If This Dietary Staple Is So Healthy, Why Can’t I Remember Anything About It?

Photo-Illustration: by Preeti Kinha; Photos: Getty

Resistant starch is an elusive and frankly forgettable nutritional concept. It’s apparently a key to good health and good bowel movements, and one of the colon’s favorite foods, but — what was it again? Unlike “carb,” “fat,” “dairy,” or even “fiber,” resistant starch defies easy categorization.

As the name implies, resistant starch is a type of starch found in certain forms of carbohydrates, like whole grains, beans, green bananas, and raw potatoes. (It’s also a kind of fiber.) But unlike most starches, it resists being digested in the small intestine, where most food gets absorbed. This allows it to arrive intact in the large intestine (the colon), where it ferments and feeds the gut bacteria there. (Think of it like a long-haul starch that doesn’t hop off the train until the very last station.) Along with supporting overall colon health, resistant starch can help promote bowel regularity and reduce intestinal inflammation as well as improve insulin sensitivity and lower blood sugar (both of which can help prevent type-2 diabetes). In 2018, the FDA included resistant starch in a review on the effects of different kinds of fiber, noting its “beneficial physiological effect on postprandial insulin levels.” In other words, it’s good for gut health.

Rhonda Witwer has been working with resistant starch for 17 years — first as the business manager for a maker of resistant-starch products (Ingredion) and then, independently, as the founder and proprietor of the educational, not-for-profit site Resistant Starch Research. (We met through a knitting event and eventually struck up a conversation about health and resistant starch. She no longer receives financial compensation from Ingredion.) “As the evidence around resistant starch’s health benefits grew and was confirmed over and over,” Witwer told me, “I came to believe that I could not make any better contribution to the world than studying and promoting it.”

Witwer encourages a return to the “ancestral” way people around the world used to eat resistant starch, emphasizing the ways we now suffer from that lack. The typical modern Western diet includes three-to-eight daily grams of resistant starch, she notes, whereas historically we were eating 30-to-50 grams a day. I was struck by an image she shared of a can of potato chips from the ’40s, marketed as being “more readily digestible.” According to Witwer, this trend toward digestibility has unintentionally contributed to the prevalence of obesity and diabetes. “Now,” she writes, “we are discovering the detrimental health consequences of this shift.”

I spoke with Witwer about what resistant starch does, who should be eating more of it, and why it’s so hard to remember what it is. While we spoke, I had the somewhat ridiculous feeling of being an early fan of a new band. We’ve been eating resistant starch since the dawn of humanity, but if we’ve processed so much of it out of our food, it’s arguably new again. The fiber revolution?

You write that people used to eat a lot more resistant starch. How were they getting it?
If they lived in the tropics, it was from bananas. If they lived in Tibet, it was from whole-grain barley. If they lived in the Andes, it was from potatoes. In Africa, they ate a lot of cooked-and-cooled porridges that contained resistant starch. Also from beans, raw starches, and other grains that have been less processed.

Resistant starch is naturally found in intact whole grains that have not been processed, so today you would find it in foods like muesli cereal, green bananas, beans, and raw potatoes. Over time, we have processed resistant starch out of the diet. In making foods more easily digestible, we’ve stripped away the seed, the shell, or the hull that protects the starch from digestion. Since resistant starch feeds our microbiome, this lack is having a tremendous negative impact on our health.

How so? And how exactly can eating resistant starch benefit people?
Resistant starch feeds the microbiome that lives in the large intestine. For some reason, resistant starch produces more of the short-chain fatty acid called butyrate than any other fiber tested. Colon cells eat butyrate as their energy source, and if they get enough butyrate, they get fat and happy, and they work really well. If they don’t get enough butyrate, they shrink and get leaky, and there are all kinds of health issues that come out of that.

Keeping the colon barrier intact and working well can help treat constipation, promote regularity, and stop diarrhea. Something about the fermentation of resistant starch also changes the expression of genes in the large intestine, some of which are directly connected to insulin sensitivity. When you add resistant starch back into the diet, insulin sensitivity improves, which can help prevent diabetes.

Resistant starch also increases satiety and changes the energy balance in the diet, so people are less hungry — in other words, they eat less when they get resistant starch. We’re also seeing that it reduces inflammation, particularly in people who have chronic kidney disease. We’re seeing that it reduces levels of toxins that are damaging to the kidneys. We’re seeing that it improves the pancreatic release of insulin, too.

So it’s like the harder to digest something is, the more beneficial it is, as long as it’s edible. And that in making foods more easily digestible, a lot has been sacrificed?
And we didn’t know it at the time! This wasn’t a malicious choice by the food industry. They thought they were doing the right thing, because people wanted these easier-to-digest foods. It wasn’t a conspiracy; we just didn’t know. And over the last ten years, the importance of the microbiome has become more obvious. It’s now recognized as a complete separate organ that’s essential for health. So many intestinal health problems, like immunity trouble and allergies, are linked to the microbiome.

I’m convinced that the lack of resistant starch in our diet is why we’re seeing so much prediabetes. And diabetes is an epidemic.

How do you consume resistant starch, personally?
I do a muesli cereal in the morning, and I mix into that a resistant starch powder from dried green bananas.

