Effects of Probiotics and Synbiotics on Weight Loss in Subjects with Overweight or Obesity: A Systematic Review

Intestinal microbiota has been shown to be a potential determining factor in the development of obesity. The objective of this systematic review is to collect and learn, based on the latest available evidence, the effect of the use of probiotics and synbiotics in randomized clinical trials on weight loss in people with overweight and obesity. A search for articles was carried out in PubMed, Web of science and Scopus until September 2021, using search strategies that included the terms “obesity”, “overweight”, “probiotic”, “synbiotic”, “Lactobacillus”, “Bifidobacterium” and “weight loss”. Of the 185 articles found, only 27 complied with the selection criteria and were analyzed in the review, of which 23 observed positive effects on weight loss. The intake of probiotics or synbiotics could lead to significant weight reductions, either maintaining habitual lifestyle habits or in combination with energy restriction and/or increased physical activity for an average of 12 weeks. Specific strains belonging to the genus Lactobacillus and Bifidobacterium were the most used and those that showed the best results in reducing body weight. Both probiotics and synbiotics have the potential to help in weight loss in overweight and obese populations.


Introduction
The Western Diet, characterized by a large consumption of processed products, saturated fats, sugars and a low fiber content, together with an increasingly sedentary lifestyle, has generated tripled levels of obesity in the world as compared to the year 1975, according to the World Health Organization [1]. In fact, obesity is currently classified as an epidemic and has become one of the greatest public health challenges in the twenty-first century [2].
Obesity is defined as an excessive accumulation of fat and hypertrophy of adipose tissue [1]. It is a chronic pathology, where the fundamental cause is the imbalance between the calories consumed and those expended [3]. However, it is considered a multi-causal and complex disease influenced by factors intrinsic and extrinsic to the individual, such as environmental, genetic, neuronal, endocrine and behavioral components [3,4]. Furthermore, overweight and obesity are risk factors for other chronic diseases, such as diabetes mellitus II, cardiovascular diseases and some types of cancer [3].
It has recently been shown that obesity and its association with other chronic noncommunicable diseases are not only the result of genetic factors, eating habits or lack of physical activity; it has also been proven that the Intestinal Microbiota (IM) is an environmental factor in its development [5][6][7][8].
People with overweight or obesity have been shown to have a specific IM profile, characterized by dysbiosis (imbalance) and lower microbial diversity compared to people with normal weight [6,9]. In this sense, a decrease has been seen in some bacterial phyla, such as the relationship between Bacteroidetes/Firmicutes [10,11], with lower proportions of Bacteroidetes and higher proportions of Firmicutes than those from people without obesity [12]. This seems to facilitate energy extraction from the ingested food and increases energy storage in the host's adipose tissue [7]. In addition, this altered microbiota also results in a suppressed production of fasting-induced adipose factor (Fiaf). This suppression leads to an increased storage of triglycerides in adipose tissue and a low release of hormones such as glucagon-like peptide 1 (GLP-1) and the peptide YY (PYY), promoting food intake. Although research in humans is incipient, there are clear indications that the intestinal microbiota is important for maintaining homeostasis of energy metabolism [13][14][15].
The International Scientific Association of Probiotics and Prebiotics (ISAPP) defines probiotics as "live microorganisms that, after ingestion in specific numbers, exert benefits for the health of the host" [16] and prebiotic as "a substrate that is selectively utilized by host microorganisms conferring a health benefit" [17]. A synbiotic, is defined as "a mixture comprising live microorganisms and substrate(s) used selectively by host microorganisms that confers a benefit to the health of the host" [18].
This systematic review evaluates the effect of probiotic and synbiotic intake on IM in reducing body weight and/or body fat in apparently healthy people with overweight or obesity in randomized clinical trials.

Materials and Methods
This systematic review was reported according to the PRISMA statement [19].

