Probiotics confer immunological protection to the host through the regulation, stimulation, and modulation of immune responses. Researchers have shifted their attention to better understand the immunomodulatory effects of probiotics, which have the potential to prevent or alleviate certain pathologies for which proper medical treatment is as yet unavailable. It has been scientifically established that immune cells (T- and B-cells) mediate adaptive immunity and confer immunological protection by developing pathogen-specific memory. However, this review is intended to present the recent studies on immunomodulatory effects of probiotics. In the early section of this review, concepts of probiotics and common probiotic strains are focused on. On a priority basis, the immune system, along with mucosal immunity in the human body, is discussed in this study. It has been summarized that a number of species of Lactobacillus and Bifidobacterium exert vital roles in innate immunity by increasing the cytotoxicity of natural killer cells and phagocytosis of macrophages and mediate adaptive immunity by interacting with enterocytes and dendritic, Th1, Th2, and Treg cells. Finally, immunomodulatory effects of probiotics on proinflammatory and anti-inflammatory cytokine production in different animal models have been extensively reviewed in this paper. Therefore, isolating new probiotic strains and investigating their immunomodulatory effects on cytokine profiles in humans remain a topical issue.

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Immunomodulatory Effects of Probiotics on Cytokine Profiles

Melatonin has been shown to improve lipid metabolism and gut microbiota communities in animals and humans; however, it remains to know whether melatonin prevents obesity through gut microbiota. Here, we found that high-fat diet promoted the lipid accumulation and intestinal microbiota dysbiosis in mice, while oral melatonin supplementation alleviated the lipid accumulation and reversed gut microbiota dysbiosis, including the diversity of intestinal microbiota, relative abundances of Bacteroides and Alistipes, and functional profiling of microbial communities, such as energy metabolism, lipid metabolism, and carbohydrate metabolism. Interestingly, melatonin failed to alleviate the high-fat-induced lipid accumulation in antibiotic-treated mice; however, microbiota transplantation from melatonin-treated mice alleviated high-fat diet-induced lipid metabolic disorders. Notably, short-chain fatty acids were decreased in high-fat diet-fed mice, while melatonin treatment improved the production of acetic acid. Correlation analysis found a marked correlation between production of acetic acid and relative abundances of Bacteroides and Alistipes. Importantly, sodium acetate treatment also alleviated high-fat diet-induced lipid metabolic disorders. Taken together, our results suggest that melatonin improves lipid metabolism in high-fat diet-fed mice, and the potential mechanisms may be associated with reprogramming gut microbiota, especially, Bacteroides and Alistipes-mediated acetic acid production. Future studies are needed for patients with metabolic syndrome to fully understand melatonin's effects on body weight and lipid profiles and the potential mechanism of gut microbiota.

Melatonin reprogramming of gut microbiota improves lipid dysmetabolism in high-fat diet-fed mice.

Optimal nutrition can improve well-being and might mitigate the risk and morbidity associated with coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This review summarizes nutritional guidelines to support dietary counseling provided by dietitians and health-related professionals. The majority of documents encouraged the consumption of fruits, vegetables, and whole grain foods. Thirty-one percent of the guidelines highlighted the importance of minerals and vitamins such as zinc and vitamins C, A, and D to maintain a well-functioning immune system. Dietary supplementation has not been linked to COVID-19 prevention. However, supplementation with vitamins C and D, as well as with zinc and selenium, was highlighted as potentially beneficial for individuals with, or at risk of, respiratory viral infections or for those in whom nutrient deficiency is detected. There was no convincing evidence that food or food packaging is associated with the transmission of COVID-19, but good hygiene practices for handling and preparing foods were recommended. No changes to breastfeeding recommendations have been made, even in women diagnosed with COVID-19.

Dietary recommendations during the COVID-19 pandemic.

