Modulation of the Gut Microbiota by Nutrients with Prebiotic and Probiotic Properties^,,

Abstract

Experimental data in animals, but also observational studies in humans, suggest that the composition of the gut microbiota differs in obese vs. lean individuals, in patients with vs. without diabetes, or in patients presenting other diseases associated with obesity or nutritional disbalance, such as non-alcoholic fatty liver disease (NAFLD) or cardiovascular diseases. In this review, we describe how changes in the composition and/or activity of the gut microbiota by administration of nutrients with probiotic or prebiotic properties can modulate host gene expression and metabolism and thereby positively influence host adipose tissue development and related metabolic disorders.

State of the Art

Obesity always relates to “a positive energy balance,” implying that the total caloric intake is greater than the total energy expenditure over a relatively long period of time. In addition, the “obesogenic” diet is often rich in fats and poor in dietary fibers and carbohydrates with a low-glycemic index.

Increased attention was focused on the potential implication of the bacteria that colonize our gut, which in ideal conditions live in symbiosis with the host. Several papers and reviews support the idea that a “dysbiosis” (inadequate changes of gut microbiota composition and/or activity related to host disease) characterizes overweight or obese individuals or those with diabetes (1, 2). Regarding the inadequate composition of the gut microbiota, obese and overweight individuals were initially characterized by a change in the Firmicutes-to-Bacteroides ratio (1, 3). Nevertheless, these results were not strictly confirmed by other papers that extend the concept of dysbiosis to other bacterial phyla, genus, or species (for review, see 3). The analysis of the microbial composition of human fecal samples revealed that the bacterial population can be stratified into 3 robust clusters termed “enterotypes,” dominated by Bacteroides, Prevotella, or Ruminococcus, respectively (4). These enterotypes seem independent of nationality, age, gender, and BMI but are influenced by long-term dietary habits (5).

It appears that, in addition to these quantitative modifications of microbial phyla, obesity and some related metabolic diseases might be associated with modifications of microbial gene expression and therefore with the modulation of metabolic functions of the gut microbiota (6, 7). Thanks to the improvement in the exploration of gut microbiota, mainly through the development of metagenomic approaches, scientists are now able to identify and quantify gut microbial genes and to stratify individuals by their “gut bacterial richness.” Le Chatelier et al. (8) reported that, among overweight and obese Danish individuals, those characterized by a low number of gut microbial genes (meaning a low bacterial richness) present adiposity, insulin resistance, dyslipidemia, and inflammatory phenotype to a larger extent than those characterized by a high bacterial richness. Furthermore, Cotillard et al. (9) highlighted that overweight and obese individuals characterized by low bacterial gene richness seem quite resistant to dietary intervention and exhibit persistent inflammation. These results suggest that alterations of bacterial functions associated with obesity could explain the differential response to dietary intervention.

The microbiome is now considered as a new therapeutic target against obesity and its linked diseases (10). In fact, changes in dietary habits and especially an enrichment in some bioactive food components present in whole grain cereals are able to modify the composition of gut microbiota and could be helpful in the prevention of chronic diseases, including obesity and related disorders, such as type 2 diabetes (11). Wu et al. (5) showed that microbiome composition may change 24 h after initiating a high-fat (HF)4/low-fiber or a high-fiber/low-fat diet but that enterotype identity remained stable during a 10-d nutritional intervention. They suggest that nutrients like dietary fibers, which are not digested by host enzymes but are fermented by gut bacteria, could modulate the gut microbiome in a relatively short period of time, independently of the effect of changes in transit time.

Nowadays, gut microbiota modulation appears as an interesting tool in the prevention and/or treatment of the dysbiosis associated with obesity and metabolic disorders. The gut microbiota may be modulated by the administration of antibiotics, prebiotics, or probiotics or by fecal transplantation. This review will focus on the interest of probiotic and prebiotic approaches in the management of obesity and related diseases.

Interest of Probiotic Approaches in the Management of Obesity and Related Diseases

The oral delivery of viable bacterial strains (probiotics) allow their integration, even if transiently, into the gut ecosystem. Both rodent and human studies reveal interesting results issued from probiotic administration in the treatment or the prevention of obesity (3, 12, 13).

