By Jamie Hulzebos
In the Netherlands, almost 50% of adults and 14% of children are overweight, according to the CBS. However, overweight is not just a problem of the Netherlands. In the United States, 70% of the adults are overweight and 17% of children have obesity, according to the National Center for Health Statistics. Overweight and obesity are risk factors for health problems such as type 2 diabetes and hypertension. Consequentially, a wide array of products are continuously being developed, aimed at helping people to lose weight. Developers of these products can make lots of money from this, however, the efficacy of these products is rarely proven and some can even be dangerous. In contrast, a subset of these products is widely tested by scientists for their effectiveness and safety: probiotics.
To understand the effect of probiotics, one must first understand some things about our gut microbiome. Our gut contains 1.1014 bacteria, from which more than 1000 different species have been identified. Multiple studies have concluded that the composition of the human microbiome of lean and overweight adults differ from each other. For example, the microbiomes of lean adults contain more Bacteroidetes and the microbiomes of obese adults contain more Firmicutes. This leads to the assumption that having relatively more Firmicutes in your microbiome increases your risk for developing obesity. This risk could be reduced by changing your diet, as a calorie-restrictive diet changes the composition of the microbiome. However, losing weight this way has shown to be difficult for many. Therefore, there is a growing interest among researchers to find ways to alter the composition of our microbiomes. Among these ways is the ingestion of live bacteria to which beneficiary health effects have been ascribed, called probiotics.
Dror et al. conducted a meta-analysis of randomized controlled trials (RCTs) reporting on the effect of probiotics on weight. In their study, they combined the results of 24 different RCTs on probiotics. They found that the ingestion of probiotics leads to minor weight loss in adults but to a minor weight gain in children and infants. However, they identified a high risk of selective reporting and attrition bias, meaning that there is a high possibility that negative results are not published and that results are distorted by the loss of participants in the studies. Moreover, Dror et al. identified a large heterogeneity in the choice of composition of the probiotics used in these RCTs.
So even though their study suggests a role for probiotics in promoting weight loss in adults, additional studies are needed to convincingly support this finding. Unfortunately, we thus may have to conclude that losing weight is not as easy as drinking a bottle of Yakult every day.
 Da Silva, S.T., Dos Santos, C.A. & Bressan, J. Intestinal microbiota; relevance to obesity and modulation by prebiotics and probiotics. Nutricion hospitalaria 28, 1039-1048 (2013).
 Dror, T., Dickstein, Y., Dubourg, G. & Paul, M. Microbiota manipulation for weight change. Microbial pathogenesis 106, 146-161 (2017).