As we have already learned, starch that we eat is digested at different rates. The starch in potatoes, cereals, and baked goods digests very rapidly. Other starchy foods, such as beans, barley, or long grained brown rice, are digested more slowly, and cause a much slower and lower blood sugar rise. Resistant starch actually goes all the way through the small intestine without being digested at all. In this way, it is more like fiber, and in some cases is classified and labeled as fiber.
What makes some starch resistant?
There are four types of resistant starch:
Starch that is difficult for the digestive process to reach, often due to a fibrous "shell". Grains and legumes which are cooked intact are an example. Also, some altered starches, such as Hi-Maize corn starch, are in both this category and the next.
Some foods, such as unripe bananas, raw potatoes, and plantains, have a type of starch which our digestive enzymes can't break down.
Small amounts of resistant starch (about 5% of the total) are produced when some starchy cooked foods, such as potatoes and rice, are allowed to cool before eating.
Manufactured resistant starch, made by various chemical processes. It is not known whether these starches have the same benefits as those in the other three groups.
Most starchy foods have at least a small amount of resistant starch in them.
Does resistant starch have calories?
Yes, but not in the way you would think, and less than regular starch. When resistant starch reaches the colon, it is used for fuel by the bacteria there. This process, called fermentation, produces a certain type of fat called short-chain fatty acids (SCFAs). It is these fatty acids which produce most of the calories from resistant starch, and many of the benefits. SCFAs are also produced by soluble fiber and oligosaccharides - this is the reason why on some food labels, some fiber is shown as having calories associated with it, but these calories do not raise blood glucose.
What are the benefits of resistant starch?
It seems that the more it is studied, the more positive effects are being found. Many of these are common to oligosaccharides and fermentable fiber. We will discuss fermentable fiber more in Part 5 of this series. Here are some of the benefits of resistant starch:
Resistant starch is especially associted with one type of SCFA, called butyrate, which is protective of colon cells and associated with less genetic damage (which can lead to cancer). Butyrate also protects the cells in other ways. This is one of the real strengths of resistant starch over oligosaccharides and soluble fiber. Their fermentation does produce butyrate, but not at the levels of resistant starch.
As with other fermentable fiber, resistant starch is associated with more mineral absorption, especially calcium and magnesium.
Perhaps most exciting for people with sugar issues, resistant starch seems to improve insulin sensitivity. In the so-called "second meal effect", fermentable fiber and resistant starch are associated with improved glucose tolerance the next day. There is evidence that this is caused by the presense of the short chain fatty acids, and by a peptide produced in the fermentation process.
Resistant starch produces more satiety, possibly partly through the release of a different peptide (PYY).
Resistant starch consumption is associated with lower cholesterol and triglyceride levels.
Promotes "good" bacteria, and supresses "bad" bacteria and their toxic products.
Promotes bowel regularity.
Resistant starch in a meal is associated with less fat storage after that meal.
What foods have resistant starch?
Beans are the very best food source. Although the types of beans and preparation methods cause varying amounts of resistant starch (canned beans are more glycemic), in general, the starch in beans is about evenly dividied between slowly-digested starch and resistant starch. Note, though, that products such as Bean-o, which increase the digestibility of beans, will also decrease the amount of resistant starch. List of Legumes
Whole, intact grains are decent sources of resistant starch. The starch in pearl barley is about 12% resistant and 43% slowly-digesting. Bulgar wheat and long grain brown rice are similar.
The starch in shirataki noodles is classified as soluble fiber, but it seems fairly close to resistant starch in composition, from what I can tell.
Hi-Maize corn starch is also a possibility. It can be used to substitute for part of the flour in baked goods. I've been experimenting with it. I've developed a recipe that is a variation on my flax meal bread. It produces a slightly lighter texture. I will report on my ongoing experiments. Hi-Maize can also be added to shakes. One source is available at King Arthur Flour. There is also resistant wheat starch, and other related products. I have not read as much research on their effects.