Most of the studies regarding the “anti-obesity” effects of probiotics performed in rodents are achieved with members of the genus Lactobacillus. Three papers (14–16) report that the administration of Lactobacillus gasseri BNR17 is able to suppress body-weight and fat-mass gain in high-sucrose diet-induced obese rodents and to reduce fasting glycemia in db/db mice. Another 3 studies (17–19) show that L. gasseri SBT2055 is able to decrease fat mass and adipocyte size in rodents. Furthermore, Miyoshi et al. (18) reveal that L. gasseri SBT2055 administration decreases adipose tissue inflammation and fat accumulation in the liver, 2 phenomena implicated in the complication of obesity, such as insulin resistance and NAFLD. These results suggest that, in addition to effects on body weight and fat mass, the administration of probiotics is able to counteract some metabolic diseases related to obesity. Other in vivo studies show that L. rhamnosus GG or L. sakei NR28 administration is able to decrease body-weight gain and adipose tissue weight in mice. Both strains are able to decrease lipogenic gene expression in the liver (20). Another study confirms that probiotic administration could modulate lipid metabolism. The administration of L. curvatus HY7601, combined or not with L. plantarum KY1032, reduces plasma cholesterol amount and hepatic lipid content (TGs and cholesterol) in diet-induced obese mice (21). However, probiotic administration is not always successful. For example, 1 study performed in ApoE^−/− mice with L. reuteri fails to show any improvement of atherosclerotic lesions, despite a decrease in body-weight gain, adipose tissue, and liver weight (22).

In addition to the studies performed with Lactobacillus species, several studies used specific Bifidobacterium strains alone, such as Bifidobacterium longum or B. adolescentis, or a combination of Bifidobacterium species (B. pseudocatenulatum SPM1204, B. longum SPM1205, and B. longum SPM1207). These studies show that Bifidobacterium spp. are able to decrease body-weight gain and adipose tissue in HF diet–induced obese rats (23–25). A recent study also demonstrated that the administration of the strain B. pseudocatenulatum CECT7765 to diet-induced obese mice can ameliorate metabolic and immunologic alterations associated with obesity (26).

Finally, a study performed with a commercial combination of probiotics (VSL#3: Lactobacillus spp., Bifidobacterium spp., Streptococcus thermophilus) demonstrated that VSL#3 administration decreases the hepatic inflammation induced by HF diet in young rats (27).

The anti-obesity effect described in the previously reported studies are not associated with any modification in food consumption, suggesting other specific effects of probiotic-strain ingestion on adipose tissue and weight gain. However, the mechanisms by which probiotics exert their beneficial effects are not elucidated yet. Some host targets, including lipid and glucose metabolism but also inflammation, are modulated by probiotic administration and could in part be implicated in the anti-obesity effect of probiotics. One study (28) showed that L. paracasei ssp. paracasei F19 administration in germ-free mice or HF diet–fed mice is able to increase circulating amounts of a lipoprotein lipase inhibitor (angiopoietin-related protein 4), leading to a decrease in fat storage. Another study (17) also suggested that probiotic administration could decrease dietary fat absorption. Insulin sensitivity may be improved by probiotic administration. Indeed, L. rhamnosus GG administration increases the production of insulin-sensitizing hormone, such as adiponectine, but also decreases gluconeogenesis in the liver (29). Finally, Lactobacillus spp. administration is associated with a decreased expression of proinflammatory genes in white adipose tissue of HF diet–fed mice (30).

In the majority of the studies mentioned above, the gut microbiota analysis after probiotic administration is lacking, so we cannot state that the selected bacteria per se are responsible for the improvement of obesity and related disorders. Furthermore, in a study reporting gut microbiota composition analysis, the authors showed that probiotic administration modifies the abundance of other bacteria. For example, the administration of L. acidophilus NCDC13 is associated with an increase of Bifidobacterium spp. (31).

