II antagonists or proton pump inhibitors, which elevate the pH within our stomach, takes away that first line of defense that helps us destroy any ingested pathogens. And then lastly, stress can really impact the gut microbiota. Not only physiological stress, which we'll talk more about, such as with critical illness, but mental stress is also associated with the gut microbiota. And this has led to a growing area of research with the gut-brain axis. The gut microbiota also helps regulate the immune function within the host's intestinal tract. So if you think about the fiber that we take in, this gets fermented by the gut microbiota in the distal gut, and it beneficially produces what's called short-chain fatty acids. These short-chain fatty acids, in particular butyrate, have been shown to help regulate immune responses, in particular induction of T regulatory cells within the intestine. Also through the interaction of these short-chain fatty acid with their receptors, they're known to be... These short-chain fatty acids are known to be anti-inflammatory. So together with this regulation of immunity and regulating inflammation, a healthy diet can help keep the gut microbiota and the host in balance. Changes in diet from a high-fiber, low-fat to one which is high-fat and low-fiber has been associated with many chronic diseases. You can see here on this slide, in all the panels, how a low-fiber diet has been associated with different chronic diseases such as intestinal diseases, autoimmune diseases, mental health disorders, and different metabolic disorders. And all of this is presumed to occur through this dysregulation of the gut microbiota, its food supply, and then the production of these beneficial metabolites get decreased and therefore dysregulates these immune responses within the host. So if you think about the patient population that may end up in the intensive care unit, many of our patients have chronic diseases or some of these intestinal disorders. So they already are likely to have a dysregulation in their gut microbiota and some of their immune responses, a crosstalk, prior to entering the intensive care unit. So what happens to them now upon entry of critical illness and everything that goes on in a critical care setting? So as I just described, you can see what's optimal is a high-fiber diet. And so what happens to patients when they enter a critical care setting? Well, number one, if we think about their diet, some patients are not fed at all. So they're NPO. So non-luminal delivery of nutrients is known to induce gut dysbiosis. So not only maybe not being fed, but for patients exclusively on parenteral nutrition, this can also impact the gut microbiota that way. If patients are being fed, oftentimes they're not being fed adequately enterally. So they're not getting adequate nutrients. But definitely, they're probably low-fiber diets are being provided. Patients are also typically on antibiotics or these acid-reducing agents. They're undergoing shock, ischemia, low flow states. So all of this, with resuscitation, can impact the oxygen supply to and from the gut, which is typically an anaerobic environment. So these shifts in oxygen to the gut can also impact the gut microbiota as well. So all of this is creating an altered metabolic environment going on within the gut microbiota, which can disrupt it. So what actually happens to the gut microbiome with critical illness? This was a NICE study that was conducted in 115 critically ill patients. On admission and discharge, they obtained fecal, skin, and oral samples. And they compared these samples with healthy controls that came from the American Gut Project. They analyzed these samples for the microbiome. And what they found here, you can see in this top panel A, that there was clear separation between the subjects that were the healthy controls in this orange bar with the fecal samples and then the fecal samples from the patients in the intensive care unit. And then if you look at the changes in the microbiome throughout the intensive care unit stay compared to healthy controls, you can see that here in these bottom panels. So this red color represents the proteobacteria phylum. Proteobacteria overgrowth is concerning because this is where the predominantly gram-negative potentially pathogenic microbes are within this phylum. And you can see here, even on admission, the patients that were critically ill had an overabundance of proteobacteria compared with the healthy controls. And this overabundance just became more pronounced throughout the intensive care unit stay. Okay, so we see that there's a change in the gut microbiome to a decrease in bacteroides and firmicutes and an overabundance of proteobacteria. So now these patients are going to be fed and require nutrition support, enteral or parenteral feeding. So how does this type of feeding impact the gut microbiome? So here's an animal study that aimed to investigate how parenteral nutrition might cause gut atrophy, dysbiosis, and leaky gut. So they aimed to