Search Strategy
In order to address the proposed objective, a systematic review has been carried out, finishing in September 2021 in three health science databases (Pubmed, Web of Science and Scopus).
Searching strategies were constructed using controlled language from the Medical Subject Headlines (MeSH) (Probiotics, Lactobacillus, Synbiotics, Overweight, Obesity, Weight Loss) and health science descriptors (DeCs) (Probiotic *, Lactobacillus, Synbiotic *, Overweight, Obesity, Weight loss, Bifidobacteria). Additionally, to define the union between the terminologies, the Boolean operators "AND", "OR" and "NOT" were used. The truncation (*) was also used in order to encompass all words related to probiotic, and synbiotic.
To further delimit the results, additional filters were applied to each database. The specific details of the aforementioned strategies are given in Table S1.

Eligibility Criteria
The selection criterion were randomized clinical trials carried out in humans, published in the last 10 years, in apparently healthy people classified as overweight or obese, according to Body Mass Index (BMI; overweight 25-29.9 kg/m 2 , obese ≥ 30 kg/m 2 ), body fat percentage (females: overweight 26.0-31.9%, obese ≥ 32%; males: overweight 21.0-24.9%, obese ≥ 25%), visceral fat area or waist circumference (obese females ≥ 80 cm, obese males ≥ 94 cm) [20,21] in all age groups, where it was evaluated the effect of taking a probiotic or synbiotic on weight loss. The published results of the study required to be written in English. Review articles, studies of people with other chronic non-communicable diseases, carried out on animals, in pregnant participants or in the breastfeeding stage were excluded. The selection process can be found in Section 3.1.

Data Extraction and Analysis
Data was extracted by one of the authors (VA-A) and contrasted with the other coauthor (SM-P). From each selected publication, information about the name of the main author, year and place where the study was carried out, population characteristics, study design, intervention, comparison, strains and doses used, period of intervention and main results were extracted.

Quality Assessment
To assess the quality of the 27 studies included in the systematic review, the Jadad scale [22] was used. It consists of 5 questions related to methodological quality. The following criteria are evaluated: whether the trial is randomized and double blind, whether a description of exclusions and dropouts is detailed, and finally if the randomization and double blind method are adequate. A score of 5 points correspond to the maximum quality level, whereas a score < 3 points is considered to indicate poor quality.

Selection Process
Based on the search carried out, a total of 204 articles (101 Pubmed, 47 Scopus, 56 Web of Science) were obtained using the strategy previously described. From the initial search results, 17 duplicate articles were discarded. After screening of the titles and abstracts of the remaining studies, 146 articles not meeting the eligibility criteria (8 studies carried out on animals, 10 reviews, 128 studies not assessing the association of our study) were excluded. From the 40 articles that were read in their entirety, 13 did not meet the selection criteria (1 study carried out on animals, 12 studies not assessing the association of our study) and were also discarded. Finally, 27 articles were included in this systematic review. A flow chart illustrating the selection process is presented in Figure 1.  Table 1 presents the collection of the selected studies in an orderly and summarized manner, including the country, author or authors, methodological design, intervention and a summary of the main results.  Table 1 presents the collection of the selected studies in an orderly and summarized manner, including the country, author or authors, methodological design, intervention and a summary of the main results.  In all three treatments, the caloric restriction induced significant changes in weight, waist circumference and BMI during the entire intervention. In the group placebo-synbiotic 2, the weight loss percentage was significantly higher than in the group of synbiotic 1-synbiotic 2 (p = 0.030)  The articles came from different countries, the majority being from Asia, where the most prevalent countries were Iran, South Korea and Japan. The second most prevalent countries were from the European continent, including Spain [39,42], Bulgaria [43,48] and Turkey [31]. The remaining countries were the United States [47], Canada [24,25] and Brazil [35]. From the 27 selected studies, 24 were conducted in adult populations and three in children [26,31,37]. Most of the studies considered both sexes, with the exceptions of [29,35,44,50], which were conducted in female only. The average age of the people included in the trials was 31.1 years, and the average BMI was 30.5 kg/m 2 .

