Last week, we told you what resistant starch was, whether it had health benefits and why it’s important to learn more about it. By now, you likely already know what this starch is, but let’s go over it again before we dive into the next part. Starting from the very beginning: you know that sugars are carbohydrates that your body uses for energy. Starches are chains of these sugars that are linked together by chemical bonds. So, depending on how long it takes your digestive system to free up those sugars from their bonds, you can think of starches as either digestible or resistant.
Some of the carbohydrates in resistant starches are not broken down and absorbed in the small intestine, but instead go into the large intestine, where they are digested by bacteria in the large intestine. Read our overview of resistant starch for more on why foods like white rice, lentils, legumes, green bananas, oats, and other types of resistant starch are a good addition to your diet. If any of this sounds familiar, it’s they are closely linked to complex carbohydrates. Similar to resistant starch, complex carbohydrates are simple sugars linked together, but unlike resistant starch, they are easily digested and absorbed in the small intestine. Insoluble fiber is something else that is similar to resistant starch since it is also fermented by bacteria in the colon.
This idea of eating something that is digested really slowly is also linked to the glycemic index (GI), which tracks how quickly a food is converted to glucose in the blood compared to a standard amount of pure glucose, which has a glycemic index of 100. So, resistant starches are complex carbohydrates by definition, act like dietary insoluble fibers, and can have lower GIs because more of their starch “escapes” digestion. And, there are four different types!
The Types of Resistant Starch
Now that we know a bit more about this fascinating type of starch, it’s time to focus on the different types. Resistant starch is divided into four types, usually called RS1, RS2, RS3, and RS4. Foods with seeds can contain resistant starch type 1, or RS1, for example. Foods like green bananas are “born” with high amounts of resistant starches, but as they ripen, these become regular starch.
Here’s more about each one:
1. Resistant Starch Type 1 (RS1): These starches resist digestion. Think of things like seeds here since your small intestine can have trouble digesting the starches within them. You can find RS1 in legumes, and whole or partially milled grains.
2. Resistant Starch Type 2 (RS2): These are non-gelatinized granules. This means that the small intestine can slowly digest them, but not before much of them escape to the large intestine. You can find RS2 in green bananas, raw potatoes, or high amylose corn.
3. Resistant Starch Type 3 (RS3): These are cooked in water until they’re fully hydrated. Their starches move to water, then re-form into structures that are difficult to digest when they’re cooled. RS3 can be found in bread, cooked and cooled potatoes, cornflakes, or processed foods.
4. Resistant Starch Type 4 (RS4): These are chemically modified by human processing (in other words, man-made!) and can be difficult to digest. RS4 is the product of using chemically modified starches, so you can find it in more or less anything, including certain types of cereal.
More About Resistant Starch in Your Food
All these categories can seem overwhelming, so let’s take a quick look at some concrete examples of the types of resistant starches in some common foods. Take white flour as our first example. It’s made from wheat that’s separated and ground into flour, so predictably only three percent of it is resistant starch, namely RS2. So, this small amount of resistant starch will likely not have much of an impact on slowing digestion.
Foods made with other types of flour have a higher percentage of RS2. Think of cookies made with banana flour, with 38 percent RS2, or those made with potato flour, with 35 percent RS2. These foods tend to digest slower, and have a lower glycemic index too.
The same food can have more or less resistant starch depending on whether it's raw vs. cooked or green vs. ripe. For instance, green bananas (mostly RS2) have a higher resistant starch content, but when the banana ripens, the resistant starch turns into regular starch (and it becomes much sweeter).
Another example is a baked potato that is taken right out of the oven. That potato has very low levels of resistant starch, but if you cool it in the refrigerator overnight it can almost triple the content of resistant starch (RS3). This is due to changes to the structure of the starch during the cooling process. Cooking foods can actually decrease the resistant starch content of food, but it is possible to recapture the resistant starch content by cooling the foods. This study suggested maximizing the resistant starch content of beans by cooling them after they have been cooked.
Cooking methods can also have an impact on the resistant starch content of a food. This study found that microwaving or grilling potatoes had a more favorable impact on the amount of resistant starch as compared with steamed potatoes.
Looking for more foods that contain resistant starches? Here’s a list to start with:
- Plantains or green bananas contain RS2, but as they ripen, this becomes digestible starch.
- Potatoes, which have RS1 when raw, and RS3 when cooked and then cooled.
- Whole grains like oats and barley contain RS2. The processing technique for grains can either increase or decrease the resistant starch content. More processed grains tend to have less resistant starch.
- Seeds like sunflower seeds or pumpkin seeds contain RS1.
- Legumes, like canned chickpeas, have RS1 or RS3. Kidney beans contain RS2 or RS3. Similar to potatoes, cooking and cooling legumes can also have a favorable increase in resistant starch.
- Rice that is first cooked, then cooled, has RS3.
By now, you’re probably already making a list of foods and thinking about experimenting to see how your body responds to it. If you’re considering including some in your diet, use a continuous glucose monitor (CGM) to track how different types of resistant starch can affect your metabolism and blood sugar levels.
Wondering what else there is to learn about resistant starch? Watch this space for more on how resistant starch is made and what similarities it shares with insoluble fibers.
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Kara Collier is the co-founder and VP of Health at Nutrisense, one of America’s fastest-growing wellness-tech startups, where she leads the health team. She is a Forbes 30 under 30 recipient, frequent podcast guest & conference speaker.