characterize the gut and blood microbiomes in response to different parenteral nutrition solutions in male mice that were randomized to the different groups for seven days. So the different feeding groups were the controls, which was a chow-fed control that had no jugular vein catheter. There was also a group with chow with a jugular vein catheter and saline. And then there were two parenteral nutrition groups that differed in the types of lipids that were provided. One group had a high ratio of omega-6 to omega-3 fatty acids. And the other one had a low ratio of omega-6 to omega-3 fatty acid ratios. After seven days of these feedings, the mice were euthanized by cardiac puncture and blood was collected and stool was collected. And both of these samples underwent 16S ribosomal RNA sequencing for microbiome analysis. So what they found in looking at the stool of these mice is at the phylum level, this red color is firmicutes, the blue is bacteroidetes, and the orange is proteobacteria. You can see here with the different feeding groups that the chow and the saline groups are relatively similar. However, with the parenteral nutrition, there seems to be a rise of proteobacteria and a decrease in the firmicutes to bacteroidetes ratio. Then looking at the blood, again, here now we have red is the proteobacteria, blue is actinobacteria, and green is firmicutes. So looking at the blood, you can see the two chow groups have relatively lower abundance of proteobacteria, but there's now an overabundance of proteobacteria starts to occur in the blood with the parenteral nutrition feeding. And looking here at the phylum level with the stool, you can see the alpha diversity here that the two parenteral nutrition treatment groups have an increase in their alpha diversity at the phylum level. And then looking at beta diversity, you can see here with this PCOA plot that there's clear separation between the animals that were fed with parenteral nutrition and those fed with chow. So overall, these data suggest that in these mice, that just with parenteral nutrition, no metabolic insult at all, but just with the parenteral nutrition, non-luminal delivery of nutrients causes a shift in the gut microbiome as well as the blood microbiome with an overabundance of proteobacteria and a decreased abundance of the beneficial phyla. So in humans now, looking at the effect of enteral versus parenteral on the gut microbiota, this is non-critically ill patients, but this was a systematic review that was conducted that included 11 studies of a total of 367 patients that received nutrition support over three years. There was increased heterogeneity amongst these studies. You can see the different clinical conditions that were included, the different types of enteral formulas that patients were being fed varied as well. And also the way that the microbiome was analyzed varied between the studies. So analysis of the microbiome results when they compared parenteral versus enterally fed patients. Looking at those with parenteral, there was a lower diversity with parenteral compared to enteral. There were seven of the 10 studies reported that. Patients fed parenterally had higher proteobacteria, seven studies reported on that. There was lower Firmicutes reported in six studies, lower Bacteroidetes reported in five studies. Five of the six studies that had patients with parenteral nutrition for Short Powell Syndrome found higher proteobacteria with parenteral nutrition. Lower Bifidobacteria was reported in one of the two studies. And there were also decreases in short chain fatty acids in four of the studies where the patients were fed parenterally. So overall, these data suggest that, again, in humans being fed enteral versus parenteral, parenteral nutrition also induced a shift in the gut microbiome to that of a higher proteobacteria and a lower abundance of beneficial microbes. So in summary of enteral versus parenteral nutrition on the gut microbiota, and actually the gut itself, this diagram is showing that the effects of enteral nutrition to be supportive of maintaining the balance of healthy, good commensal microbes, which then help maintain the gut barrier, which helps also to maintain the immune responses within the gut. Parenteral nutrition, however, increases proteobacteria, alters the immune responses, decreasing secretory IgA secretion, increasing lipopolysaccharide or endotoxins, which together these can allow for impaired gut barrier, which can allow for endotoxin to penetrate the epithelial barrier into the lamina propria, and dysregulates a lot of these immune responses going on within the gut. So the question really lies as to whether it's the parenteral nutrition solution itself, or is it really the fact that when we give exclusively parenteral nutrition, there's a lack of nutrients delivered to the gut. So non-luminal delivery of nutrients is what's associated with the parenteral nutrition, which causes these negative effects on the gut microbiome. But then we have to think about, if we're feeding the