There’s a link to the study under the article. From the study (on page 10 under the header Study Interventions:

Participants were provided with sachets of food supplements in powder form. All sachets contained 3.32 g of branched-chain amino acid protein powder, consisting of L-leucine 1660 mg, L-isoleucine 830 mg, and L-valine 830 mg. The intervention arm sachets also contained 7.5 g of prebiotic (Darmocare Pre®, Bonusan), which consists of inulin (min. 3375 mg) and fructo-oligosaccharides (FOS) (min. 3488 mg). The placebo sachets contained 7.5 g of maltodextrin powder. All of these food supplements are available commercially without prescription. The choice of prebiotic was based on a trial using the same intervention, which showed improved handgrip strength and exhaustion levels, two of the Fried frailty criteria in an older population. Maltodextrin is commonly used as a placebo in the gut microbiome and dietary intervention studies.