Resistant starch and colorectal neoplasia

There are several approaches to examining the relationship between resistant starch (RS) and development of colorectal cancer (CRC). These include examination of epidemiological relationships, objective testing of effects of RS given to humans on biological events of relevance to CRC, and studies in animal models where protection and mechanisms of protection can be directly tested. Nine epidemiological studies have examined the relationship between starch and CRC and/or adenomas. Most show a significant protective effect. However, epidemiological tools for measuring consumption of RS are poorly developed and so a benefit for RS can only be inferred. On balance, the magnitude of protection by starch appears to be in the order of 25-50%. Human intervention studies have examined the effect of various types and amounts of RS consumption on colonic biology. To generalize from these studies, RS softens stools and increases stool bulk, decreases pH, increases short-chain fatty acids (SCFAs) including butyrate, reduces products of protein fermentation, and decreases bile salts in fecal water. Such changes seem to be achieved within about 4 weeks of commencing consumption. The greatest effects are seen with the highest doses where increased fecal starch recovery is observed. A modest number of animal studies have been undertaken. Those examining effects of RS on colonic biology and biomarkers for CRC confirm and extend the results in humans. RS modifies the lumenal environment, largely through altered fermentation of polysaccharides and proteins. RS also affects epithelial biology in that it increases apoptotic deletion of genetically damaged cells. More work is needed to define what types and combinations of RS, perhaps with probiotics, exert the greatest effects on colonic environment and epithelial biology, and then to test these in the cancer models for their protective effect. A few studies have examined effect of RS on cancer as an end point in several rodent models, but the results are not clear cut. In conclusion, consumption of RS dramatically affects the colonic lumenal environment and facilitates apoptotic deletion of genetically damaged cells in the colon, several of which are considered to be biomarkers associated with risk for CRC. These effects can be interpreted as reflecting improved colonic health, which might be of benefit in protection against CRC. Direct evidence for protection is still not available.