gut, what are we actually providing? What are the sources of the nutrients? And we talked about earlier showing that in people who are non-critically ill, a high-fiber, low-fat diet is preferred to help support the gut microbiota. So what do we actually feed our critically ill patients when we feed them enterally, and is this affecting the gut microbiome? So let's just discuss fiber for a moment. We know that fiber is an important nutrient for the gut microbiota. And if we think about just in healthy individuals, what is recommended for fiber intake? There is no recommended daily intake for fiber. However, the Institute of Medicine does recommend adequate intake, which is based on age and sex. And you can see here on the table here on the right, the recommended amounts for men is 38 grams per day. For those under 50, 30 grams per day. For those over 50. And for women, it's 25 and 21 grams, respectively. Well, surveys have been done every five years when they do the USDA dietary guidelines. And they find that most Americans consume only half of the recommended amount of fiber every day. So on average, people are consuming about 12 grams of fiber daily. So these low intakes of fiber are associated with health concerns. And the low intakes of fiber are because our diets of the Americans tend to be more of the Western style diet, high fat, high sugar and low fiber. So we, so many studies, both animal and human have shown that this Western style diet is associated with gut dysbiosis. So trying to understand the role of fiber and its impact on the gut microbiome and its recovery. This study looked at 30 healthy subjects between the ages of 18 and 60 years. And they looked at what the subjects normally consume, what types of diets. And they found they had 10 subjects that typically consumed a vegan diet and 20 that consumed omnivores. The subjects that consumed an omnivore diet were divided 50-50. And then they were provided an exclusive enteral nutrition diet versus an omnivore diet. What they did is they actually engineered an omnivore diet to replicate the nutrient composition of the exclusive enteral nutrition composition. So this study, the subjects underwent three phases. There was a baseline phase, a microbiota purge and a recovery phase. What they allowed the patients or the subjects that normally consumed a vegan diet to continue following that same vegan diet as an outpatient. The omnivores were admitted to an inpatient research unit so that their diets could be controlled. At the end of the study period, they collected stool and blood samples and they analyzed this for microbiome and metabolomics. Here you can see here in this panel, the differences in fiber intake and the types of fiber that the different groups consumed. So you can easily see here that those that follow the vegan diet had the highest amount of daily fiber intake. Okay, so looking at the results of this study, this shows several data information on this slide. So here in this graph here, we're looking at baseline, the copy number of total bacteria. And you can see during the baseline phase that the three groups all had similar amounts, numbers of bacteria. During the purge phase, the subjects all decreased the total number of bacteria, but then with recovery, you can see here the subjects that followed the exclusive enteral nutrition had a lot lower recovery of total bacteria compared to the other two dietary groups, the vegan and the omnivore diet. Now, looking at the alpha diversity using the Shannon index, again, at baseline, you can see in all three groups, the alpha diversity was very similar. Alpha diversity decreased with the purge. However, it didn't decrease as drastically in those that followed a vegan diet. And then you can also see the recovery phase was very delayed in those following the exclusive enteral nutrition diet. Looking at the phyla level, for bacteroides, firmicutes, and proteobacteria, again, for the different dietary groups, all of these were similar at baseline. And then following the purge, you can see how these shifted. And then I really wanna point out the recovery of these in the exclusive enteral nutrition. You can see that there were variances in their recovery. Now, remember I said one of the beneficial metabolites of fermentation of dietary fiber is a short-chain fatty acid, butyrate. So they looked at stool butyrate levels and found they were all similar at baseline, decreased with the purge, but those that followed the exclusive enteral nutrition did not recover their butyrate levels like the vegans and the omnivore diet. So these data are suggesting that the lack of fiber that was associated with exclusive enteral nutrition diet really altered not only the way that the gut microbiome responded to the purge, which the purge was antibiotics and PEG, but also its recovery was delayed, not only with the bacteria, but also these beneficial metabolites. So in summary so far, we've seen that the delivery of nutrients to the gastrointestinal tract is important to support the gut