Protease inhibitor content of human dietary samples

A large body of experimental work has revealed that protease inhibitors (PI) are highly effective suppressors of carcinogenesis. Little is known about the level of PI activity in the diet of the US population. In the present study, we assayed the levels of PI activity in dietary samples from 31 free-living subjects who saved duplicate portions of all foods consumed over two 24-hour periods, six months apart. The majority of samples (90%) contained detectable PI activity; 82% contained trypsin inhibitory activity; 61% contained chymotrypsin inhibitory activity. Of those samples containing chymotrypsin inhibitory activity, 87% also contained trypsin inhibitory activity. The median concentration of soluble chymotrypsin inhibitory activity present in these samples was 6.5 micrograms/g food (range 0-150 micrograms/g food), whereas the median concentration of soluble trypsin inhibitory activity was 14.5 micrograms/g food (range 0-465 micrograms/g food). We conclude that a) human diet samples contain both chymotrypsin and trypsin inhibitory activity, b) the levels of PI in some of these samples was similar to that found to be anticarcinogenic in animal studies, and c) due to the large within-subject variation in PI intake, assessment of long-term dietary intake in epidemiological studies will be necessary to accurately classify subjects according to PI intake.