The present invention relates to means and methods for modulating the gut microbiota composition of a subject, preferably a human. In particular, the present invention provides an agent capable of reducing the proportion of Lachnospiraceae in the gut microbiota composition of a subject which agent is useful for promoting weight loss or preventing weight gain in a subject, preferably an obese or overweight subject. Lactobacillus rhamnosus CGMCC 1.3724 is particularly preferred as the agent provided by and used in the present invention.