The number of clinical studies remains limited Table 1. Only very few human intervention studies were designed to analyze the effect of probiotic administration on body fat and weight (3). In fact, the ingestion of L. gasseri SBT2055 allows for the reduction of fat-mass gain, body weight, BMI, waist, hip, and waist-to-hip ratio in the probiotic group compared with placebo group after 12 wk of intervention in overweight adults (32). These results were confirmed in a recent study with a lower dose of the same bacterial strain than the 1 used in the first study (10⁸ vs. 10¹¹ LG2055 CFU/d) (33). Another interventional study showed that the perinatal modulation of the gut microbiota by a probiotic [L. rhamnosus GG (ATCC53103; American Type Culture Collection)] is able to avoid excessive weight gain during the first years of life (34). Even if they are controversial, some papers suggested that probiotic administration might increase the growth of children and promote obesity (35, 36). However, a recent study counteracts this idea, reporting no effect on anthropometric variables or the serum lipoproteins profile after the administration of L. paracasei ssp. paracasei F19 from 4 to 13 mo of age (37). This study also shows that probiotic administration is associated with a decrease of palmitoleic acid (MUFA strongly linked to visceral obesity) and an increase of putrescin (polyamine with importance for gut integrity) amounts in the plasma (37). The effects on the health of the modifications of bacterial-derived metabolites need to be further evaluated to propose a mechanism by which selected probiotics are able to counteract obesity-related metabolic disorders. One trial suggested that the administration of L. acidophilus NCFM is able to prevent the decrease in insulin sensitivity observed in the placebo group, suggesting that this Lactobacillus strain is able to prevent the establishment of metabolic disorders associated with obesity, such as insulin resistance (38). However, other studies conducted with the strain L. casei Shirota fail to show any improvement of the gut permeability or low-grade inflammation in patients with the metabolic syndrome. The authors suggest that the lack of effect is probably due to the too-short duration of the study or the underdosing of the probiotic strain (39, 40). Another recent study (41) reports that the administration of L. salivarius Ls-33 in obese adolescents fails to show any improvement of inflammatory markers or metabolic variable linked to the metabolic syndrome.

TABLE 1.

Probiotic administration in human studies¹

Probiotic and Patients	Design	Dose/duration	Outcome	Reference
L. rhamnosus GG				(36)
Healthy, term infants	Double blind	107 CFU/g powdered milk	Increased weight and length
	Placebo-controlled	Until the age of 6 mo
L. paracasei ssp. paracasei F19				(37)
Healthy, term infants	Double blind	108 CFU/d	Length, weight, head circumference
	Placebo-controlled	From 4 to 13 mo of age
L. rhamnosus GG				(34)
Pregnant women and children	Double blind	1010 CFU/d	BMI and BWG in children
	Placebo-controlled	Mothers: 4 wk before delivery
		Children: 6 mo after birth
L. rhamnosus GG and B. lactis Bb12
Pregnant women	Double blind	1010 CFU/d each strain	Over the pregnancy: BWG, adiposity, insulin sensitivity	(80)
	Placebo-controlled	From trimester 1 of pregnancy until the end of exclusive breastfeeding	In the postpartum period: adiposity	(81)
L. acidophilus L-1				(43)
Normal-weight adults	Open trial	4.9 × 109 to 2.7 × 1010/d	BW, blood lipid profile
	Placebo-controlled	6 wk
B. breve CBG-C2 and Enterococcus faecalis FK-23				(82)
Healthy adults	Double blind	Doses not mentioned	BW, BMI, LDL cholesterol
	Placebo-controlled	8 wk
L. acidophilus La5 and B. lactis Bb12				(42)
Women with BMI < 30	Open trial	3.9 × 107/g each strain	BW, BMI, plasma lipid profile
	Placebo-controlled	300 g/d
		6 wk
L. gasseri SBT2055				(32, 33)
Overweight and obese adults	Double blind	108–1011 CFU/d	BWG, BMI, waist-to-hip ratio, adiposity
	Placebo-controlled	12 wk
L. salivarius Ls-33				(41)
Obese adolescents	Double blind	1010 CFU/d	BW, height, BMI, waist-to-hip ratio
	Placebo-controlled	12 wk
L. casei Shirota				(39)
Obese with metabolic syndrome	Open trial	3 × 6.5 × 109 CFU/d	No effect on body weight, BMI, and gut permeability
	Control group without placebo	3 mo
L. casei Shirota				(40)
Obese with metabolic syndrome	Open trial	3 × 6.5 × 109 CFU/d	Insulin sensitivity, endothelial function, low-grade inflammation
	Control group without placebo	12 wk
L. gasseri BNR17				(83)
Overweight and obese adults	Double blind	6 × 1010 CFU/d	BW, BMI, adiposity
	Placebo-controlled	12 wk
Lactobacillus spp.				(84)
Morbidly obese individuals after Roux-en-Y gastric bypass	Open trial	2.4 × 109/d	Weight loss
	Control group without placebo	6 mo
L. plantarum 299v				(85)
Males with moderately elevated blood cholesterol	Double blind	5 × 107 CFU/mL	Plasma lipids (TG, total, LDL cholesterol, HDL cholesterol) and glucose
	Placebo-controlled	200 mL/d
E. faecium and S. thermophilus				(86)
Healthy overweight and obese adults	Double blind	6 × 107/mL for E. faecium	BW and fat mass, LDL cholesterol
	Placebo-controlled	1 × 109/mL for S. thermophilus
		450 mL/d
		8 wk
L. plantarum 299v				(44)
Smokers	Double blind	5 × 107 CFU/mL	BMI, systolic blood pressure and other atherosclerosis risk factors
	Placebo-controlled	400 mL/d
		6 wk
L. acidophilus NCFM				(38)
Males with type 2 diabetes or impaired/normal glucose tolerance	Double blind	1010 CFU/d	Insulin sensitivity, systemic inflammation
	Placebo-controlled	4 wk
L. gasseri SBT2055				(87)
Adults with hypertriacylglycerolemia	Single blind	5 × 1010 CFU/100 g	BW, BMI, NEFA after oral fat-loading test, Hb A1c
	Placebo-controlled	200 g/d
		4 wk

