Does digestibility of meat protein help explain large bowel cancer risk?

An association between meat eating and large bowel cancer risk has been shown in a variety of epidemiologic studies. One reason could be that meat is less well digested than other protein foods and leads to greater amounts of protein entering the large bowel and being metabolized by colonic bacteria to potential carcinogens. To test this hypothesis, five subjects with ileostomies were fed, for five days, a basal diet to which were added test meals of cheese, a small or a large fried beef steak, and a large steak with resistant starch (RS). Ileal true nitrogen digestibility was similar for all five diets: control, 86.3%; cheese, 89.4%; low beef, 88.6%; high beef, 89.6%; and high beef + RS, 88.7%. Beef, at both low and high intake levels, was as well digested as cheese, suggesting that poor digestibility of meat does not explain the association between meat intake and large bowel cancer risk. Ileal starch output on the high beef + RS diet was 27% greater than expected on the basis of the measurement of dietary RS in vitro (p = 0.005 for linear trend), and this was confirmed by a meta analysis with eight other published studies. The relation between meat and large bowel cancer may reflect higher protein intakes in meat eaters or may be explained by other mechanisms.