Probiotics, Prebiotics, Gut Microbiota, and Obesity

| November 17, 2009

by Margaret Furtado, MS, RD, LDN

Bariatric Times
. 2009;6(11):27–30.

INTRODUCTION
Probiotics are nonpathogenic live microorganisms that are believed to confer health benefits to the host when ingested. Researchers have suggested weight loss and/or antiobesity effects are among these benefits. Prebiotics are nondigestible oligosaccharides believed to act as “fertilizers” of colonic microbiota, enhancing growth of beneficial commensal organisms (e.g. Bifidobacterium, Lactobacillus). Prebiotics are believed to confer health benefits on the host, including modulation of lipid metabolism, and researchers have suggested potential antiobesity effects as well, possibly by modulating gut hormones, such as peptide YY (PYY) and glucagon-like peptide-1(GLP-1). The combination of prebiotics and probiotics, termed synbiotics, are believed to possess enhanced health benefits acting as a “functional food.” This article reviews the role of prebiotics and probiotics on obesity and discusses research that suggests that individuals with obesity may have differing amounts of particular microbiota (e.g. Firmicutes) and that gastric bypass surgery may alter gut microbiota in a favorable way.

ABSTRACT
This article provides an overview of probiotics and prebiotics as well as discussion on their relationship to gut microbiota and connection with current research, which suggests a correlation between particular types of gut bacteria and obesity. The role of genomics in further elucidating gut microbiota function will also be reviewed. A provocative look into the possible future applications in obesity medicine will also be provided

PROBIOTICS
Probiotics are nonpathogenic live microorganisms that, when ingested, confer health benefits to the host. The anti-obesity effect of Lactobacillus rhamnosus (L. rhamnosus) PL60, a bacterium of human origin that produces conjugated linoleic acid (CLA), was studied in diet-induced obese mice.[1] After eight weeks of oral feeding with L. rhamnosus PL60, mice lost weight without reducing energy intake. CLA has been suggested to possess a number of potential health benefits based on animal studies, including the ability to reduce body fat. Therefore, this is one way in which probiotics are believed to possess antiobesity effects.

Ingestion of probiotics is recommended as a preventative approach to maintaining the balance of the intestinal microflora and thereby enhancing health and well being. A survey of the literature indicates positive results in over 50 human clinical trials, with prevention/treatment of infections the most frequently reported output.[2] Increased levels of probiotics may induce a “barrier” influence against common pathogens. Mechanisms of effect are likely to include excretion of acids (lactate, acetate), competition for nutrients and gut receptor sites, immunomodulation, and formation of specific antimicrobial agents.[3]

Recent evidence suggests a compelling role for probiotics in enhancing liver health, in particular nonalcoholic fatty liver disease (NAFLD).[4] Intestinal microflora are believed to play a large role in the progression from NAFLD to cirrhosis of the liver. When gut microbiota are high in facultative microbes (e.g., Enterobacteriaceae) and low in anaerobes (e.g., Bifidobacteria), higher levels of ammonia and endotoxins result.[4] Probiotics have been shown to modulate intestinal microflora and increase the anaerobic population. The genomics era is now providing tools to more effectively understand probiotics’ interaction in the gut.

Probiotics have been linked to weight loss in humans as well as in animals. Celleno et al[5] examined the effects of the Phaseolus vulgaris (P. vulgaris) extract, which is derived from white kidney beans. This probiotic has previously been shown to inhibit the activity of the digestive enzyme alpha amylase. These researchers examined the effect of P. vulgaris on the body composition of human subjects who were overweight in a randomized, double-blinded, placebo-controlled study of 60 preselected subjects with moderate overweight and stable weight for at least six months. Two groups of subjects, matched to be homogeneous for age, gender, and body weight, were involved in the study. One group took 445mgs of P. vulgaris daily prior to their main meal rich in carbohydrates for 30 days, while the other group, the control group, received a placebo. Body weight, fat, and nonfat mass, and skin-fold measures were taken in both groups.

