The effect of meat protein and dietary fiber on colonic function and metabolism II. Bacterial metabolites in feces and urine

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Abstract
The association of high meat protein intakes with a high incidence of large bowel cancer cannot be explained on the basis of current hypotheses for the cause of this cancer. A variety of protein metabolites including the volatile phenols, tryptophan, and ammonia have been implicated in the etiology of cancer. We have, therefore, investigated by using controlled diet studies in four subjects the effect of increasing meat protein intake on bacterial metabolism in the gut as indicated by urinary volatile phenol excretion and fecal ammonia and short-chain fatty acid concentration and tryptophan excretion. The possible protective role of dietary fiber from wheat has also been assessed in relation to these products. Increasing protein intake from 62.7 to 136 g/day increased urinary volatile phenol excretion from 74 ± 14.5 to 108 ± 14.6 mg/day and fecal ammonia concentration from 14.8 ± 1.3 to 30.4 ± 1.1 mmole/liter but did not significantly alter fecal nitrogen excretion. Adding 29.8 g dietary fiber per day to the high protein diet did not alter fecal ammonia concentration despite a large increase in stool output. Urinary total volatile phenol excretion fell (81 ± 4.8 mg/day) and fecal nitrogen excretion almost doubled. The dietary changes did not alter fecal short-chain fatty acid concentrations. Increasing meat intake, therefore, increases the concentration in the feces and excretion in the urine of certain protein metabolites some of which may be carcinogenic. The role of dietary fiber in protecting against large bowel cancer cannot be related to any one single effect on colonic metabolism and may be due to a combination of dilution of colonic contents, shortened transit time, altered bacterial metabolism, or possibly other properties such as adsorption of potentially harmful materials.