The effect of meat protein and dietary fiber on colonic function and metabolism I. Changes in bowel habit, bile acid excretion, and calcium absorption

Abstract

No satisfactory hypothesis has been put forward to explain the association internationally of high meat protein intakes with the high incidence of large bowel cancer. Since current evidence suggests that the development of colonic cancer is related to high fecal bile acid levels, low fecal weight, and slow transit through the gut we have measured these in four healthy subjects taking metabolically controlled diets of different protein and fiber content through three successive 3-week periods. The diets were low protein (LP) (62.7 g protein per day), high protein (HP) (136 g protein per day) meat being the main source of the extra protein, and high protein with added wheat fiber (HP + F). The change in protein intake was without significant effect on bowel habit (fecal frequency LP: 3.5 ± 0.7 HP 4.3 ± 0.8 stools per week; fecal weight LP 79.7 ± 10, HP 80.5 ± 11 g/day), mean transit time (MTT: LP 75 ± 11 hr, HP 70 ± 6 hr); or fecal bile acid excretion (LP 196 ± 13, HP 221 ± 19 mg/day). By contrast, adding 31 g dietary fiber from wheat to the HP diet significantly increased fecal weight to 210 ± 9 g/day, diluted 3-fold an inert marker in the stool, shortened MTT (50 ± 3 hr) and increased fecal bile acid excretion to 444 ± 27 mg/day. The change from LP to HP diet altered the route of calcium excretion, more appearing in the urine, while overall balance was maintained. When fiber was added to the HP diet all the subjects went into negative calcium balance although the period of observation was only 3 weeks. Meat protein cannot be related to the etiology of bowel cancer through an effect on stool weight, transit time, or total fecal bile acid excretion. The epidemiological associations either need reexamining or alternatively current theories of the cause of large bowel cancer should be revised.