Study Characteristics
Regarding the typology of the selected articles, they all had a quantitative approach, being randomized controlled clinical trials. Although most of the studies were doubleblind, we found also single [31,42,44,47,50] and triple blind [26] studies. The most of the selected articles used probiotics except seven studies which investigated the effect of synbiotics [26,31,37,41,42,47,51]. Intervention duration average was 12 weeks, being 1 week the minimum [31] and 36 weeks the maximum [48] duration. Finally, the minimum sample size included 20 subjects [47] and the maximum, 225 [43].
Regarding the probiotic bacteria used in both probiotics and synbiotics, the genus Lactobacillus standed out, which included strains from the species L. rhamnosus, L. gasseri, L. plantarum, L. casei, L. lactis, L. acidophilus, L. delbrueckii, L. reuteri and L. curvatus. Regarding Bifidobacterium, it was common the use of strains belonging to the species B. animalis, B. bifidum, B. lactis and B. breve.

Probiotic Strains, Daily Doses and Total Intervention Doses
Most of the studies using fermented foods for the intake of probiotics contained lactic acid bacteria (LAB) as starter cultures, coming from the genus Lactobacillus and Bifidobacterium mainly.
The multi-strain combinations were multiple and are detailed in Table 1. The probiotic amount and duration of the intervention studies varied, from a maximum dose of 5 × 10 10 [19,20] and the minimum dose of 1 × 10 6 [24], and from 4 [21] to 36 [20] weeks, respectively. Table S2 shows the evaluation of the methodological quality of the 27 randomized clinical trials included in this systematic review, carried out using the Jadad scale. A total of seven studies were rated with poor methodological quality [31,37,42,44,46,47,50] (<3 points), and six studies obtained an acceptable score [23,24,[26][27][28][29]36] (3 or 4 points). The remaining studies obtained the maximum score (5 points). Most of the studies clearly described the method of randomization used in the study, as well as the blinding procedure. Others, however, did not explain the method of allocation concealment or blinding, and some were not defined as double-blind randomized trials, thus scoring lower.