microbiome. But also what's important is the quality of the nutrients that we provide enterally also drives the composition and function of the gut microbiome. So I just wanted to show this slide, bear with me, it looks a little busy, but it really is comparing two different types of enteral formulations. So more recently, real food formulas have started to arise on the market for our patients. So we think about what these real food enteral formulations are. So the macronutrients are to be provided as food-based ingredients. And when the recommended volumes are consumed, they provide the RDI for nutrients. What's important to know is that all types of enteral feedings also contain food additives. So even these real food formulations, they contain extra vitamins and minerals, and they contain thickening and emulsifying agents. There's only been a small number of studies showing a positive that have been done. And of these studies have been done on the impact of these real food formulas on GI tolerance and microbiome in pediatric outpatients, they've shown to be well tolerated, improve GI tolerance, and then also to have a positive impact on the gut microbiome. However, studies are lacking on the safety and the effects of these real food formulations in critically ill patients. And here I've just put two different tables here for you to look at later, but you can see for the standard enteral formulas versus the real food formulas for the different macronutrients and the fiber, and what the sources are for these different nutrients that the macronutrients provided in these formulas. Okay, so I just wanna now take a few moments to discuss some of the evidence for using other means, nutrition supplementation of prebiotics, probiotics, symbiotics for manipulating the gut microbiome and impacting outcomes in critically ill patients. So before we can begin discussing the evidence, it's really important that you understand the difference between a probiotic, a prebiotic, and a symbiotic. So a probiotic is a live microorganism, which when consumed in adequate amounts confers a health benefit on the host. A prebiotic is really the food substrate for the gut microbiota. It's typically these foods, these natural fibers occurring in foods that we've been talking about and that the host cannot digest, and therefore the gut microbiota can metabolize them, and then as a result, it helps enhance the composition of the gut microbiota as well as the function of the gut microbiota. And a symbiotic is just a physical combination of a pre and a probiotic. What's really important to understand is that probiotics and prebiotics are not all the same. So not every probiotic is the same. It's not a one size fits all. So even within the Lactobacillus species, there's hundreds of different strains of Lactobacillus that may behave differently. So their mechanism of action can differ. And so it's not equal to replace one and utilize one Lactobacillus probiotic for the other. And you can see here listed on this slide are just some of the mechanisms of action that have been shown by strain specificity for different probiotics and prebiotics. So some of them have been shown to have antimicrobial effects, some have been shown to enhance a mucosal barrier, and also some have been shown to have some immune modulatory effects. But it's important again to remember the strains and substrate specificity. So the same thing with prebiotics, not every prebiotic will affect the composition of the microbiome the same or its function the same. So due to lack of time, I'm not able to go into individual studies, but I found this nice systematic review and meta-analysis on fiber and prebiotic supplementation in enteral nutrition. This recent analysis looked at a total of 22 studies. Eight studies were in the intensive care unit patients. And the studies had enteral feeding that was supplemented with fiber, and they looked at the effects on diarrhea, the microbiota, and short-chain fatty acids. What the meta-analysis found is that fiber reduces diarrhea in adult patients with enteral nutrition, but it did not have an effect on ICU patients with diarrhea. And there was limited data amongst these studies for fiber and prebiotics on the effect on the microbiome and for short-chain fatty acid effect. So overall, there's just limited data studies that have been done looking at fiber and prebiotics in the critically ill, on the microbiome, and short-chain fatty acids. So looking at the effect of probiotics or symbiotics in the adult critically ill population, there have been a number of meta-analyses of randomized control trials over the past several years. This was a recent meta-analysis that was performed in 2023. And what they did and why this meta-analysis differs from the prior ones is they actually looked at the quality of the studies, and they rated the outcomes of the meta-analysis based on overall quality. So this meta-analysis had randomized control trials that occurred between 1983 and 2022. Overall, there are 71 studies included. There