Not surprisingly, there has been a lot of interest in manipulating the gut microbiota to facilitate weight loss. How might probiotics play a role?
|| Response from Philip J. Gregory, PharmD
Associate Professor, Pharmacy Practice, Center for Drug Information & Evidence-Based Practice, Creighton University, Omaha, Nebraska
The role of the gut microbiota in health and disease is one of the hottest topics in medical research. Some have referred to the microbiota as the “hidden organ,” reflecting its critical role in numerous bodily functions.
An area of increasing interest is the role of the gut microbiota on weight loss and obesity.[1,2,3] The gut flora seems to play an important role in metabolic processes, with one indicator being that diet substantially influences the composition of gut flora. For example, a more diverse or complex diet results in a more diverse flora.
Of note, the makeup of gut flora differs between lean and obese people, with overweight and obese people tending to have a less diverse gut flora. The ratio of the dominant gut phyla, Firmicutes and Bacteroidetes, also changes in obese people.[2,3] Specifically, Firmicutes increases while Bacteroidetes decreases.
The alterations in gut flora in obese people also can have an impact on gut function and metabolic processes, such as harvesting and utilization of energy. In fact, some research shows that the microbiota in obese people has an increased energy-harvesting capacity. Other research suggests that changes in the microbiota of obese people is associated with such adverse consequences as increased body fat, breakdown in intestinal epithelium, pathogenic colonization, insulin resistance, lipogenesis, and inflammation.[2,3]
Not surprisingly, there has been a lot of interest in manipulating the gut microbiota to help facilitate weight or fat loss.
Animal studies have shown that probiotics can have an impact on weight. Some probiotics have been linked to weight gain and some to weight loss. Several preliminary clinical studies have begun to evaluate the impact of probiotic supplements and foods on measures of weight and body fat in humans; however, most are limited by small study populations and short treatment durations. Individual study findings have been largely inconsistent.
A meta-analysis by Park and Bae  identified nine randomized controlled clinical trials evaluating the impact of probiotics on weight and body fat outcomes in overweight patients. All of these studies evaluated probiotic products (including supplements and foods) that provided Lactobacillus species. However, only four of the studies provided adequate data to be included in a meta-analysis. The overall summary effect of this analysis found no significant impact on body weight, body mass index (BMI), or measures of body fat.
While the previous analysis focused on overweight and obese adults, Dror and colleagues conducted a more comprehensive meta-analysis of randomized controlled trials that evaluated adults and children who were lean, normal weight, or obese at baseline. The analysis included 14 randomized controlled trials in adults and found that probiotic interventions brought about significant changes in weight-related outcomes. The results, reported as a standardized mean difference (SMD) with a confidence interval of 95%, suggested a medium treatment effect of reducing weight in adults (SMD -0.54 [-0.83, -0.25]). In a subset of eight studies, which consistently reported BMI outcomes, an absolute mean difference was calculated and determined to be -0.43 (-0.54, -0.33). Of note, an analysis of studies evaluating probiotics in children found the opposite effect: Weight was significantly increased, with an SMD of 0.2 (0.04, 0.36), suggesting a small treatment effect. The majority of included studies evaluated patients who were overweight or obese.
A wide variety of different probiotic products were used in these studies, with no single product evaluated in more than one trial. Nine studies evaluated a dairy product (eg, yogurt, milk, cheese) containing a probiotic, and five studies evaluated a supplement product. Probiotic species included were Lactobacillus acidophilus, L casei, L plantarum, L gasseri, L rhamnosus, . bulgaricus, Bifidobacterium infantis, B lactis, B longum, B breve, Streptococcus thermophilus, and Saccharomyces cerevisiae. Most studies used products providing multiple probiotic species, with only six trials using a single probiotic species. Owing to the different products and probiotics evaluated, it is difficult to generalize these findings to probiotics.
A recent randomized controlled trial, which was not included in either meta-analysis described above, evaluated the use of a probiotic powder, 2 g twice daily, providing L curvatus and L plantarum species, in overweight patients. After 12 weeks of treatment, those patients receiving the probiotic had a significantly (but modestly) reduced weight, BMI, and fat mass. Weight decreased by an average of approximately 0.6 kg.
At this point, it is clear that the gut microbiota plays a role in a variety of physiologic processes and also probably affects body weight and fat. The question remains, though, as to whether taking a probiotic-containing product can alter the microbiota in such a way as to cause desirable anthropomorphic changes. Although some evidence shows an overall modest impact on weight, it is based on preliminary data using a heterogeneous mix of probiotic species. Higher-quality data involving larger patient populations and longer durations of treatment are needed to better understand which specific probiotics might be beneficial and who might benefit from them.
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Probiotics and Weight Loss