¹The table was designed as follows: the first part describes studies performed with children, the second part describes studies performed with pregnant women, the third part describes studies performed with healthy or normal-weight adults, and the last part describes studies performed with obese individuals or patients with metabolic syndrome. Outcomes are indicated as follows: beneficial, bold; null, normal font; harmful, italics. BW, body weight; BWG, body-weight gain; Hb A_1C, hemoglobin A1c; NEFA, non-esterified FA.

Some trials testing the effect of Lactobacillus spp. administration on serum lipids profile or on markers of cardiovascular risk do not allow showing any significant changes in BMI during probiotic administration, which is comprehensive in view of the short duration of the treatment (6–8 wk). Those studies did not highlight major modifications in the serum lipoproteins profile (42, 43). Interestingly, another study (44) demonstrated a reduction in cardiovascular disease risk factors (decreased systolic blood pressure and plasma fibrinogen) by administration of L. plantarum 299v (2 × 10¹⁰ CFU/d) in smokers.

Some studies were performed to analyze the effect of probiotic administration on liver damage associated with obesity: NAFLD (45). A mixture of L. bulgaricus and S. thermophilus decreases aminostransferase activity in patients treated with the probiotic mixture, whereas in the placebo group, the amounts of aminotransferases remain unchanged (46). Another study (47) reports a decrease in alanine aminotransferase activity after 8 wk of treatment with L. rhamnosus GG in children with obesity-related liver disease.

Most of the authors reporting the studies mentioned above suggest a strain-specific ability of probiotics to modulate obesity and the associated metabolic disorders. However, the comparison between strains is rarely performed, and the mechanisms by which probiotics exert their beneficial effects remain to be characterized. Additional investigation is needed to assess the potential role of probiotics in body-weight regulation, which remains modest in term of kilograms lost during intervention studies.

Interest of Prebiotic-Type Nutrients in the Management of Obesity and Related Diseases

Would it be possible to link the properties of dietary fibers that specifically modulate the gut microbiota with host functions related to obesity and overfeeding? Some fermentable carbohydrates were initially defined as prebiotics, because they are preferentially fermented by specific types of bacteria, generally recognized as beneficial for the host (48, 49). Bifidobacterium spp. represent an important and complex group of bacteria whose presence is often associated with interesting health effects (50). In the context of obesity, several studies (2, 51, 52) reported that a low number of Bifidobacterium spp. correlated with the development of obesity and/or diabetes. Several fermentable carbohydrates (glucans, galactans, and fructans) are easily and widely fermented by bifidobacteria and promote their development. Several data had the bifidogenic effect of dietary inulin-type fructans (ITF) or arabinoxylans added in the diet of obese mice or rats (53–55).