Hah. And I put sliced green bananas on top of that, so I do both whole-food sources as well as supplement sources. I’ve baked with resistant cornstarch for 15 years; it’s one of the very few resistant starches that you can bake with. I can substitute about 20 percent of the flour in any recipe with resistant cornstarch. Every cake that I’ve taken to church, every cookie I’ve made for Christmas, every birthday cake, pancakes — they’ve all had resistant starch.

In supplement form, you can get green-banana flour, or green-banana powder. You can get a raw potato starch that has a high quantity of resistant starch in it — but those need to be consumed raw, because the resistant starch will cook out.

I tried a green banana recently, but it was hard to eat! It was a little grosser than I’d anticipated.
Yep! It’s astringent. It’s harsh. The flavor is earthy from the polyphenols. That’s why I mix the powder into my cereal and put the bananas on top of cereal. But I don’t really notice the texture anymore.

I read somewhere that increasing resistant starch intake can affect the quality of your dreams, making them more vivid. Is that real? 
Absolutely. A lot of our hormone production is out of the large intestine. We first heard the dream benefit from low-carbers who had added resistant starch back into their diet. They attributed it to the hormones that are produced out of the fermentation of resistant starch. I haven’t heard that benefit from other groups, and I’ve not seen it in the research. But it has been a very strong benefit communicated anecdotally in the low-carb communities.

Does eating more resistant starch cause gas?
Resistant starch is insoluble, so it ferments very slowly. And yes, there is some gas that is produced, but it’s not as much as people commonly experience with soluble, fermentable fibers like inulin and fructooligosaccharides (FOS), which are common in many vegetables and are well known for causing gas, bloating, and potentially even cramping — and their dose tolerance is about ten grams a day. The dose tolerance of resistant starch, by contrast, is about 45 grams a day, before people experience significant increases in gas and bloating.

Tell me more about how certain cooked-and-cooled foods — like cooked-and-cooled rice — contain resistant starch, whereas cooked (and not-cooled) foods don’t.
Starch is nothing more than tightly bound chains of glucose in a granule. As long as that granule is intact, it resists digestion. Once the granule swells — as it does when it’s cooked — to the point that it bursts, it releases all those glucose chains. In the cooling-down phase, some of those glucose chains form bonds and then become resistant to digestion.

And then what if you reheat that all over again? 
If food is reheated and then cooled down again, you actually get increasingly more resistant starch — even more than you had at the previous cycle.

Who would benefit from increasing resistant starch in their diet right now?
I think every single one of us can benefit from getting more resistant starch. As a population, we are grossly underconsuming dietary fiber, specifically fermentable fiber.

Nearly half the population is prediabetic or type-2 diabetic, and those people in particular could use more resistant starch. There is actually an FDA-approved health claim saying that limited evidence shows that resistant cornstarch could help reduce the risk of type-2 diabetes.

And then there are the many people who have digestive issues, which can include irritable bowel disease (IBD), celiac disease, major constipation, allergies, and immune issues. These people may also benefit from eating more resistant starch.

One challenging thing about resistant starch is that the concept is blurry and imprecise enough that I mostly forget what it is later. 

When we’re talking, it feels clear, but afterward it floats away. Do you have any recommendations for how to better remember what it is? 
I keep coming back to: Our ancestral starches have always been in our diet. And ancestral starches were resistant starches. We’ve largely taken them out of our foods, but they help our bodies work better, in lots of different ways.

Also: Resistant starch is a “good” carb that used to be in our diet, but we’ve taken it out, and we need to put it back in.

Right now there’s so much discussion around eating low carb and what qualify as “safe” starches. Different approaches have tried to come up with different work-arounds, but the simplest work-around is: Put ancestral starches back into your diet.

It seems like an awkward time to be advocating for resistant starch in powdered supplement form, given the culturewide anti-processed-food and anti-supplement moment. But I’m gathering that powdered resistant starch, in particular, may be a paradox: Its function is to negate the downsides of processed foods while being itself a processed food?
Yeah. It depends. If somebody is focused on that, then eat muesli that’s unprocessed. And green bananas or raw potato flour.

Some people who get into this will completely change their diet, not eat processed foods, and cook everything from scratch. But most of us won’t. And so the food and supplement industries have a responsibility to develop better options. If isolating or creating resistant starch and putting it back into bread, crackers, cookies, and pancakes is going to help people eat better, we need to be doing that. The diabetes epidemic and the health consequences that we’re facing are too big, and too severe, not to use every single tool at our disposal.

What drives you to share resistant starch information? Has that drive evolved over time? 
In the beginning, it was professional, as I was the business manager for the Hi-maize resistant cornstarch. As I learned more, it became personal. I believe that my dad, for example, does not have type-2 diabetes in large part because my mother feeds him resistant starch. My cousin is better managing his diabetes with resistant-starch supplements and closely follows the latest research, which we discuss over our occasional family dinners. I have many friends who’ve also experienced these types of benefits. People get resistant starch primarily from supplements today, but in the future, more resistant-starch-based foods will be created and introduced into the market, making consumption more convenient. It is imperative that we continue to make foods not only convenient and tasty but also healthier. I am working to create those healthier choices, which is a worthwhile goal in life.

This conversation has been lightly edited for length and clarity.

If This Food Is So Healthy, Why Can’t I Remember What It Is?