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.

Improved technique for measuring fecal energy loss in normal and malabsorbing humans.

Fecal energy concentration is measured by bomb calorimetry on freeze-dried stool samples. Some of the energy-containing fecal compounds are volatile in the pH ranges of normal stool and hence may be lost during sample preparation. We found that significant amounts of volatile fatty acids and lactic acid are lost during lyophilization. Fecal alkalization caused an increase of 9.8% of measurable energy in stools from normal individuals and 25% in stools from patients with untreated exocrine pancreatic insufficiency. We conclude that previous reports of fecal energy concentration that did not use an alkalization procedure are probably underestimations. We recommend fecal alkalization before lyophilization in future measurements of fecal energy excretion.