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Resistant Starch and Fiber: What’s the Difference?

Payton Baker, RD, MS, LMNT

Published in Nutrition

5 min read

February 2, 2022
two eggs and a bowl of flour
two eggs and a bowl of flour

We’ve been talking about resistant starch for a while now, so you probably already know what it is. Just in case you missed our breakdown of this type of starch, here’s a refresher. First off, starches are long chains of glucose that most carbohydrates in your diet would qualify as (think potatoes and grains, as an example). To explain it just a little more: starch is a chain of simple sugars like glucose, fructose, or galactose, linked together by chemical bonds that your body has to break down to digest.

A Refresher on Resistant Starch and Dietary Fiber

a cut potato

But what’s all this talk about 'resistant' starches? They’re a specific type of starch that your body usually takes a lot longer to break down than it would with any other kind. They can take over two hours to digest and are primarily broken down by fermentation in your large intestine by the bacteria that live there. 

There are many differences between resistant and regular starches: their structure, where they’re digested, and perhaps most importantly, how they’re digested. Since they “escape” the small intestine, the sugars in their starch chain are not rapidly taken up into the blood the same way as those found in other starches are. If you haven’t already, take a more in-depth look into resistant starch here to see why you should care about them.

Remember, there are four different types of resistant starch, and it’s possible for a kind of food to have more than one type of resistant starch in it. If you’re curious, here’s more about commercial (or manmade) resistant starch.

Resistant starches share characteristics with something else you may have heard of—insoluble fibers. This type of dietary fiber does not dissolve in water, and so when your food moves through your digestive tract, these fibers stay intact. 

If you need a refresher on dietary fiber—it’s the part of fruits, grains, and vegetables that your body can’t break down and digest. So, in a nutshell: While your body can digest the rest of your food and absorb it for nutrients, fiber remains relatively intact as it works its way through your stomach, intestines, and finally, colon. This sounds like a bad thing, but it isn’t! Here’s more about dietary fiber and why you should include it in your diet.

Resistant starches, insoluble fibers, and soluble fibers have a lot in common, but they’re not exactly the same. Read on to find out more!

First, More About Soluble and Insoluble Fibers

a bowl of almonds

Fibers act in a similar way to resistant starches. Soluble fiber, which can be found in fresh and dried fruit, can increase the intestinal contents’ viscosity and reduce cholesterol absorption by the intestine. Some other sources of soluble fiber include vegetables, oats, and legumes.

Since it also “escapes” the small intestine, bacteria can ferment it in the large intestine. It also has probiotic properties, can help maintain the health of colon bacteria.

On the other hand, Insoluble fiber is in the cell walls of plant cells, for example, in the form of cellulose. While the small intestine doesn’t digest it, it isn’t fermented. Instead, it acts as “roughage” and quickly passes through the digestive tract. Interestingly, humans lack the enzymes to digest cellulose, however, other mammals like cows do have the ability to digest cellulose.  

Insoluble fiber is the sort of fiber you find in things like whole-grain bread, cereals, fruits, and vegetables. Insoluble fiber can also be found in foods like psyllium, which is usually added to supplements. 

Dietary fibers like insoluble fiber have many benefits; one of the most common is that they help aid digestion. This is because you have to chew more before you swallow them, which means you likely eat them slower. They also may help with weight loss, cholesterol and reduce risk factors for colon and breast cancer. They can also sometimes help reduce your risk of type 2 diabetes by slowing down sugar absorption and helping to improve blood glucose levels. 

Resistant Starch vs. Insoluble Fiber

oatmeal with fruits and yogurt

Now that you’re well-versed with all the different types of starch and fiber, how does insoluble fiber stack up when you compare it with resistant starch? For starters, resistant starch has similar properties to both soluble and insoluble fiber. Some examples include green bananas and legumes and unprocessed whole grains. 

However, one key difference between resistant starch and insoluble fiber is in what happens after these starches escape the small intestine. Insoluble fibers continue to pass through the colon, but resistant starches are fermented by bacteria in the colon. 

So they can both be used as dietary supplements. Still, resistant starch will have the added benefit of nourishing the bacteria in the colon, similar to soluble fibers. Some resistant starches are also soluble, like those formed by cooking and cooling starches like pasta or rice. 

So you can basically think of certain types of resistant starch as a cross between insoluble and soluble fiber. Resistant starch may also have some additional benefits. It acts as a prebiotic, and some foods (like cooked and then cooled pasta) can cause smaller glucose spikes. 

Both fiber and resistant starch can help lower blood sugar. Food containing resistant starch can lower the glycemic response to a meal and thus also insulin secretion. There is evidence that resistant starches can also increase insulin sensitivity. Fiber (both soluble and insoluble) also has a beneficial effect on glucose levels. This includes enhancing insulin sensitivity and it slows down the digestion of glucose, which can have an impact on the blood glucose response to a meal. 

Here's a handy guide to the differences between resistant starch, soluble and insoluble fiber, from Nutrisense dietitian, Katherine Kissane. 

a table comparing resistant starch, soluble fiber, insoluble fiber

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Katie Kissane, MS, RD

Reviewed by: Katie Kissane, MS, RD

Katie is a dietitian at Nutrisense. With over 11 years of experience as a dietitian in many areas of nutrition, Katie has worked as a clinical dietitian within a hospital, as well as in the fields of diabetes, sports and performance nutrition, recovery from addiction, and general wellness. She’s also an athlete and has run 8 marathons, including the Boston Marathon.

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