Prebiotics, gut microbiota and metabolic risks: Unveiling the relationship
Introduction
Prebiotics are non-digestible food components that selectively stimulate the growth or activity of specific indigenous bacteria in the digestive tract in a manner claimed to be beneficial for the host. They were identified in the early 1950s by Gyorgy (1953) as “bifidus factor,” a bifidogenic substance that selectively improved the growth of bifidobacteria. The term prebiotic was first devised in 1995 by Gibson and co-workers (Gibson, Roberfroid, 1995, Sharma et al, 2012). Earlier work on prebiotics only elaborated on the microbial changes in the human digestive ecosystem. Later work provided evidence that prebiotics allow desirable changes in the composition as well as the activity of the gastrointestinal microflora and confer health benefits to the host (Kanakupt et al, 2011, Nagpal et al, 2014).
Prebiotics are primarily carbohydrates (oligosaccharides and polysaccharides), but may include some non-carbohydrate moieties. Soluble fibres are the most prevailing type of prebiotics. Nevertheless, various other forms of dietary fibre may serve the purpose. In this context, criteria have been established to categorize any food ingredient as a candidate prebiotics: they have the ability to resist gastric hydrolysis by digestive enzymes and remain unabsorbed in the upper gastrointestinal tract; they undergo fermentation by resident microbiota in the large intestine; and they stimulate the activity/growth of potentially beneficial intestinal bacteria (Xiao et al, 2014, Yeo et al, 2009).
Mechanistically, prebiotics are non-viable food constituents having pronounced effect on human health by modulating colonic microflora (FAO, 2007). They induce specific changes in the composition of gut microbiota, increase the number of bifidobacteria and lactobacilli. Alongside, prebiotics decrease the toxin-producing bacteria like bacteroides, proteolytic clostridia and Escherichia coli (Ogueke, Owuamanam, Ihediohanma, & Iwouno, 2010). According to the criteria, many non-digestible food oligosaccharides exhibit prebiotic activities. Likewise, fructooligosaccharides, xylooligosaccharides, isomaltooligosaccharides and glucooligosaccharides as well as some sugar alcohols and polysaccharides (modified and resistant starch) are also included in this category. They can either be produced enzymatically or found naturally in some plants (Cummings, Macfarlane, & Englyst, 2001).
The promising health benefits associated with prebiotics are improvement in the gastrointestinal microflora, enhanced mineral absorption, stimulation of the immune system, reduced risk of irritable bowel syndrome and of constipation (Gibson, Probert, van Loo, Rastall, & Roberfroid, 2004). They also prevent colorectal cancer and exhibit cholesterol lowering potential (Kelly, 2003). Nevertheless, the health enhancing effects of prebiotics are not direct as they selectively nourish the microbial community i.e. lactobacilli and bifidobacteria that in turn improve gut health. Approximately 300 to 500 species of bacteria occur in the human gastrointestinal tract that becomes denser in the large intestine, approaching a concentration of microbial cells 1011/g of luminal content (Guarner & Malagelada, 2003). It has been established that almost 55% of faecal bulk consists of microorganisms. The microbial colonization in the digestive tract is markedly influenced by transit time and luminal pH (Paineau et al., 2008).
The majority of colon bacteria are anaerobes that avail energy through fermentation. In this context, main fermentative substrates from dietary source are non-digestible carbohydrates i.e. oligosaccharides, fibres, resistant starches and non-starch polysaccharides that escape digestion in the small intestine. Nonetheless, carbohydrate fermentation products are effective substrates leading to gradient utilization by the colon (Macfarlane, Steed, & Macfarlane, 2008). The ascending colon can break down the sugars and the mass of incoming carbohydrates is being fermented to short chain fatty acids (SCFAs), mainly acetate, propionate and butyrate. Besides, other metabolites like pyruvate, lactate, succinate and ethanol as well as gases H2, CO2, H2S and CH4 are produced (Whelan, Judd, & Preedy, 2005).
SCFAs are captured by the colonic mucosa and contribute towards energy needs of the host. Acetate is primarily metabolized in the kidney, heart, brain and human muscles, whilst propionate, a glucogenic precursor, suppresses cholesterol production. Butyrate is utilized by the colonic epithelium where it helps in cell growth regulation and differentiation (Sangwan, Tomar, Singh, Singh, & Ali, 2011).
Prebiotics exhibit imperative technological properties as well as attractive nutritional value. In food formulations, they appreciably upgrade sensory features and improve taste and mouth feel. In order to become part of functional food, prebiotics must be stable to processing conditions like heat, pH and Maillard reaction because degraded mono- and disaccharides are not available for bacterial fermentation. Previous investigations on prebiotics have shown that heating at low pH causes reduction in prebiotic activity whilst other conditions did not alter stability (Al-Sheraji et al, 2013, Bohm et al, 2006).
Inclusion of prebiotics in food is a natural way to provide healthy ingredients to the consumers. Most of the prebiotics are easily consumable and give desired functionality to the food items (Courtin, Swennen, Verjans, & Delcour, 2009). For instance, short chain prebiotics act like sugars and contribute to crispiness and browning of the end product. Long chain prebiotics work as fat replacer, escalate the texture and mouthfeel. The majority of the prebiotics are not considerably distorted or damaged by processing treatments, thus retaining their functionality throughout the alimentary tract. Contrarily, most of the probiotics in the finished products have been killed due to harsh processing conditions that are required to eradicate microbes for food safety reasons (Bohm et al., 2006).
Section snippets
Prebiotics and metabolic syndromes
Recent advances to curtail disease progression have opened new avenues for the development of prebiotic-based dietary interventions to halt the incidence of metabolic dysfunction (Riccioni et al., 2012). In the present scenario, production of designer foods with natural ingredients like prebiotics is a pragmatic approach. A number of epidemiological studies have illuminated the cardioprotective effect of diets high in phytochemicals (vitamins, minerals, antioxidants) and fibres (Ignarro,
Bifidogenic effects of prebiotics
Dietary patterns based on therapeutic ingredients influence consumers health in multifarious ways. Likewise, the intestinal probiotics affects numerous physiological aspects and are helpful to generate desirable constituents. The association between colonic microorganisms and vulnerability to diseases has set forth the demand of new functional products for healthy intestinal activity (Peng et al, 2015, Sonnenburg et al, 2010). Diet can alter the functional metabolism of the intestinal
Hypocholesterolaemic perspectives
Increased serum cholesterol and triacylglycerols (TAG) along with (abdominal) obesity, insulin resistance and arterial hypertension are components of the metabolic syndrome. Cholesterol is the main cause for the accumulation of fatty deposits in the inner lining of arteries leading to atherosclerosis and onset of heart disease, stroke and other vascular diseases. It is estimated that about 18% of the stroke events and 56% of the heart diseases worldwide are ascribed to high cholesterol level (
Hypoglycaemic facets
The high prevalence of diabetes in the developing world is shifting towards an epidemic. The striking increase in obesity and weight gain are major risk factors contributing to various metabolic complications. The risk of insulin resistance increases with advancing obesity (Deguchi & Miyazaki, 2010). Obesity may lead to an insulin resistant state in the liver, muscle cells and adipose tissue, resulting in distorted function of insulin targeted cells and buildup of macrophages that secrete
Conclusion
Results from these studies support the potential of prebiotics especially fructooligosaccharides against physiological threats including hypercholesterolaemia, hyperglycaemia and intestinal disorders. Prebiotics appear to exert their beneficial effects through various mechanisms. The studies discussed here provide clinical and experimental evidence for supplementation of diet with prebiotics. However, there are only few human studies which support this hypothesis that warrant the need for
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