are different ICU types that were included in this as well. And like I mentioned, studies were rated on the quality, high versus low, risk of bias, and then they did a trial sequential analysis, a TSA. They had studies with only probiotics, and then there were 27 studies that also had, were only symbiotics. Prebiotics were used in the control group in 18 of the studies. The most frequently used probiotics that were studied amongst the studies are listed here. The probiotic typically was started within three days of the ICU or hospital admission or the inciting event in 31 of the studies. The probiotic was provided a range of one to four times per day, and the colony forming the units that were provided varied between 10 to the fifth to 10 to the 11th CFU. Duration varied between five to 60 days, but overall outcomes that were analyzed for this analysis were ventilator-associated pneumonia, other infectious complications, overall mortality, diarrhea, duration of mechanical ventilation, and adverse events. So the first outcome I'm going to present is the effect of the probiotics on ventilator-associated pneumonia and ventilator-associated pneumonia and lung infection. So there are 16 studies with VAP, and there were 32 for VAP and lung infection. Overall, based on lower quality and small sample size studies, probiotics significantly reduced these two conditions. But looking at the TSA, this suggested a type one error in pooled relative risk of the lower quality studies and the single center studies. So in analysis of just the higher quality studies suggested there was futility in the trend in trying to find a difference in the future if only high quality studies were conducted. So looking at the other outcomes that were measured, infectious complications, there were 34 studies that looked at this and found that probiotics did reduce the infectious complication. But this benefit was not found in the higher quality studies or multi-center trials. And the TSA indicated futility in finding difference in higher quality or multi-center trials. And the TSA indicated futility in finding a difference in higher quality or multi-center studies. There were 49 studies that looked at overall mortality, and probiotics did not affect this outcome. And TSA indicated futility in finding a difference in higher quality or multi-center studies. Duration of mechanical ventilation was analyzed in 11 studies, and the duration of ventilation significantly, p-value 0.04. The length of ICU stay was assessed in... in 26 studies and probiotics were found to reduce ICU stay, but these benefits were only found in the lower quality studies. In looking at adverse or serious adverse events, there were 15 higher quality studies that assess for this. Probiotics were associated with higher adverse events than the control. And isolation of the probiotic species was found from a sterile site culture or as a predominant organism from a non-sterile site culture in 13 incidences. And this is what these adverse events were associated with. Bowel ischemia or intestinal ischemia and necrosis was reported in 11 of the studies. In summary of this meta-analysis, probiotics or symbiotics in the critically ill patient, high quality studies did not support beneficial effects of probiotics on clinical or diarrheal outcomes in the critically ill. And there were increased adverse events with probiotics compared to the control groups. Therefore, the authors concluded that probiotics should not be routinely provided to critically ill patients. So overall, in summary, and my takeaways today are that the gut microbiome composition and function are both disturbed in the critically ill patient. And then a patient coming to the ICU is already likely to have gut dysbiosis based on the fact that many of our patients have chronic diseases, which is also associated with gut dysbiosis. Also, enteral nutrition supports the gut microbiome compared to parenteral nutrition. However, when we think about feeding fiber to our patients, we have to realize that the general American population does not consume the adequate amounts of fiber that's recommended. Therefore, fiber introduction needs to be gradual and not overburden their gut with something that it's not used to seeing. So a gradual titration in the fiber is warranted. There is no evidence to support the routine use of probiotics in the critically ill patients for ventilator-associated pneumonia, infections, diarrhea, or length of stay, or mortality. More data is needed regarding the type of enteral formulation on the gut microbiome and outcomes in critically ill patients. So with the release of these newer real food formulations, investigations into their beneficial effect in a critically ill population, further research is definitely needed. Thank you so much for your attention.

Gail Cresci

gut microbiome

nutrition planning

critically ill patients

beneficial bacteria

parenteral nutrition

probiotics

nutritional planning

enteral feeding

dysbiosis

fiber supplementation

nutrition support