Supplements may contain live yeast cells, killed yeast or yeast extract. The most common killed form is Brewers Yeast which is a good source of B Vitamins and some essential amino acids. Live yeast cultures generally include Saccharomyces cerevisiae, S boulardii or Aspergillus oryzae yeast species intended to colonise the gut and act as probiotics conferring some benefit to the host horse, particularly in stabilising the hind gut microbiota and improving fibre digestion. Whilst it seems that yeasts can survive passage through the highly acidic stomach environment into the large intestine, whether they do any good or not remains difficult to evaluate. Trials have yielded conflicting results and more independent research is needed before a firm recommendation can be made.