The Specific Carbohydrate Diet and the GAPs diet both rule out all starches, including sweet potatoes, because they say that the starch will ferment in your gut and allow the unbalanced growth of "bad" bacteria. I'm wondering: are there any studies to back this up?
I have done some research and there are studies that talk about starch being digested in the colon--but none of them suggest that starch consumption will lead to gut dysbiosis (leaky gut), at least not directly.
How do we digest starch? Most of the job is done by bacterial fermentation:
The predominant amylolytic bacteria belonged to the genera Bifidobacterium, Bacteroides, Fusobacterium and Butyrivibrio. Mixed populations of gut bacteria rapidly fermented starch with the production of volatile fatty acids and organic acids. Lactate was observed to be a major, though transient intermediate during starch fermentation by these cultures. Approximately 60% of starch utilized was converted to volatile fatty acids, which in the human colon would be potentially available for absorption.
But starch happens to be a complicated subject as there is normal starch, such as that in a legume or tuber, and then resistant starch. To quote one study:
You can get one kind of resistant starch by cooking and then cooling starchy legumes, cereals, and tubers.
RS starch is starch and starch degradation products that resist the action of amylolytic enzymes.
And it seems that resistant starch may have beneficial qualities for the stomach.
To learn about resistant starch, I'd direct you here:
This article argues that the microbiotic balance of the colon is highly dependent on diet. Individuals fed resistant starch have gut microflora "blooms" of Ruminococcus bromii, Oscillibacter, and Eubacterium rectale. So it seems clear that resistant starch diets will have an effect on microflora. To make a long story short, based on the two following studies, these appear to be" good" bacteria.
F. prausnitzii, R. bromii, Eubacterium rectale, Ruminococcus albus, Bifidobacterium bifidum, P. distasonis, and Alistipes putredinis
were more abundant in healthy than in CD microbiota.
The other study stated,
Eubacterium rectale, Bacteroides fragilis group, B. vulgatus, Ruminococcus albus, R. callidus, R. bromii, and Faecalibacterium prausnitzii were 5-10-fold more abundant in the healthy subjects than in the CD patients, while Enterococcus sp., Clostridium difficile, Escherichia coli, Shigella flexneri, and Listeria sp. were more abundant in the CD group.
In summary, "I'lll just quote one sentence from one more study: "RS [Resistant starch] enhanced the growth of intestinal bacteria assumed to promote health."
Resistant starch (RS) is a dietary fibre component and its fermentation generally favours butyrate production. Dietary RS intakes and faecal butyrate levels are high in populations at low risk of diet-related large bowel diseases. Conversely, RS intakes and faecal butyrate levels are very low in high risk groups. This raises the possibility that greater RS consumption could be of health benefit.
Also, why you should feed Bifidobacteria:
And in Pigs, high amylose starch fed Bifidobacteria species:
Starch with high amylose content and low in vitro digestibility increases intestinal nutrient flow and microbial fermentation and selectively promotes bifidobacteria in pigs.
One IMPORTANT caveat of course is that resistant starch only makes up some of the starch that you eat--not necessarily high percentages of it.
However, there is another line of argument that suggests that starches are bad, namely, that starch feeds the bacteria Klebsiella.
As one study puts it,
A link exists between high dietary starch intake and the growth of intestinal microflora, involving especially Klebsiella microbes.
(I would love it if I could read past the abstract to learn more about that link.)
And Klebsiella is a front runner, alongside Candida, for dysbiosis.
Anyway, there is some food for thought. I'd be interested to see what other sort of research can be found on the subject.
Edited by NewBrigade, 13 April 2012 - 08:44 AM.