After 30 days, subjects receiving the P. vulgaris extract with a carbohydrate-rich, 2,000-to-2,200kcal-a-day diet had significantly greater reduction of body weight (p<0.001), with decreases in body mass index (BMI), fat mass, adipose tissue thickness, and waist/hip/thigh circumferences, while maintaining lean body mass (LBM), compared to subjects who received the placebo. The researchers’ conclusions suggest that P. vulgaris produces significant decrements in body weight and suggests decrements in fat mass in the face of maintained LBM.

PREBIOTICS
Prebiotics are nondigestible oligosaccharides that act as “fertilizers” of colonic microbiota, enhancing growth of beneficial commensal organisms (e.g., Bifidobacterium, Lactobacillus). Inulin and oligofructose (naturally occurring fructooligosaccharides [FOS]) are prebiotics that have gained more exposure lately as possible health-enhancing agents. Prebiotics modulate lipid metabolism, most likely via fermentation products. The most common sources are wheat, onions, bananas, garlic, and leeks. Americans are said to consume anywhere from 1 to 4 grams of prebiotics per day, while Europeans are estimated to ingest 3 to 11 grams per day. Beta configuration of anomeric C-2 in fructose monomers result in their decreased absorption in the gastrointestinal (GI) tract without affecting mineral absorption.[6] In high-fat-oligofructose-treated mice, a significant positive correlation between the Bifidobacterium species and improved glucose tolerance, glucose-induced insulin secretion, and normalized inflammatory tone have been observed.[6]
Inulin-type fructans claims, based on research,[6] include constipation relief due to increased intestinal motility as well as suppression of diarrhea, especially intestinal infections. Reduction of osteoporosis risk has also been found and is believed to be related to the inulin-type fructans’ ability to improve the bioavailability of calcium and cause a physiologic change in peak bone density and muscle mass. These fructans are also suggested to reduce the risk of atherosclerotic heart disease, as well as reduce the risk of obesity. Finally, they are believed to possibly lower type 2 diabetes risk via improvements in insulin resistance.[6]

PREBIOTICS AND GUT HORMONES
The effects of prebiotics on abdominal adiposity and gut peptides (gut peptide YY [PYY]) and glucagon-like peptide [GLP-1]) were examined in three rat studies[6] to determine if fermentable and nonfermentable fibers differed in metabolic effects. In Study 1, rats were fed one of three diets with different metabolizable energy densities. In Study 2, they were fed diets with similar fiber levels, using high-amylose-resistant cornstarch (RS). Finally, in Study 3, the rats were fed diets with a similar dilution of metabolizable energy using cellulose or RS. Measurements were taken of their food intake, body weights, abdominal fat, plasma PYY and GLP-1, GI tract weights, and gene transcription of PYY and proglucagon. Study 2 rats fed RS had increased cecal weights and plasma PYY and GLP-1 and increased gene transcription of PYY and proglucagon. Study 3 rats fed RS had increased short-chain-fatty-acids (SCFAs) in cecal contents, increased plasma PYY, and gene transcription for PYY and proglucagon. The investigators concluded that RS inclusion in the diet may affect energy balance through their effect as a fiber or as a stimulator of PYY and GLP-1 expression. In addition, increasing gut hormone signaling with a bioactive functional food, such as RS, may be an effective natural approach to the treatment of obesity.[7]

THE PREBIOTICS/PROBIOTICS COMBINATION
The combination of pre- and probiotics, known as synbiotics, has been proposed to characterize some colonic foods with interesting nutritional properties that make these compounds candidates for classification as health-enhancing, functional food ingredients that are capable of altering the gut microbiota.[8]

THE HUMAN GUT
The human gut includes 100 billion microorganisms per milliliter of material in the colon, with a total of 100 trillion microorganisms in the GI tract. Compare that with the estimated 10 trillion cells in the human body, and we are actually more bacteria than cells.

