Changes in the gut microbiota are associated with metabolic disorders, such as overweight and elevated blood glucose. Mouse studies have shown that gut microbiota can regulate metabolism with a mechanism related to gut barrier function. An impaired gut barrier permits the translocation of bacteria and their components which, when in contact with the sub-mucosal immune system, evoke metabolic inflammation and distract signalling in metabolically active tissues. Despite thorough research of the topic in animals, the hypothesis is yet to be proven in humans. Cross-sectional studies have shown that certain bacterial populations - such as Akkermansia muciniphila, Faecalibacterium prausnitzii, Methanobrevibacter smithii and Christensenellaceae - are better represented in lean individuals compared to those who are overweight or metabolically unhealthy. Although these differences reflect those seen in mice, it is possible that they are caused by different dietary or other lifestyle habits. Diet has an indisputable influence on gut microbiota making it very difficult to draw conclusions on microbiota-host interactions from cross-sectional studies. Certain research areas do, however, indicate that gut microbiota could causally influence metabolism. Several studies show that antibiotic use in infancy increases body weight in later childhood. Also, probiotics are emerging as a potential therapy for metabolic syndrome. In fact, a handful of human studies and numerous animal studies show promise for probiotics in reducing blood glucose levels or improving insulin sensitivity. For weight management human evidence is scarcer. Nevertheless, it is becoming increasingly recognised that gut microbiota plays a part regulating metabolism, also in humans, which gives rise to novel opportunities for preventative and treatment strategies.