My source is "Dr. John H. CUMMINGS: The Large Intestine in Nutrition and Disease"
Around 95% of the SCFAs produced in the colon are absorbed. Of the produced butyrate, 80% is used by the cells of the colon itself. Did you see the table I posted? You have the conversion rates for starch and pectin there. You'll find some more in the above reference.
Yes, that's the table I was asking about. I checked the original table at
http://www.angelfire.com/folk/cusp/images/large_intestine.pdf and it looks like it's implying that 23g of butyrate is converted from 100 g of pure starch (a potato is not pure starch). Should that then be multiplied by the 49% "yield" to give 11.27 g that is actually absorbed by the body?
The 63 SCFA/100 g figure is listed elsewhere in the report as 63 SCFA/100 g carbohydrate, but according to Dr. Eades, less SCFA is produced by carbohydrate than starch, and the table lists "starch," so I'm not sure how that fraction applies to the table.
If the 23% conversion rate is correct and I use that with the USDA database
http://ndb.nal.usda.gov, I get:
100 g Potatoes, Russet, flesh and skin, baked
contains 21.44 g starch
23% of 21.44 g starch converts to 4.9 g butyrate
Individual variation in gut microbiota, amylase, and health theoretically might affect these figures.
I'm not sure I understand all the figures, as there isn't much explanation of them, so let me know how you think it's supposed to be calculated.
Here's how Dr. Eades calculated butyrate at
http://www.proteinpower.com/drmike/metabolism/resistant-starch/:consume a half cup of cooked potato
produces 12.9 grams of carbohydrate (almost three teaspoons)
of which 10.5 grams are starch
If we go by our RD’s estimate that 5 percent of the total starch is resistant starch, we calculate that our half cup of potato contains about half a gram of resistant starch (0.5265 g to be exact)
[I think he makes an error here, so I'm replacing the rest of the calculation]
assuming 100 percent conversion to butyrate (which Eades says "isn’t the case because some is converted to other short chain fatty acids") results in .5 g of butyrate
Eades comments, "So, we eat our half cup of cooked potato, and what do we get? We get almost three teaspoons of sugar and carb that convert almost immediately to glucose and head directly into the bloodstream. The blood volume of a person with a normal blood sugar contains about a teaspoon of sugar, which means that consuming the potato almost quadruples the amount of sugar in the blood. The pancreas then secretes insulin to drive this excess sugar into the cells. This extra insulin then does all the things excess insulin is famous (or infamous) for doing.
But what about the butyrate from the resistant starch? Oh yeah, the 2.3 grams of butyrate. I don’t see how the butyrate is going to do much to stop the insulin spike resulting from the ingestion of the sugars and starch from the non-resistant starch part of the potato. And even if butyrate really does all it is cracked up to do, we wouldn’t really need the potato with all its accessory easily absorbed carb because we can get the equivalent amount of butyrate from a single pat of butter. (Or almost the same – a pat of butter contains 1.45 g butyrate. Two pats of butter contain 3 g or about 1.5 times the amount generated by the resistant starch component of the potato.)"
And he says, "In my opinion, it’s ‘resistant’ for a reason – it’s an anti-nutrient. I’ll post about anti-nutrients in the future. I would avoid resistant starch myself."
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@ Muhammad: Thanks. Yes, I know about soluble and insoluble fiber. Dr. Eades actually said that it's a third type of fiber, "resistant starch" (
http://en.wikipedia.org/wiki/Resistant_starch), that gets converted to butyrate.