**1. Introduction**

Inflammatory bowel disease (IBD) is represented by ulcerative colitis (UC) and Crohn's disease (CD), which manifest as chronic inflammation of the gut [1]. It has a higher prevalence and a rising incidence in developing countries in recent years [2]. The clinical features of IBD include pathological damage, ulceration of intestinal mucosa, diarrhea, as well as bloody stool [3]. Various factors have been involved in the pathogenesis of IBD, such as environmental exposure, diet shortage, innate and required immunity deficiency, or gut microbiota dysregulation [4,5]. Until now, the conventional treatment of IBD has still used drugs such as methylprednisolone, infliximab, and cyclosporine. However, the continued refractory as well as the serious adverse effects are strongly troubling to the patients [6]. Recently, it has been indicated that polysaccharides from the natural source could alleviate IBD symptoms, which might be an alternative element in the treatment of IBD [7].

Natural polysaccharides can improve intestine damage recovery, through their structural characteristics, molecular weight, and bioactivities. The diversity of microbiota is significantly reduced comparing the IBD patient with normal people. The microbiota dysbiosis in the gut is recognized as the increase of *Proteobacteria* and *Bacteroidetes*, and simultaneously a decrease in the abundance of *Firmicutes*, *Lactobacillis* and *Clostridia* [8,9]. Polysaccharides can be metabolized by gut microbiota and finally fermented into SCFAs, then SCFAs modulate the gut microbiota ratio via increase the abundance of probiotics and decrease the ratio of bacteria harmful to the gut mucosa.

**Citation:** Wang, Y.; Zhu, H.; Wang, X.; Yu, Y.; Xie, J. Natural Food Polysaccharides Ameliorate Inflammatory Bowel Disease and Its Mechanisms. *Foods* **2021**, *10*, 1288. https://doi.org/10.3390/foods10061288

Academic Editor: Barry J. Parsons

Received: 4 May 2021 Accepted: 2 June 2021 Published: 4 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Recently, more and more research has attached great importance to the immunity effect in the IBD development [10]. On the one hand, the supplement of natural polysaccharides up-regulates the tight junction proteins, which not only decreases the neutrophil infiltration through intestine mucosa, but also promotes the recovery of the epithelial barrier of the colon. On the other hand, administrating polysaccharides can influence the production of inflammatory cytokines in the IBD patients' gut. As the indispensable component of the immune responses, inflammatory cytokines send signals to the numerous immune cells, which are critical to cellular immunity regulation. Excessive pro-inflammatory cytokines, like interleukin (IL)-6, IL-1β, tumor necrosis factor (TNF)-α, induce diseases associated with immunity reaction, while anti-inflammatory cytokines ameliorate the inflammation and promote recovery [9]. Thus, recovering the balance of inflammatory cytokine production via natural polysaccharides is an essential mechanism to the gut defensive response.

The excretion of inflammatory cytokines is regulated by the inflammatory signaling pathways including toll-like receptors (TLR), mitogen-activated protein kinase (MAPK), nuclear factor-κB (NF-κB), and G-protein coupled receptors, which participate in regulating the phosphorylation level of TLR2/4/6, ERK, IκB, GPR41, and GPR43 [11]. By regulating these signal transduction pathways, natural polysaccharides and their metabolites ameliorate IBD symptoms in the damaged tissues. Polysaccharides alleviate oxidative stress through down-regulating the production level of oxygen free radicals including myeloperoxidase (MPO), nitric oxide (NO), malondialdehyde (MDA), which subsequently facilitate IBD symptoms [12].

Most of the reviews related to natural polysaccharides have paid attention to their structures and anti-inflammatory applications in pharmaceuticals. However, few discussed the effects of natural polysaccharides and their mechanism involved in colitis in detail and in its entirety. This current review summarizes the latest linkages of natural polysaccharides and IBD, including the types of natural polysaccharides that alleviate the IBD symptoms and the mechanisms of moderation of this disease. The main objective is to discuss the relationship between natural polysaccharides and IBD treatment.

#### **2. Polysaccharides and Their Impact on Inflammatory Bowel Diseases**
