The Role of Diet in Inflammatory Bowel Diseases – Facts and Myths
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The topic of gut health and the role of diet and the gut microbiome in maintaining and improving gut health has been receiving a tremendous amount of attention. Catchwords like leaky gut, gut inflammation, and anti-inflammatory diets fill social media and have entered the general conversation where influencers make endless recommendations about what healthy people can or cannot eat to fight the symptoms thought to arise from these gut problems.
On the scientific level, increased permeability of the gut and resulting systemic immune activation as a consequence of the unhealthy Standard American Diet (SAD) has been recognized as a potential risk factor underlying the epidemic of chronic non-communicable diseases plaguing the developed world, including a wide range of common diseases including cardiovascular disease, Alzheimer’s and Parkinson’s disease, chronic liver disease, colon cancer and inflammatory bowel diseases. Considerable epidemiological evidence supports the benefits of a largely plant-based diet, like the traditional Mediterranean diet in improving gut health, reducing systemic inflammation, and reducing the risk of common chronic non communicable diseases.
But how good is the evidence for a role of such a diet in the development and in the treatment of inflammatory bowel diseases ulcerative colitis and Crohn’s disease?
Although the cause of inflammatory bowel diseases (IBD with its two major diagnosis ulcerative colitis, UC, and Crohn’s disease, CD) remains incompletely understood, diet has been suggested to play a pivotal role in their pathogenesis and in the course of the disease. Various dietary components may impact on the disease course due to their regulatory effects on the intestinal microbiota, mucosal barrier function, nutritional status, and intestinal immunity.
A large body of preclinical and clinical evidence has implicated the SAD, which is high in sugar, ultra-processed foods, animal-derived protein, saturated fats, altered omega-6 to omega-3 ratio, and diets low in fruits and vegetables as an important factor underlying to intestinal and systemic inflammation.
The SAD is likely to contribute to intestinal inflammation through alterations in the intestinal microbiota, decreased production of anti-inflammatory metabolites such as short-chain fatty acids (SCFAs) and an increased production of microbial protein fermentation metabolites. Together, these factors may compromise the intestinal barrier function leading to inappropriate activation of the gut associated immune system. Although the nutrient content and composition of plant foods vary greatly, the consumption of largely plant-based diets often result in a reduced intake of saturated fatty acids (SFAs) and animal-derived dietary protein and a concomitant higher intake of dietary fiber and phytochemicals due to intakes of fruits and vegetables.
“The Mediterranean diet had a beneficial effect on gut inflammation in CD patients with mild to moderate disease activity”
Several studies have identified a lower risk of Crohn’s disease (CD) among populations consuming a diet consistent with the traditional Mediterranean diet, a diet high in fresh fruits, vegetables, nuts, fish, and whole grains and use of olive oil as the predominant fat source. Consumption of such a diet has also been associated with reduced symptoms and improved quality of life after diagnosis of CD. A recent study by Lewis et al, published in Gastroenterology in 2021 showed that the Mediterranean diet had a beneficial effect on gut inflammation in CD patients with mild to moderate disease activity, even though the benefit was not different from another often-prescribed diet in IBD, the so-called Specific Carbohydrate Diet (SCD).
The SCD was popularized by Elaine Gottschall in the book Breaking the Vicious Cycle. It is characterized by a number of allowed and excluded foods, the former category containing the majority of fresh fruits and vegetables which are universally acceptable, with the exception of certain starchy vegetables. The second category includes grains and several animal products including processed, canned, and most smoked meats, and milk. In the Lewis study, both diets were equally effective in achieving symptomatic and biological remission. Based on these results, the authors suggested that “ the greater ease of following the Mediterranean diet and other health benefits associated with it, the Mediterranean diet may be preferred to the SCD for most patients with CD with mild to moderate symptoms.”
Even though we now know about the effectiveness of a diet high in fresh fruits and vegetables and low in animal products, how much do we know about the mechanisms underlying these clinical benefits?
It remains to be elucidated if dietary plant-derived fatty acids are beneficial in relation to disease activity in UC and CD patients. It seems most likely that reducing the saturated fatty acids content and the level of n-6 fatty acids (relatively to n- 3 fatty acids) in the diet by the reduction of animal products and by an increased consumption of plant-based foods and seafoods high in omega-3 fatty acids (mussels, sardines, salmon) could potentially minimize disease activity via the incorporation of n-3 acids into the host membrane phospholipids or due to their influence on the intestinal microbiota.
Carbohydrates can be classified as simple sugars (glucose, sucrose, fructose) and digestible starch, which are all rapidly absorbed in the small intestine, and in complex carbohydrates which require the gut microbes to break them down into absorbable molecules such as fiber molecules, or so-called microbiota accessible carbohydrates (MACs).
Plant-based foods provide large amounts of MACs that have been suggested to play a significant regulatory effect on the gut and provide the major source of energy for the gut microbes. Microbial metabolism of MACs is the major source for short chain fatty acids like butyrate (different from the saturated fatty acids coming from animal products), which has a local and systemic anti-inflammatory effect, as well as being an important nutrient for the cells lining the gut. Although the consumption of a diet high in MACs has been linked to reduced disease activity in IBD patients, there seems to be only limited evidence available on the impact of a high fiber diet on disease activity from high quality clinical trials.
The consumption of various plant foods provides many phytochemicals or so-called bio-actives that may significantly impact intestinal and gut microbial health. Several fruits and vegetables contribute considerable amounts of polyphenols (there are up to 8000 different polyphenol molecules), with spices and herbs as one of the richest sources. Several well-designed clinical studies of curcumin in UC patients with mild to moderate disease have shown promising results, even though the number of studies is limited. A substantial number of factors influence the actual intake of polyphenols, such as environmental factors (e.g., time of harvest, organic vs. conventional agriculture) and food processing methods such as cooking and boiling. Thus, the actual intake of polyphenols with the diet may vary considerably according to these factors and may not reflect the actual amount of polyphenols reaching the gut. As only a limited number of studies have investigated the efficacy of polyphenols in IBD patients, the optimal safe dose and efficacy of these bioactive substances still need to be determined based on adequate powered clinical studies.
Many IBD patients have an increased protein requirement depending on their disease status (active vs. remission). At present, little is known about the role of proteins originating from plants in regard to actual measures of disease activity and nutritional status of IBD patients, as most of the research has been conducted in other populations or using animal protein. One large epidemiological study clearly demonstrated that the much demonized gluten, a structural protein naturally found in certain cereal grains, in particular in wheat, does not play a role in IBD pathophysiology. Thus, the role of other plant-derived protein on IBD-related outcomes (i.e., measure of disease activity and nutritional status) remains to be determined.
In summary, there is strong evidence that plant-based dietary components may impact various physiological mechanisms of intestinal inflammation, and that a diet optimized for these components, like the Mediterranean diet (and to a certain degree the Specific Carbohydrate Diet) may be beneficial in reducing the number of disease flares and reducing disease severity. A key question is whether it is possible to support and maintain clinical remission of IBD by adopting a largely plant-based eating pattern, and/or by adding supplements to the diet like the flavonoid curcumin. Plant-based foods are major sources of complex carbohydrates and phytochemicals that have bidirectional effects with the gut microbiota and exert a direct anti-inflammatory effect on the gut. Awaiting further corroborative evidence from well-designed clinical trials in subpopulations of IBD patients, I strongly recommend a personalized, traditional Mediterranean type diet as an important component in the treatment of IBD patients with mild to moderate disease activity.
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