Discussion
In recent years, the effect of pre-and probiotics has gained a great deal of interest in the treatment of overweight and obesity. The results of this systematic review indicate that probiotics and synbiotics, whether used as single-strain or multi-strain, could have a favorable effect on weight loss and other related anthropometric markers in people with overweight or obesity. In particular L. gasseri [23,27], different strains of L. acidophilus alone or together with different strains of the genera Bifidobacterium [31,43,48] or Lactobacillus [30,40] showed reducing effects even when the participants did not undergo energy restriction; however these interventions had in common an intervention period ≥ 8 weeks.
Trials finding positive results also at anthropometric level combined part or all of the intervention with a hypocaloric diet, calorie restriction, and/or increased physical activity. Consequently, weight loss was not assigned solely to the effect of the probiotic. Therefore, the real results of the strain(s) used in these trials are somewhat biased, especially in those trials where the interventions were very short (4 weeks) [21].
Studies with L. gasseri showed a decrease in body weight [27], BMI [23,27], waist circumference [23,27,38] and areas of visceral [23,27] and subcutaneous fat [23]. High body mass has been reported to be strongly associated with risk factors for cardiovascular diseases in both childhood and adulthood [53,54]. L. gasseri BNR17 was associated with a decrease in visceral adipose tissue in waist and hip circumferences post-consumption [38].
In another trial carried out in adults with diabetes with the same strain, only a slight reduction was observed (without being significant) in body weight and waist circumference; a result possibly associated with the sample size [55]. Kadooka et al. [27] used L. gasseri SBT2055 and observed a decrease in visceral fat area, BMI and waist and hip circumference at doses of 10 6 CFU/g. When the same authors [23] used the same probiotic strain in a higher dose, observed also a reduction in the abdominal subcutaneous fat area, which suggests that at lower doses this strain can reduce its effectiveness.
The probiotics L. curvatus HY7601 with L. plantarum KY1032 showed reductions in body weight, BMI and waist circumference [30], what reinforces previous results carried out in mice, where they showed a reduction in weight gain and accumulation of fat, through the modulation of the intestinal microbiota [56].
It is worth mentioning that the doses of probiotics were different between the included studies, the minimum dose being 10 6 and the maximum dose 5 × 10 10 CFU. One of the main reasons for this variations in the doses may be the characteristics of each probiotic. Some strains are more resistant to storage, and the dosage can also vary depending on how it is administered, for example either in dairy products or via capsule [61].
Another important point is the duration of the studies, which ranged from 4 to 36 weeks. However, most of the studies showing positive effects presented an intervention duration of 12 weeks. Therefore, 12 weeks seems to be the trend to start observing positive effects on body weight and/or fat mass by using probiotics. It should be noted that a reduction of ≥5% in initial body weight in a period of 6 months is clinically relevant and this would be associated with a significant reduction in some cardiovascular risk factors such as a reduction in blood pressure, lipids and blood glucose [62]. In the trial of Michael et al. [48], 40% of the participants in the probiotic group achieved this reduction after 9 months of supplementation, without dietary limitations.
Probiotics and synbiotics have been proposed to exert a decrease in body weight through different mechanisms (reviewed by [63,64]). Probiotics help in the recovery of the tight junctions between epithelial cells, thus reducing intestinal permeability, preventing the translocation of bacteria and reducing inflammation derived from lipopolysaccharides (LPS). The reduction in inflammation leads to an increase in insulin sensitivity in the hypothalamus, which improves satiety. Additionally, increased concentrations of leptin in adipose tissue, glucagon-like peptide 1 (GLP-1) and pancreatic polypeptide (PPY) in the intestine leads to a reduction in food intake due to an increase in satiety [15]. Recently, oral microbiota has also been associated to the development of obesity due to its modulating effects on the intestinal microbiota [65,66] and could have had an effect in the some of the observed results, especially in those where the intervention implied the ingestion of fermented foods.
Despite the observed beneficial effects, probiotics and synbiotics are not yet considered an alternative strategy in the treatment of obesity, probably due to the lack of regulation of this market [67]. In fact, in these products essential details such as the type of strain-or its combinations when more than one used-, the number of microorganisms included, the treatment duration, the route of administration, the formulation or the shelf-life and storage conditions, are often missing. Consequently, medical providers and the public are faced with a plethora of probiotic products with not proved health claims. In fact, the European Food Safety Authority (EFSA) has rejected all submitted health claims for probiotics so far. More evidence-based trials that support their use are needed, taking into account that even when evidence exists, not all probiotic products are equally effective for all disease prevention or treatment indications.
The present study had some limitations. First of all, in many of the studies the probiotic/synbiotic intervention was accompanied by dietary or physical activity interventions, which may have hidden the real effect of the probiotic strain(s) used. In addition, there were also variations in the populations included in the different studies regarding sex and age, which can introduce bias. The strengths of this study are that only randomized clinical trials were included in order to compilate the highest degree of evidence, conducted in otherwise healthy people with overweight or obesity in order to minimize biases. Additionally, the review includes recent studies and provides specific strains, doses and intervention times.

Conclusions
From the analyzed randomized clinical trials, this systematic review indicates that both probiotics and synbiotics, specifically certain strains of Lactobacillus gasseri, L. rhamnosus, L. plantarum, L. curvatus associated with other Lactobacillus species and/or with species from the Bifidobacterium genus, have the potential to aid in weight and fat mass loss in overweight and obese populations. There is still a need, though, for clinical trials, in order to state more accurate recommendations in terms of strains, doses and intervention times. It is also suggested to carry out studies in homogeneous populations in terms of sex and age. In addition to this, it would be ideal that future trials would be carried out in the absence of weight loss techniques (such as dietary recommendations for weight loss and physical activity programs), in order to evaluate the specific effect of the strain/s.

Supplementary Materials:
The following are available online at https://www.mdpi.com/article/10.3 390/nu13103627/s1, Table S1: Search strategies used for this systematic review, Table S2: Assessment of the methodological quality of the included clinical trials, using the Jadad scale. Institutional Review Board Statement: Ethical review and approval were waived for this study, because the Systematic Review relies on retrieval and synthesis of data from existing approved human trials.
Informed Consent Statement: Patient consent was waived because the Systematic Review relies on retrieval and synthesis of data from existing approved human trials.

Data Availability Statement:
The publications analyzed for this systematic study can be accessed from their respective journals, whereby access restrictions may apply.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
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