OK, you made me feel obligated to learn more. I've just read all 14 pages of a fascinating recent review article on resistant starch. The title is "Resistant Starch -- A Review" and it was published in Vol. 5, 2006 of "Comprehensive Reviews in Food Science and Food Safety. Here's a link to the PDF file:
http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1541-4337.2006.tb00076.xHere's my summary of this article.
"Resistant starch [RS] [a term first coined in 1982] is the component of starch that is not digested in the small intestine and has physiological properties similar to that of dietary fiber."
Starchy foods have three kinds of starch: rapidly digested starch (RDS), slowly digested starch (SDS), and RS. RDS and SDS are both digested in the small intestine; RDS is completely digested in about 20 minutes and SDS is completely digested in about 2 hours.
Metabolism of RS in the large intestine occurs 5 to 7 hours after consumption. RS is fermented by the colonic flora to produce short-chain fatty acids. It provides energy to the cells in the colon and also the rest of the body. 30%-70% of RS is metabolized and the balance excreted.
In one study comparing human subjects, there was considerable variation in the per cent metabolized. On average, the subjects extracted about 3 Cal from a gram of RS in comparison to about 4 Cal from a gram of ordinary carbohydrates.
There are actually three kinds of resistant starch that occur naturally in foods. The amount of RS3 present can be considerably increased by cooking and related processing. There are many different factors that affect the formation of RS3 starch produced by processing a starchy food. These include the amount of water, the temperature, processing conditions such as pressure cooking or autoclaving, the nature of other compounds naturally present in the material being processed, and certain added compounds. The effects of these factors are so varied that it isn't possible to generalize. However it is true that for most starchy foods, cold storage after processing generally increases the RS for storage times of up to two to four days.
Because of all these factors, it is difficult to control the amount of RS3 present in a given sample. It is also difficult to analyze food for resistant starch. However, the figure of 50% resistant starch that Trina gave for navy beans is certainly within the realm of possibility.
Resistant starch is what you might call a "good carb" in that your body doesn't metabolize it like ordinary carbs so eating resistant starch doesn't have the bad effects that eating ordinary carbs does. If you are familiar with the idea of subtracting fiber carbs from total carbs to get ECC or effective carb count, you could do the same thing with RS carbs. In other words, RS carbs count as calories but not grams of carbs if you are counting carbs.
Unfortunately, in order to get RS from ordinary food, you have to eat the ordinary carbs along with it. So you are eating something bad for you together with something good for you. For example, if you wanted to eat 20 g of RS and you were lucky enough to have a starchy food that was 50% RS, you would also have to eat 20 g of ordinary carbs, which is the max allowed per day on a really low carb diet.
There have been some negative effects of RS seen in rats that may or may not affect humans. Further studied is required.
There are definitely some good effects on the colon. RS can have a positive impact on colonic health similar to fiber; it can reduce cholesterol and tryglycerides; and it may help prevent colon cancer. Some studies show increased benefit of combining RS with insoluble fiber like wheat bran or with psyllium.
It is not clear to me whether there is any benefit from RS that you don't get from fiber. However, there are a number of companies working on methods to develop pure RS. I believe manufactured RS is currently used in special foods for diabetics.
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