The gut microbiota (GM) is defined as the community of microorganisms (bacteria, archaea, fungi, viruses) colonizing the gastrointestinal tract. GM regulates various metabolic pathways in the host, including those involved in energy homeostasis, glucose and lipid metabolism, and bile acid metabolism. The relationship between alterations in intestinal microbiota and diseases associated with civilization is well documented. GM dysbiosis is involved in the pathogenesis of diverse diseases, such as metabolic syndrome, cardiovascular diseases, celiac disease, inflammatory bowel disease, and neurological disorders. Multiple factors modulate the composition of the microbiota and how it physically functions, but one of the major factors triggering GM establishment is diet. In this paper, we reviewed the current knowledge about the relationship between nutrition, gut microbiota, and host metabolic status. We described how macronutrients (proteins, carbohydrates, fat) and different dietary patterns (e.g., Western-style diet, vegetarian diet, Mediterranean diet) interact with the composition and activity of GM, and how gut bacterial dysbiosis has an influence on metabolic disorders, such as obesity, type 2 diabetes, and hyperlipidemia.

https://www.mdpi.com/2072-6643/12/4/1096/pdf

You Are What You Eat-The Relationship between Diet, Microbiota, and Metabolic Disorders-A Review.

Dietary protein and its metabolites, amino acids, are essential nutrients for humans and animals. Accumulated research has revealed that the gut microbiota mediate the crosstalk between protein metabolism and host immune response. Gut microbes are involved in the digestion, absorption, metabolism and transformation process of dietary protein in the gastrointestinal tract. Amino acids can be metabolized into numerous microbial metabolites, and these metabolites participate in various physiological functions related to host health and diseases. The components of dietary protein impact the gut microbiota composition and microbial metabolites. The source, concentration, and amino acid balance of dietary protein are primary factors which contribute to the composition, structure and function of gut microbes. A suitable ratio between protein and carbohydrate or even a low protein diet is recommended over a diet with protein in excess of requirements. Greater levels and undigested protein lead to an increase of pathogenic microorganism with associated higher risk of metabolic diseases. Herein, the crosstalk between dietary protein and gut microbiota composition and function is summarized, which will help to reveal the potential mechanism of gut microbes on the gastrointestinal tract health.

Dietary Protein and Gut Microbiota Composition and Function

Probiotics are microbial strains that are beneficial to health, and their potential has recently led to a significant increase in research interest in their use to modulate the gut microbiota. The animal gut is a complex ecosystem of host cells, microbiota, and available nutrients, and the microbiota prevents several degenerative diseases in humans and animals via immunomodulation. The gut microbiota and its influence on human nutrition, metabolism, physiology, and immunity are addressed, and several probiotic species and strains are discussed to improve the understanding of modulation of gut microbiota. This paper provides a broad review of several Lactobacillus spp., Bifidobacterium spp., and other coliform bacteria as the most promising probiotic species and their role in the prevention of degenerative diseases, such as obesity, diabetes, cancer, cardiovascular diseases, malignancy, liver disease, and inflammatory bowel disease. This review also discusses a recent study of Saccharomyces spp. in which inflammation was prevented by promotion of proinflammatory immune function via the production of short-chain fatty acids. A summary of gut microbiota alteration with future perspectives is also provided.

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Probiotic Species in the Modulation of Gut Microbiota: An Overview.

Piglets experience severe growth challenges and diarrhea after weaning due to nutritional, social, psychological, environmental, and physiological changes. Among these changes, the nutritional factor plays a key role in postweaning health. Dietary protein, fibre, starch, and electrolyte levels are highly associated with postweaning nutrition diarrhea (PWND). In this review, we mainly discuss the high protein, fibre, resistant starch, and electrolyte imbalance in diets that induce PWND, with a focus on potential mechanisms in weaned piglets.

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What Is the Impact of Diet on Nutritional Diarrhea Associated with Gut Microbiota in Weaning Piglets: A System Review.

Inflammatory Bowel Disease (IBD) is a kind of chronic inflammation, which has increasing incidence and prevalence in recent years. IBD mainly divides into Crohn's disease (CD) and ulcerative colitis (UC). It is hard to cure IBD completely, and novel therapies are urgently needed. Amino acids (AAs) and their metabolites are regarded as important nutrients for humans and animals and also play an important role in IBD amelioration. In the present study, the potential protective effects of AAs and their metabolites on IBD had been summarized with the objective to provide insights into IBD moderating using dietary AAs and their metabolites as a potential adjuvant therapy.

