Although carbs tend to get a bad rap these days, abundant research suggests that starch has been a staple in our diets for millions of years.

From the days of our ancestors, the hunter-gatherer diet included a variety of starches in the form of tubers, fruits, roots, and nuts. Almost 250,000 years ago, Neanderthals in France created tools to dig starchy roots up from the ground for sustenance.

It's no secret that our diets have shifted dramatically since the time of our Paleolithic ancestors. Over the past century, our starch consumption has increased significantly due to modern food processing and industrialization.

Beginning in the 1940s, the advent of processed foods embodied a shift towards making foods more easily digestible. We began to strip away the seed, the shell, or the hull that protects the starch from digestion (which our microbiome has grown evolutionarily accustomed to).

As a result, we removed the vast majority of resistant starch, one of the three types of complex carbs, from our diets and replaced it with high-glycemic, digestible starch to the detriment of our health. To put it into perspective, our ancestors consumed around 30-50 grams of resistant starch each day, while the resistant starch intake of a typical modern Western diet includes a meager 3-8 grams per day.

In today's edition, we'll dive into resistant starches, examining how the fermentable fiber produces beneficial byproducts in your gut and why you might want to consider making sushi with your rice leftovers.

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What is Resistant Starch and Why Should I Care?

It's a type of carb resistant to digestion.

‍Unlike regular starch, which is broken down into sugar when digested, resistant starch (RS) is one of three types of complex carbohydrates that resists digestion in the small intestine. Once it reaches the large intestine, it's fermented and used as fuel by specific bacteria, producing beneficial compounds like short-chain fatty acids and other metabolites. (Source)

It has prebiotic effects.

‍Resistant starch is a highly fermentable type of insoluble fiber. As the fibers ferment in the large intestine, they act as a prebiotic and feed the good bacteria in the gut. By lowering the pH level and producing short-chain fatty acids in the large intestines, resistant starches create an environment where beneficial bacteria can thrive. Studies have shown that this prebiotic environment can help keep colon cancer at bay, reduce the severity of inflammatory bowel disease (IBD), and help maintain a healthy weight or promote weight loss. (Source, Source)

There are four different types.

Type 1 (RS1): starchy foods coated with seeds or germ (e.g., unprocessed whole grains, legumes including soybean seeds, beans, lentils, and dried peas)

Type 2 (RS2): naturally resistant starchy foods (e.g., uncooked potatoes, green banana flour, and high-amylose corn flour). This is also the most common supplemental form of RS.

Type 3 (RS3): retrograded starch — starchy food that has been cooked and then cooled, which increases its resistant starch content (e.g., potatoes or pasta cooked-and-cooled, sushi rice, etc.)

Type 4 (RS4): synthetic starchy foods chemically modified by manufacturers to be digestion-proof (e.g., food additives derived from corn, potatoes, or rice) (Source)

Studies have demonstrated the numerous potential health benefits of resistant starch.

Some of the beneficial effects of RS backed by research include improved insulin sensitivity, reduced blood glucose after meals, and increased satiety. Resistant starch also supports the gut by boosting the production of short-chain fatty acids, increasing uptake of minerals from the gut, and supporting colonic health. (Source, Source)

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What Does the Research Show About Resistant Starch Foods?

Responses to resistant starch supplementation vary based on the individual and type of starch

‍Researchers have found that responses to RS are highly individualized based on the baseline gut microbiota composition (this adds to the complexity of studying its effects on the body). For example, one study reported a 50% increase in butyrate levels after treatment with potato starch, but a further examination of the results reveals that individual responses were quite varied. Evidence currently suggests that we may need to transition from the message of “eat more resistant starch” to “eat the resistant starch that is right for your microbiome.” (Source, Source)

It improves the integrity and function of the gut

‍Resistant starch improves the integrity of the cell lining in the colon, reducing cell damage and immune reactivity. It also prevents bacterial toxins from circulating in the body and reduces leaky gut, which could have positive effects on inflammatory conditions, such as allergies and autoimmune diseases. (Source)

Not all resistant starches behave the same

‍Several studies have suggested that raw RS2 may be damaging to the microbiome (even in healthy individuals) and may increase gut inflammation in populations with poor gut health. A 2015 study found that supplementing children's diets with 8.5 grams per day of high amylose corn starch resulted in reduced microbial diversity and increased calprotectin, a clinical sign of gut inflammation, after four weeks. (Source)

Maybe there's something to potato salad?

‍A 2016 study in the journal Food Chemistry found that chilled potatoes had more resistant starch than reheated potatoes, although both contained higher levels than hot potatoes. Researchers also found that baked potatoes had higher RS content than boiled potatoes. (Source)

Synthetic applications may be used for improved medication uptake in the future

‍Scientists are currently researching ways to develop resistant starch capsule coatings to release medications farther along the digestive tract (i.e., in the colon). (Source)

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So What Should I Do About Resistant Starch Foods?

Incorporate whole foods rich in resistant starch

‍Some of the food sources high in resistant starch include raw potatoes, green bananas, plantains, beans, cooked-and-cooled potatoes, oats, and cooked-and-cooled rice. Raw potato starch is one of the buzziest foods with the highest resistant starch content, with 72% of its starches being resistant starch. In general, you'll want to opt for cooked and cooled RS3 in the form of whole foods when possible and take caution with RS2 sources, as studies have shown how this type of RS may potentially damage the gut (see above). (Source)

Green bananas > yellow bananas

‍Bananas contain the most resistant fiber when they’re green. As they ripen, resistant starch transforms into regular starch (i.e., sugar). For comparison, a small ripe banana has 4 grams of resistant starch, whereas green bananas have up to 80% more. (Source)

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Cook then cool

‍Interestingly, how you prepare starch-containing foods affects their starch content. As a general rule of thumb, starchy foods have more resistant starch when they're raw. Heat breaks down most resistant starches, causing starch molecules to relax during the cooking process. Once a food cools, starch molecules return to their hardened state (often becoming more dense and difficult to break down), which slows down the digestion process and helps to stabilize blood sugar levels upon consumption. On a molecular level, this is why cooked-and-cooled rice is higher in resistant starch than simply cooked rice. (Source)

Start low, go slow

‍When you're first incorporating more resistant starch into your diet, it's common to experience mild bloating or increased gas, as with other dietary fiber sources. As your gut flora adapts, this should occur less frequently, but you'll want to start small and gradually increase the amount as tolerated. Studies show that the benefits of resistant starch may be seen when consuming around 15 to 30 grams daily. If your GI symptoms worsen, then decrease the amount you’re taking for a few days until they resolve, and try increasing again gradually. If you continue experiencing symptoms, this can be a sign of SIBO (small intestinal bacterial overgrowth), gut dysbiosis, or IBS (Irritable Bowel Syndrome) and you should seek out support from your healthcare provider to test your gut for imbalances. (Source)

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Where Can I Go to Learn More About Resistant Starch?

For a fun, informative TLDR

‍If you're more of a visual learner, this quick 4-minute animated video shows how resistant starch moves through the intestine and feeds the healthy bacteria of the gut. It also highlights the role of butyrate, one of the critical short-chain fatty acids produced in the process. (Source)