CaPPtain's Blog: John Burn, October 2022

CaPPtains Blog -  CAPP2 Resistant Starch Update

A diet supplement of resistant starch doesn't reduce colorectal cancers in Lynch syndrome but does reduce other types of Lynch syndrome cancer by more than half.  This is the important result from "the other half" of CAPP2, our international research trial which began more than 20 years ago.  It appeared in the highly regarded research journal, Cancer Prevention Research in September 2022.  It has taken a long time to reach publication because the result is surprising.

Our CAPP2 trial was double blind, which means no-one knew whether they were taking the resistant starch or ordinary corn starch.  They mixed 2 sachets with their food each day for between 2 and 4 years.  Some people weren't able to complete the study, but the effect was so big that the difference was clear even if the "early dropouts" were included.  This is called an "intention to treat" analysis. The biggest effect was on cancers in the upper gastrointestinal tract near the stomach and including cancers in the pipes that let bile out of the liver.

Resistant starch is turned into short chain fatty acids by bacteria in the gut.  The most important of these short chain fatty acids is butyrate which has been known for a long time to help reduce cancer.  In addition, we think the supplement of resistant starch may have caused a long-term change in the types of bacteria in the gut and reduced the production of chemicals called secondary bile acids which are released in the bile and can cause genetic damage.

Whatever the reason, the effect on cancer risk is real and it lasts a long time.  There are no important side effects so we think that everyone with Lynch syndrome should try to increase the amount of resistant starch in their diet.

The name resistant starch comes from the fact that some starch is not broken down to glucose in the upper gut.  This can be because they are inside seeds, for example, or because their chemical structure prevents the digestive enzymes from working.  The best known example is in bananas where the starch is crystalline but gradually breaks down to glucose as the banana ripens; hence our advice to eat a green tipped banana a day.  There are lots of other ways to increase resistant starch.  The simple answer is to aim for a "high fibre" diet.

The important type is resistant starch, also called fermentable fibre in America because it reaches the bowel bacteria to be broken down by fermentation.  When starches are heated they become easier to break down in the upper gut.  I once suggested that yesterday's pizza or cold mashed potato would work too which caused quite a social media flurry!

The take home message is that a high fibre diet is good for you, reduces cancer risk and can help control your weight because resistant starch has fewer useful calories.  It takes us nearer to the wild diet eaten by our ancestors. 

Speaking of ancestors, the whole CAPP story began when Professor John Mathers invited me over 30 years ago to speak at a Newcastle University nutrition seminar where the late great Dennis Burkitt, who first showed that fibre is good for you, was in the audience. I think he would approve.

John Burn
October 2022