An adult human’s GI tract contains approximately 1012 microorganisms per milliliter of luminal content, with at least 1,800 genera and between 15,000 and 36,000 distinct species of bacteria. Gut microorganisms, also termed gut microbiota, have been classified based on phylogeny (e.g., 16sRNA sequence similarities and differences). Microbiota are defined as a collection of microorganisms normally associated with a particular tissue or organ. Microbioma may be defined as the collection of the microbial genomes.[9]

Until recently, understanding of the human gut microbiota was limited due to the reliance on conventional microbiological techniques (e.g., selective culturing). However, with the development of advanced methods (e.g., molecular fingerprinting and ecological statistical approaches), much more thorough and reliable assessment is possible. In particular, sequencing of 16S ribosomal ribonucleic acid (rRNA) genes from amplified bacterial nucleic acids extracted from fecal material or mucosal samples has greatly facilitated the identification and classification of bacteria.[10]

FUNCTIONS OF GUT MICROBIOTA
Gut microbiota are believed to have numerous functions, including breakdown of dietary toxins and carcinogens, synthesis of micronutrients, fermentation of indigestible food substances, absorption of electrolytes and trace minerals, growth and differentiation of enterocytes and colonocytes through the production of short-chain fatty acids (SCFAs), and assistance in the prevention of luminal colonization by pathogenic bacteria, such as Escherichia coli (E. coli) and Firmicutes clostridia (F. clostridia), as well as Salmonella and Shigella species.[11]

GUT MICROBIOTA AND OBESITY
Trillions of bacteria normally reside in the human GI tract, collectively referred to as the gut microbiota, and affect nutrient acquisition and energy regulation.  It has been proposed that people of “normal” weight possess different gut microbiota than those with overweight or obesity.[12] These findings raise the possibility that the gut microbiota plays an important role in regulating weight and may be a factor in the development of obesity.
Metabolic activities of the gut microbiota facilitate the extraction of calories from ingested dietary substances and help store calories in host adipose tissue for later use. Research investigating the gut bacterial flora of obese mice and humans has discovered fewer Bacteroidetes and more Firmicutes than lean counterparts, suggesting differences in caloric extraction of ingested food substances due to the composition of the gut microbiota.[13] Therefore, modifying gut microbiota may have a role in the future treatment of obesity.

COULD GUT MICROBIOTA EXPLAIN WHY SOME PEOPLE WITH OBESITY DO NOT OVEREAT?
Conventionally reared mice have 40-percent higher body fat content and 47-percent higher gonadal fat content than germ-free mice, even though they consumed less food than their germ-free counterparts.[14] The distal gut microbiota of normal mice was transplanted into gnotobiotic mice (a process called conventionalization). There was a 60-percent increase in body fat within two weeks, without any increase in food consumption or obvious differences in energy expenditure.

To assess the relative abundance of various types of gut bacteria in obese and lean mice, Ley et al[15] analyzed bacterial 16S rRNA gene sequences from cecal microbiota of genetically obese (ob/ob) mice, their lean (ob/+ and +/+) siblings, and their (ob/+) mothers), all fed the same polysaccharide-rich diet. Results showed that ob/ob mice had 50-percent fewer Bacteroidetes and more Firmicutes than their lean littermates. This was unrelated to differences in food consumption.

GUT MICROBIOTA RESEARCH: BACTEROIDETES AND FIRMICUTES
Two divisions of beneficial bacteria—Bacteroidetes and Firmicutes—reside in the gut, and the balance of the two appears to be important in determining an individual’s propensity for obesity. An association has been found between obesity and an increased relative abundance of Firmicutes, compared with Bacteroidetes. As has been suggested from the studies cited previously, lean invidividuals possess more Bacteroidetes and those with obesity possess more Firmicutes.[16]

GUT MICROBIOTA AND OBESITY RESEARCH IN CHILDREN
Kalliomaki et al[17] sought to establish whether early gut microbiota composition can guide weight development throughout early childhood. Their research design included the formation of two groups, one with overweight and obese children (N=25; 7 with obesity, 18 overweight), and another group of normal-weight children (N=24). They were selected from a prospective follow-up study on probiotics and allergies at the age of seven years. The two groups were chosen from the same cohort and matched for gestational age, BMI, mode of delivery, probiotic supplementation, duration of breastfeeding, use of antiobiotics during infancy, and frequencies of atopic diseases and sensitization. Early fecal microbiota composition analysis was performed by fluorescent in-situ hybridization (FISH) with microscopic and flow cytometry detection and quantitative real-time polymerase chain reaction (qRT-PCR).