In fact, promoting bifidobacteria is not the sole consequence of the prebiotic treatment. The pyrosequencing and microarray analysis of the caecal 16S rDNA of ob/ob mice treated or not with prebiotics allow us to point out >100 sequences that were different after prebiotic treatment, with some bacteria being particularly increased and other decreased by >10-fold (56). This allows for the identification of interesting bacteria that are promoted with the ITF prebiotic approach in this particular context. This is the case for Faecalibacterium prausnitzii, which exhibits interesting anti-inflammatory properties and is potentially involved in diabetes-related inflammation (57), or Akkermansia muciniphila, which was shown to be inversely correlated with body weight in pregnant women and preschool children (58, 59). Concerning human data, we confirmed recently, in an intervention study with ITF prebiotics in obese women, that, even if the increase in bifidobacteria remains the major and common signature of the prebiotic approach, a complex modulation of the gut microbial ecology at the phylum and genus–like taxonomic level also occurs during prebiotic treatment in obese women (60).

In obese animals (ob/ob mice, diet-induced obesity, obese Zucker or JCR:LA-cp rats), the dietary supplementation with nondigestible/fermentable carbohydrates, such as ITF or arabinoxylans, is able to lessen adiposity (49). Prebiotic treatment changes the gene expression pattern in the white adipose tissue of obese mice (by acting on peroxisome proliferator-activated receptor γ and G-protein-coupled receptor 43), leading to an increased lipolysis, a decreased adipogenesis, and an increased metabolic response to hormones such as leptin, and all those phenomenon contributing to a lower adiposity (56, 61). In obese animal fed ITF, an increase in anorexigenic peptides [peptide YY (PYY) and glucagon-like peptide-1 (GLP-1)] and a decrease in the orexigenic peptide ghrelin occurs, contributing to the satietogenic effect of the ITF (62, 63). This satietogenic effect was confirmed in a human pilot study showing that oligofructose increases satiety and reduces hunger and prospective food consumption and in another human study in which prebiotic supplementation was associated with an increase in plasma anorexigenic gut peptide concentration (PYY and GLP-1) (64, 65). The underlying mechanism of these effects is the modulation of the gut endocrine function by prebiotics in obese rats or mice that involves an increase in the number of L endocrine cells in the intestine, an effect that is correlated with bacterial changes in the gut (66). However, it remains rather difficult to know by which mechanism the gut microbial environment influences L-cell differentiation. The production of short-chain FAs (SCFAs) (namely acetate and propionate) during prebiotic fermentation could promote the increase in secretion of gut peptides by the endocrine cells (67).

Several substrates with prebiotic properties (fructans and arabinoxylans) are also able to counteract proinflammatory processes linked to obesity (54, 68). The decrease in LPS absorption occurs in prebiotic-treated animals through an improvement of the expression and activity of proteins involved in gut-barrier function, including glucagon-like peptide-2 (GLP-2), which is cosecreted with GLP-1 by endocrine L cells (54).

In addition to the beneficial effects mentioned above, in most of the studies performed, the administration of prebiotics leads to an improvement of fasting and/or post-oral glucose load glycemia (56, 69). The mechanisms by which this effect occurs could involve the secretion of gut peptides with incretin function, such as GLP-1, which participates in the improvement of hepatic insulin resistance (69). Two intervention studies with ITF prebiotics were reported in patients exhibiting hepatic diseases, suggesting that ITF administration is able to improve markers, such as LPS or aminotransferases (70, 71).

The different ways of the actions of prebiotics were not identified exhaustively. One of them is the modification of metabolites produced by the gut bacteria. The main and well-studied metabolites are SCFAs. However, these SCFAs are not the sole metabolites produced by the gut microbiota. Indeed, in view of the very large size of the microbiome (100-fold more genes than the human genome), the gut microbiota have a near-infinite metabolic potential. Many of the metabolites could be produced by the gut microbiota, and many of them are probably still unknown for the moment (72, 73). Among the metabolites identified and potentially related to human health, we can cite bile acid metabolites, choline metabolites, vitamins, polyamines, and lipid metabolites (72).

