Background. Brewer’s spent grain (BSG) is a by-product of beer production, constituting approx. 85 % of all generated by-products in the brewing industry, primarily utilized for livestock feed. It is characterized by high levels of fiber (40 ÷ 60 %), protein (19 ÷ 30 %), and fat (10 %). The protein occurring in BSG contains all amino acids, including exogenous ones, which are not synthesized by the human body.
Results and conclusions. Brewer’s spent grain fiber mainly comprises cellulose, hemicellulose and lignin. Due to various fiber fractions, including arabinoxylan (AXs), it is suspected to possess  the ability to modulate the human gut microbiota. In vitro studies conducted so far have demonstrated that BSG and extracted AXs fractions stimulate the growth of health-promoting bacteria  such as Bifidobacterium and Lactobacillus, however, they also have the capacity to stimulate bacteria of the Enterobacteriaceae family. Moreover, under the influence of brewer’s spent grain, the  abundance of Bacteroides and Firmicutes bacteria decreases, while the abundance of Actinobacteria increases. Additionally, it has been proven in each of the studies that the addition of  BSG stimulates the synthesis of short-chain fatty acids, including propionic acid and acetic acid, with acetic acid being the most prominently affected. Brewer’s spent grain may enhance the  scavenging of free radicals due to the presence of phenolic compounds and increase the antioxidant activity of food. Further research, including studies utilizing a dynamic in vitro digestive system and in vivo investigations, is necessary to confirm the beneficial impact of BSG on human health.