It goes without saying that dietary fiber is good for digestive health. Everybody knows it, so what’s left to say?

Resistant starch is a type of dietary fiber, so it’s good for digestive health. In reality, this is just the beginning of the story and if this is all you know about resistant starch, settle in for a long ride.  This is the first article in a series examining the health benefits of resistant starch on various organs and systems within the body.  Naturally, we’re starting with digestive health.

1. Starting with the oldest data – several clinical trials have shown that resistant starch helps to restore and maintain proper digestive function:

• It mildly increases regularity, (i.e., Cummings BJN 1996)
• It stops diarrhea in individuals with rotavitus, cholera or other causes of watery diarrhea. (i.e., Ramakrishna, NEJM 2000)
• It mildly increases bulking by about 1.1 gram of bulk per gram of resistant starch consumed, (calculated by meta-analysis). Most of this bulk is increased bacteria.

2. Now for the slightly more recent data – resistant starch is fully fermented by the bacteria (called the gut microbiome) that live in our large intestines. Biomarkers of this fermentation include:

• Increased breath hydrogen,
• Reduced pH,
• Increased short-chain fatty acids, most notably butyrate. Butyrate is an important SCFA because it is a primary fuel for colon cells and the digestive tract gets leaky without it.
• Reduced secondary bile acids, ammonia and phenolics (i.e., Birkett AJCN 1996)
• Typically softer stools
• Mild increase in gas (flatulence). One study showed that people would have to eat 45 grams of resistant starch each day to get a statistically significant increase in flatulence. The dose tolerance is high because the fiber is insoluble and slowly fermented.

For example, one of the Australian studies fed healthy adults almost 40 grams of resistant starch/day from high amylose corn, unprocessed wheat seeds and uncooked green banana flour (Phillips AJCN 1995). They demonstrated significant increased fecal weight, reduced pH, increased ease of defecation and increased short chain fatty acids. Other studies used lower quantities and generally found similar effects, although they were not always statistically significant at the lower doses.

Treatment of digestive diseases

Many studies are correlating imbalances in the gut microbiome with numerous digestive diseases. Researchers are busy around the world to figure this one out, as there may be lot of versions of different digestive diseases, including ulcerative colitis, Crohn’s Disease, Celiac Disease, etc.

Some individuals are very sensitive to intestinal fermentation, and may not be able to tolerate fermentable fiber in any quantity. That’s too bad, because fermentable fibers may be critical and essential to maintaining colon health. Not a lot of resistant starch research has been published in individuals with digestive diseases.

However, a new study published by Dr. Peter Gibson, Dr. Jane Muir and their colleagues in Australia in Gut this year found that increasing resistant starch and wheat bran (another insoluble dietary fiber which is minimally fermented) was well tolerated in individuals with ulcerative colitis in remission. This was an eight-week, randomized, single-blind, cross-over clinical study with 29 individuals (10 healthy controls and 19 with ulcerative colitis in remission). They found that the fiber tended to normalize gut transit time in individuals with UC. It also improved the delivery of potentially beneficial fermentation products, such as butyrate, to the colon cells, but it did not correct the fermentative deficiency. Overall, the guts of people with ulcerative colitis were significantly different than the guts of healthy controls, making comparisons less meaningful.

3. The most recent data is the most interesting – the fermentation of resistant starch in the gut shifts the composition of the gut microbiome. It appears that Ruminococcus bromii is very important in starch breakdown, as this species usually increases upon resistant starch consumption and is absent in resistant starch non-responders.

The fermentation of resistant starch actually shifts the expression of more than 200 genes located in the large intestine. At this point in time, it is not clear what is the exact pathway for the changes in genetic expression (could be the bacterial composition, the short-chain fatty acids, the starch byproducts themselves, or any number of biochemical shifts that occur in the intestinal soup), but several animal studies have documented these large-scale shifts in gene expression. Some of the genes are involved in muscular contractions (i.e. the kind that move the contents of the intestinal tract along for regularity). Others are related to inflammation, lipid metabolism and sugar metabolism (read more about these benefits in upcoming articles).

In summary, at least 36 clinical studies have been published examining the digestive health benefits of resistant starch (not including the hundreds of animal studies). Resistant starch promotes a healthy digestive tract – it helps to keep everything working well and functioning properly. More importantly, however, it directly contributes to changes in gene expression in important areas such as inflammation, lipid metabolism and sugar metabolism. Stay tuned.