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Roles of Dietary Amino Acids and Their Metabolites in Pathogenesis of Inflammatory Bowel Disease.

Background. Oxidative stress is associated with infertility. This study was conducted to determine the effects of glutamate and aspartate on serum antioxidative enzymes, sex hormones, and genital inflammation in boars suffering from oxidative stress. Methods. Boars were randomly divided into 4 groups: the nonchallenged control (CON) and H2O2-challenged control (BD) groups were fed a basal diet supplemented with 2% alanine; the other two groups were fed the basal diet supplemented with 2% glutamate (GLU) or 2% aspartate (ASP). The BD, GLU, and ASP groups were injected with hydrogen peroxide (H2O2) on day 15. The CON group was injected with 0.9% sodium chloride solution on the same day. Results. Dietary aspartate decreased the malondialdehyde (MDA) level in serum (P < 0.05) compared with the BD group. Additionally, aspartate maintained serum luteinizing hormone (LH) at a relatively stable level. Moreover, glutamate and aspartate increased transforming growth factor-β1 (TGF-β1) and interleukin-10 (IL-10) levels in the epididymis and testis (P < 0.05) compared with the BD group. Conclusion. Both glutamate and aspartate promoted genital mRNA expressions of anti-inflammatory factors after oxidative stress. Aspartate more effectively decreased serum MDA and prevented fluctuations in serum sex hormones after H2O2 challenge than did glutamate.

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Effects of Glutamate and Aspartate on Serum Antioxidative Enzyme, Sex Hormones, and Genital Inflammation in Boars Challenged with Hydrogen Peroxide

Zinc (Zn) plays a crucial role in reducing oxidative stress and diarrhea in postweanling piglets. This study is aimed at comparing the effects of zinc chelate of 2-hydroxy-4 methyl-thio butanoic acid (HMZn) and ZnSO4 on the oxidative stress in weaned piglets. A total of 32 piglets were randomly divided into 4 treatments: CON: basal diet+80 mg/kg Zn as ZnSO4; DIQ: basal diet+80 mg/kg Zn as ZnSO4; HMZn: basal diet+200 mg/kg Zn as HMZn; and ZnSO4: basal diet+200 mg/kg Zn as ZnSO4. On day 15, the DIQ, HMZn, and ZnSO4 groups were injected intraperitoneally with diquat except for the CON group. The trial lasted 21 days. The results showed that zinc sources did not influence the growth performance during the first 14 days. But HMZn increased activities of superoxide dismutase (SOD), glutathione peroxidase (GPX), and total antioxidant capacity (T-AOC) in serum ( ). After diquat injection, the fecal score was decreased in the HMZn group. Both HMZn and ZnSO4 increased the activities of GPX and T-AOC in serum and the relative mRNA expressions of hepatic and renal Nrf2, SOD1, and GPX compared with the DIQ group ( ). Moreover, the relative mRNA expression of inflammatory factors in the small intestine, liver, and kidney was downregulated; the phosphorylation of NF-κB protein was inhibited in the HMZn group compared with the DIQ and ZnSO4 groups ( ). In general, HMZn showed notable advantage over ZnSO4 in reducing diarrhea and improving antioxidant and anti-inflammatory ability in piglets challenged with diquat.

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Tiejun Li

Papers

Probiotic Species in the Modulation of Gut Microbiota: An Overview.

What Is the Impact of Diet on Nutritional Diarrhea Associated with Gut Microbiota in Weaning Piglets: A System Review.

Roles of Dietary Amino Acids and Their Metabolites in Pathogenesis of Inflammatory Bowel Disease.

Effects of Glutamate and Aspartate on Serum Antioxidative Enzyme, Sex Hormones, and Genital Inflammation in Boars Challenged with Hydrogen Peroxide

Antioxidant and Anti-Inflammatory Effects of Different Zinc Sources on Diquat-Induced Oxidant Stress in a Piglet Model