Up to 75% of the starch in red kidney beans may be resistant to digestion.
RS is merely starch that resists digestion - digestion, that is, by human enzymes. Dietary fiber resists digestion too, but RS is not a fiber in the traditional soluble and insoluble sense; you likely won't see it listed as dietary fiber on the Nutrition Facts label. You will see it lumped into the Total Carbohydrate amount - although that may change.
How Much RS Is In Food?
The amount of RS in a food is found by subtracting its rapidly digested starch (RDS: starch which is reduced to glucose units within 20 minutes - bread has a lot of RDS) and slowly digested starch (SDS: starch which is reduced to glucose units within 120 minutes - some cereals) from total starch (TS):
In an earlier post (Bacteria and Blood Sugar) I listed some foods known to contain RS: legumes, raw potato, green banana, cooled cooked potatoes/grains/pasta, and some types of corn and rice that contain higher amounts of amylose. (See the bottom of this post for a picture and description of amylose starch.)
But not all RS is resistant to the same degree.
Below is the current classification used for RS. There are 4 groups, varying among other qualities by degree of digestibility:
RS1 - Starch that is resistant to digestion because it exists in a physically dense, or physically protected form. Examples are whole- or partly-milled grains, seeds, and legumes. Milling, grinding (including chewing), and homogenization free this starch for digestion.
RS2 - Starch that is resistant to digestion because it exists in a physically dense and relatively dehydrated form. This lack of water is internal to the structure of the starch granule. It's not evident by looking at it. Examples are raw potatoes and unripe banana. Boiling and homogenization free this starch for digestion.
RS3 - The most resistant kind. Starch, mostly amylose, that becomes resistant to digestion when heated then cooled. Also known as retrograded starch. Examples are cooled cooked potatoes and beans. Amounts in pasta vary and are dependant upon the structure of the pasta, and heating and cooling times.
Boiling RS3 will not easily free it for digestion, as it will RS2. In fact, moist heating will encourage the starch molecules to swell then rearrange themselves as they cool, making this starch almost entirely resistant to digestion by pancreatic amylases. Another unique feature of RS3 ... repeated heating and cooling cycles will further increase the RS content.
RS4 - Starch that is resistant to digestion because it has been chemically modified. Bonds other than naturally occurring α-(1-4) and α-(1-6) are formed. Examples are commercially made breads, cakes, crackers, etc. that contain "modified food starch".
I expect to see, as the benefits of RS accrue, more processed foods containing higher amounts of RS4 and advertising this fact prominently. Since RS4 does not occur in nature, I'm wary of it. It reminds me of the early excitement over partially hydrogenated oils ... whose trans-configurated bonds were later found to be unhealthful.
The starch in a food can consist of all forms - rapidly digested starch, slowly digested starch, and various types of resistant starch.
Here's an excerpt of a table from Sajilata et al's, Resistant Starch - A Review:
Click for larger.
Look at that red kidney bean. Up to 75% of its starch may be undigestible. Not only will the presence of RS decrease stated caloric content, but it also decreases expected post-meal blood glucose levels. And, as I noted previously, consumption of RS can lead to decreased post-meal blood glucose for up to a day or two afterwards.
Note: Just because we (human enzymes) don't digest it doesn't mean that RS exits our body completely undigested. The bacteria in our colon have enzymes which break down RS, freeing bits of glucose (which we absorb), and producing bits of fat (which we absorb). So RS is not calorie-free. When compared to digestible starch, RS provides about 50% fewer calories. (DS: 4 cal/gram, RS: 2 cal/gram)
Snapshot Of A Moving Target
The numbers in the above table are only a snapshot of a moving target. It's difficult to pinpoint the exact amount of RS in a food because so many factors affect it:
- Chewing decreases RS.
- Adding oil as the starch cools decreases RS.
- Adding spices as the starch cools decreases RS.
- Reheating cooled starch decreases or increases RS depending on the type.
- Germination and fermentation decrease RS.
- Lower-temperature, longer-time baking increases RS.
- Storage, especially low-temperature storage, increases RS.