The bile acids signature is heavily dependent on microbial activities and the bile acid profiles of different tissues (liver, kidney, and heart) and in the plasma are modified by gut microbiota modulation (74). This modification of bile acid profile in host tissues may modulate the host physiologic response, for example, by modifying dietary lipid absorption or by changing host gene expression through interactions with specific nuclear receptors (farnesoid-X receptor and transmembrane G protein-coupled receptor TGR5) (75).

A recent study (76) reports that an HF diet and prebiotic supplementation both increase PUFA-derived bacterial metabolites in host tissues, such as cecal tissue and subcutaneous adipose tissue. We highlighted an increase in rumenic acid (conjugated linoleic acid cis-9,trans-11-18:2) but also in vaccenic acid (trans-11-18:1), with both metabolites being produced during the reduction of linoleic acid by the gut microbiota. Indeed, bacteria are able to metabolize PUFAs, such as linoleic acid and α-linolenic acid into SFAs, to lessen the potential toxicity of those PUFAs (77). In vitro studies show that human gut isolated bacteria are also able to produce these PUFA-derived bacterial metabolites (78, 79). However, the physiologic relevance of these endogenously produced PUFA-derived metabolites on human health remains to be determined.

Conclusions

Highly fermentable carbohydrates, such as prebiotics, are able to counteract several metabolic alterations linked to obesity, including hyperglycemia, inflammation, and hepatic steatosis, at least in animal models (Fig. 1). The mechanistic studies suggest that the changes in the gut microbiota occurring on prebiotics can be related to an improvement of gut bacterial functions implicated in the regulation of host energy homeostasis. The promotion of gut hormone release, changes in the gut-barrier integrity, and/or the release of bacterial-derived metabolites could all participate in the improvement of host health in the particular context of overfeeding and obesity. Appropriate human intervention studies with “colonic” nutrients (dietary fibers, prebiotics, and others), which allow for the selective promotion of beneficial bacteria, or with food containing colonic nutrients are essential to confirm the relevance of those nutrients in the nutritional management of overweight and obesity. Administration of live microorganisms (probiotics) seems also able to lessen obesity and related metabolic disorders. However, the mechanisms implicated in the beneficial effects of probiotics are not completely known. Animal studies suggest that regulation of lipid and glucose metabolism, reduction of adipose cell size and inflammation in adipose tissue, and reduction of inflammation in the liver could in part be implicated in the anti-obesity effects of probiotics (Fig. 1). These hypotheses need to be confirmed in human trials. Furthermore, regarding these probiotics, a clarification of the strain and the dose able to counteract obesity and related disorders is necessary before the generalization of the use of these microorganisms.

FIGURE 1.

Effect of prebiotics and probiotics on host pathophysiology related to obesity. Dietary carbohydrates with prebiotic properties change the gut microbiota composition by favoring bacteria involved in the control of gut-barrier function and host immunity. In the gut, prebiotics help to improve the gut-barrier function, a phenomenon that decreases LPS translocation in the host and decreases low-grade inflammation associated with obesity. Prebiotics also promote the production of gut hormones that control appetite (increase satiety and decrease hunger) and glucose homeostasis (improve glucose tolerance). The prebiotic approach also counteracts hepatic steatosis, hepatic insulin resistance, and adiposity by modifying gene expression at the tissue level. Administration of probiotics (live microorganisms) could affect host metabolism in different ways: 1) a direct effect of these microorganisms on the intestinal epithelium; 2) inducing a modulation of the composition of the gut microbiota that can also act on the intestinal epithelium; and finally 3) acting directly on the host tissue. The interaction with host tissues can be mediated by the production of bioactive compounds by the probiotics, such as SCFAs, PUFA-derived bacterial metabolites, or bile acid metabolites. The data available suggest that probiotic administration is associated with a decrease of body weight and adipose tissue weight, a decrease of the adipocyte size, a modulation of glucose and lipid metabolism, an improvement of glucose tolerance, and a decrease of systemic inflammation in adipose tissue and the liver. GLP-1, glucagon-like peptide-1; PYY, peptide YY.