Correlation between faecal iso-butyric and iso-valeric acids in different species

It is generally accepted that gastrointestinal short-chain fatty acids (SCFAs) – acetic, propionic and butyric acid – are mostly derived from carbohydrates, while iso-butyric and iso-valeric acids are from protein sources. We have investigated the faecal SCFAs and the correlation between the branched-chain fatty acids iso-butyric and iso-valeric in humans, rats, horses and pigs. The subjects were of different ages, fed on different diets and housed in different environments. High differences in the total output of SCFAs were observed within and between species. Despite these differences, a remarkable correlation between the iso-butyric and the iso-valeric acids was found. The fact that the correlation is strong irrespective of species, age, diet and living conditions indicates a common source actually reaching the hindgut. We hypothesize that this source is intestinal sloughed cells.