ReviewImportance of digestive mucus and mucins for designing new functional food ingredients
Graphical abstract
Introduction
The human digestive tract consists of a stable alliance among the epithelial barrier, the immune system and the resident microbiota. Besides its implication in absorption and assimilation of nutrients, exclusion of undigested food, protective role against harmful molecules and pathogenic microorganisms, the contribution of the digestive tract to sensory functions including the innate and adaptive immune responses have also been well documented. The epithelium of the gastrointestinal tract (GIT) consists of enterocytes, goblet cells, enteroendocrine cells, Paneth cells, microfold cells (M cells), cup and tuft cells, and is coated with a gel layer, called mucus (Ahluwalia et al., 2017, McGuckin et al., 2015). Furthermore, the human GIT also harbors more than 1013 of resident bacteria per gram of feces colonizing the mucus layer (Sender, Fuchs, & Milo, 2016). The number of microbial cells as well as the number of species varies throughout the GIT (Li et al., 2015) with few resident bacteria in the stomach and small intestine (Bik et al., 2006), and a dramatic increase in the distal ileum and the colon (Ermund et al., 2013, Li et al., 2015). The association of bacteria with the host have led to co-evolution with mutual benefits and impacts (Bäckhed et al., 2005, Liang et al., 2018). Hence, the resident bacteria could influence the composition and structuration of the mucus layer and finally the mucus properties (Jakobsson et al., 2015).
The digestive mucus, mostly consisting of mucins, plays a major role in the good functioning of the digestive tract and thereby the human health. Whatever the reasons, alterations in composition or structure of the mucus result in a wide variety of diseases including inflammatory bowel deseases or colorectal cancer (Johansson et al., 2013, Velcich et al., 2002). Hence, mucins, such as MUC2 (“MUC” refers to human mucins whereas “Muc” corresponds to mucins in mice and other animals), constitute a target for the design of new therapeutic molecules because of their involvement in tumorigenesis (Aithal et al., 2018, Li et al., 2018).
Scientific community and public are now more interested in natural products to maintain consumer health. This is reflected by the ever-growing market of functional food products containing health-promoting ingredients and/or bacteria (Douillard and de Vos, 2019, Vicentini et al., 2016). Therefore, it would be interesting to explore the possibility of identifying functional ingredients able to modulate mucus production as the drug treatments are usually associated with adverse effects. After a brief description of the mucus, its beneficial role in maintaining human health and the control of this layer, this review will address the effects of food ingredients on mucus or mucins production and the possible ways to design/improve functional ingredients, a first step in developing functional foods, summarizing thereby the recent insights in this domain.
Section snippets
Overview of digestive mucus and its beneficial role
Mucus, secreted by intestinal goblets cells, is characterized by its viscosity and rheological properties. It consists in water (≈95%), electrolytes, enzymes, salts, DNA, lipids, growth factors, antibacterial peptides and proteins such as defensins, immunoglobulins, lysozymes and trefoil factors (McGuckin et al., 2015), and especially glycoproteins called mucins. It not only constitutes a lubricant facilitating the transport of the chyme along the GIT (Corfield, 2015, Lai et al., 2009) but also
Control of mucus layer, mucin synthesis and associated deseases
Under normal physiological conditions, mucus is regularly renewed and this phenomenon results from a balance between mucus degradation and synthesis. Intestinal bacteria take important part in regenerating the mucus layer. The establishment of mucus-degrading bacteria in the host intestine starts few months after birth and is completed gradually with age (Palmer, Bik, DiGiulio, Relman, & Brown, 2007). This process is not only linked with the individual age but also with the diet. Midtvedt,
Impact of food components on mucus and mucins
As discussed before, the control of mucus layer plays an essential role in maintaining health and should constitute a major target for conception of preventive strategies, including specific diets. Although knowledge about the impact of food ingredients on the digestive mucus layer remain relatively scarce. However, different dietary components such as milk proteins, bacteria or their metabolites, bioactive molecules (released either by bacteria in certain fermented foods or derived directly
Research approaches to improve/design new functional ingredients/foods
The increasing costs of health as well as the desire of numerous consumers to support a preventive rather than a curative approach have created favourable conditions to develop functional foods, which are claimed to display health specific properties because of the presence of some specific functional and/or absence of some harmful components (Gupta & Abu-Ghannam, 2012). Although the informations presented above remain scarce, the evidence provided through the action of probiotics, bioactive
Acknowledgements/funding sources
Mounira Kebouchi is the recipient of a PhD fellowship from the “Ministère de l’Enseignement Supérieur et de la Recherche” of France.
Declaration of Competing Interest
None.
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Co-senior authors.