The results of this study revealed that Bifidobacterial numbers in fecal samples during infancy, as assessed by FISH with flow cytometry, was higher in children remaining normal weight versus children becoming overweight (p=0.02). In addition, there were increased Bifidobacteria with breastfeeding. A similar tendency was found by FISH with microscopic detection and qRT-PCR. Microbiota aberrancy during infancy in children becoming overweight was associated with greater numbers of Staphylococcus aureus (S. aureus) than in children remaining normal weight (p=0.013). The authors concluded that changes in gut microbiota may be linked not only to allergies, but also to other chronic inflammatory conditions, such as obesity. This has been termed the hygiene hypothesis.

GASTRIC BYPASS SURGERY (GBP) AND GUT MICROBIOTA
Utilizing genomic science, Zhang et al[18] examined 184,094 sequences of microbial 16S rRNA from PCR amplicons using 454 pyrosequencing technology in order to compare the microbial community structures of nine human subjects, three in each of the following categories: 1) normal weight, 2) severe obesity, and 3) status post (s/p)-GBP surgery. Although phylogenic analysis revealed that the bacteria in the human intestinal community are diverse, they fell into six bacterial divisions that had distinct differences in the three study groups. The results of this study revealed that Firmicutes were dominant in individuals of normal weight and individuals, but significantly decreased in the postoperative GBP subjects. In addition, GBP subjects had increased Gammaproteobacteria. The group with obesity had increased H2-utilizing methanogenic Archaea versus normal and postoperative GBP subjects. The authors of this study concluded that interspecies H2 transfer between bacterial and Archael species are believed to be an important mechanism for increasing energy uptake by the human colon in people with obesity. In addition, a large bacterial population shift s/p GBP may reflect the double-impact of gut alteration caused by the surgical procedure and consequent changes in food ingestion and digestion (Figure 1).[18]

POSSIBLE IMPLICATIONS OF PROBITOIC, PREBITOTOC, AND GUT MICROBIOTA AND GENOMIC RESEARCH
As depicted in Figure 1 and Figure 2, phylogenic clustering reveals subjects s/p GBP had more similarities in their gut microbiota to those of normal weight than those with obesity. Since genomic science and phylogeny interpretation is beyond the scope of this article, suffice it to say that this study, albeit small in sample size, appears to suggest that GBP surgery may change the types of gut microbiota, making the “bacterial neighborhoods” closer to those of people of normal BMIs, as compared with people with severe obesity who have not had GBP surgery. However, many questions remain that will require more research, including the following:
•    Are differences in gut microbiota in normal-weight individuals versus those with overweight or obesity the cause or result of obesity?
•    What are hormonal and/or other signals that potentially direct changes in make-up of gut microbiota? Can they be induced by probiotics and/or prebiotics?
•    What are the mechanisms responsible for the relative proportions of Bacteroidetes and Firmicutes in mice and humans?
•    Could gut microbiota and genomic research one day make GBP surgery obsolete?
•    Could the extermination of a obesity-related bacteria virtually eliminate obesity?

In the interim, what is known thus far is that probiotics and prebiotics appear to modify the gut microbiota in a positive way. Some of the modifications of the gut microbiota include an increase in the production of PYY and GLP-1, which may have weight loss properties. Probiotics and prebiotics also may be helpful in terms of decreasing inflammation, so it appears prudent to include them as part of a healthy, balanced diet.

Although researchers have been utilizing the science of genomics to make strides in gut microbiota research, many more questions remain to be elucidated. Randomized, double-blinded, prospective, clinical trials are needed to assess the efficacy of probiotics and prebiotics on gut microbiota and obesity. In addition, the effects of metabolic surgeries, such as GBP, need to be investigated, in terms of the changing gut microbiota and effects on long-term weight regulation.

Author’s Note

REFERENCES
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