Protein origin of the volatile fatty acids isobutyrate and isovalerate in human stool

Normal human stool contains five commonly recognized volatile fatty acids, of which three (acetate, propionate, and butyrate) are the product of carbohydrate degradation. We investigated whether the remaining two volatile fatty acids, isobutyrate and isovalerate, are produced by degradation of amino acids. Stools from six healthy subjects were incubated with albumin, valine, leucine, tryptophan, phenylalanine, glucose, or sucrose under atmospheres containing oxygen in the range of 0% to 5%. Isobutyrate was produced only from albumin and valine, and isovalerate was produced only from albumin and leucine. Glucose- and sucrose-containing incubates produced only acetate, propionate, and butyrate. The rate of production for each of the volatile fatty acids was constant over the various oxygen concentrations tested. We conclude that isobutyrate and isovalerate in stool are formed from valine and leucine degradation respectively. Quantitation of individual fecal volatile fatty acid excretions may reflect intracolonic degradation of protein and carbohydrate.