Here is the quotation:
In support of his thesis, Dr Wrangham, who is an anthropologist, has ransacked other fields and come up with an impressive array of material. Cooking increases the share of food digested in the stomach and small intestine, where it can be absorbed, from 50% to 95% according to work done on people fitted for medical reasons with collection bags at the ends of their small intestines. Previous studies had suggested raw food was digested equally well as cooked food because they looked at faeces as being the end product. These, however, have been exposed to the digestive mercies of bacteria in the large intestine, and any residual goodies have been removed from them that way.
— The Economist, 19 February 2009
My question is how to understand the red part? Which one does "these" refer to? "studies", "faeces", or"intestines"? If it means "studies", does this sentence means "the results of the experiment are rendered inaccurately because of the digestive effect of the bacteria"?