The Best Source of Resistant Starch

Resistant starch wasn’t discovered until 1982. Before that, we thought all starch could be digested by the digestive enzymes in our small intestine. Subsequent studies confirmed that there are indeed starches that resist digestion and end up in our large intestine, where they can feed our good bacteria, just like fiber does. Resistant starch is found naturally in many common foods, including grains, vegetables, beans, seeds, and some nuts, but in small quantities, just a few percent of the total. As I discuss in my video Getting Starch to Take the Path of Most Resistance, there are a few ways, though, to get some of the rest of the starch to join the resistance.

When regular starches are cooked and then cooled, some of the starch recrystallizes into resistant starch. For this reason, pasta salad can be healthier than hot pasta and potato salad can be healthier than a baked potato, but the effect isn’t huge. The resistant starch goes from about 3 percent up to 4 percent. The best source of resistant starch is not from eating cold starches, but from eating beans, which start at 4 or 5 percent and go up from there.

If you mix cooked black beans with a “fresh fecal” sample, there’s so much fiber and resistant starch in the beans that the pH drops as good bacteria churn out beneficial short-chain fatty acids, which are associated both directly and indirectly with lower colon cancer risk. (See Stool pH and Colon Cancer.) The more of this poopy black bean mixture you smear on human colon cancer, the fewer cancer cells survive.

Better yet, we can eat berries with our meals that act as starch blockers. Raspberries, for example, completely inhibit the enzyme that we use to digest starch, leaving more for our friendly flora. So, putting raspberry jam on your toast, strawberries on your corn flakes, or making blueberry pancakes may allow your good bacteria to share in some of the breakfast bounty.

Another way to feed our good bacteria is to eat intact grains, beans, nuts, and seeds. In one study, researchers split people into two groups and had them eat the same food, but in one group, the seeds, grains, beans, and chickpeas were eaten more or less in a whole form, while they were ground up for the other group. For example, for breakfast, the whole-grain group got muesli, and the ground-grain group had the same muesli, but it was blended into a porridge. Similarly, beans were added to salads for the whole-grain group, whereas they were blended into hummus for the ground-grain group. Note that both groups were eating whole grains—not refined—that is, they were eating whole foods. In the ground-grain group, though, those whole grains, beans, and seeds were made into flour or blended up.

What happened? Those on the intact whole-grain diet “resulted in a doubling of the amount excreted compared to the usual diet and produced an additional and statistically significant increase in stool mass” compared with those on the ground whole-grain diet, even though they were eating the same food and the same amount of food. Why? On the whole-grain diet, there was so much more for our good bacteria to eat that they grew so well and appeared to bulk up the stool. Even though people chewed their food, “[l]arge amounts of apparently whole seeds were recovered from stools,” but on closer inspection, they weren’t whole at all. Our bacteria were having a smorgasbord. The little bits and pieces left after chewing transport all this wonderful starch straight down to our good bacteria. As a result, stool pH dropped as our bacteria were able to churn out so many of those short-chain fatty acids. Whole grains are great, but intact whole grains may be even better, allowing us to feed our good gut bacteria with the leftovers.

Once in our colon, resistant starches have been found to have the same benefits as fiber: softening and bulking stools, reducing colon cancer risk by decreasing pH, increasing short-chain fatty acid production, reducing products of protein fermentation (also known as products of putrefaction), and decreasing secondary bile products.

Well, if resistant starch is so great, why not just take resistant starch pills? It should come as no surprise that commercial preparations of resistant starch are now available and “food scientists have developed a number of RS-enriched products.” After all, some find it “difficult to recommend a high-fiber diet to the general public.” Wouldn’t be easier to just enrich some junk food? And, indeed, you now can buy pop tarts bragging they contain “resistant corn starch.”

Just taking resistant starch supplements does not work, however. There have been two trials so far trying to prevent cancer in people with genetic disorders that put them at extremely high risk, with virtually a 100-percent chance of getting cancer, and resistant starch supplements didn’t help. A similar result was found in another study. So, we’re either barking up the wrong tree, the development of hereditary colon cancer is somehow different than regular colon cancer, or you simply can’t emulate the effects of naturally occurring dietary fiber in plant-rich diets just by giving people some resistant starch supplements.

For resistant starch to work, it has to get all the way to the end of the colon, which is where most tumors form. But, if the bacteria higher up eat it all, then resistant starch may not be protective. So, we also may have to eat fiber to push it along. Thus, we either eat huge amounts of resistant starch—up near the level consumed in Africa, which is twice as much as were tried in the two cancer trials—or we consume foods rich in both resistant starch and fiber. In other words, “[f]rom a public health perspective, eating more of a variety of food rich in dietary fibre including wholegrains, vegetables, fruits, and pulses [such as chickpeas and lentils] is a preferable strategy for reducing cancer risk.”


What’s so great about resistant starch? See my video Resistant Starch and Colon Cancer.

I first broached the subject of intact grains in Are Green Smoothies Bad for You?.

Why should we care about what our gut flora eats? See Gut Dysbiosis: Starving Our Microbial Self.

Did I say putrefaction? See Putrefying Protein and “Toxifying” Enzymes.

Berries don’t just help block starch digestion, but sugar digestion as well. See If Fructose Is Bad, What About Fruit?.

The whole attitude that we can just stuff the effects into a pill is a perfect example of reductionism at work. See Reductionism and the Deficiency Mentality and Why is Nutrition So Commercialized? for more on this.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Change Your Diet; Change Your Microbiome

If whatever gut flora enterotype we are could play an important role in our risk of developing chronic diet-associated diseases (see What’s Your Gut Microbiome Enterotype?), can we alter our gut microbiome by altering our diet? Yes. Indeed, diet can rapidly and reproducibly alter the bacteria in our gut, as I discuss in my video How to Change Your Enterotype.

Concern has been growing that recent lifestyle trends––most notably the high-fat and high-sugar “Western diet”––have altered the composition and activity of our resident gut flora. “Such diet-induced changes to gut-associated microbial communities are now suspected of contributing to growing epidemics of chronic illness in the developed world,” yet it has remained unclear how quickly our gut bacteria could respond to dietary change. So, researchers prepared two diets: a “plant-based diet” rich in grains, beans, fruits, and vegetables, and an “animal-based diet” composed of meats, eggs, and cheeses. Neither diet contained refined sugars, as the researchers just wanted to test diets consisting of plant versus animal products. Within just one day of the animal-based diet hitting the gut, there was a significant shift.

What happens when you put a lifelong vegetarian on an animal-based diet? The vegetarian’s baseline microbiota was dominated by Prevotella, unlike everyone else eating a more standard American diet, who had large Bacteroides populations. Remarkably, the animal-based diet inverted the vegetarian’s Prevotella-to-Bacteroides ratio, causing the Bacteroides to outnumber the Prevotella within just four days on the animal-based diet. The entire gut flora got turned on its head and got completely reversed.

The fact that our gut can so rapidly switch between herbivorous and carnivorous functional profiles is probably a good thing in terms of evolution. If you bring down a mammoth and eat meat for a couple of days before switching back to plants, you want your gut to be able to deal with it. This flexibility is manifest in the diversity of human diets to this day, but what is the healthier state to be in most of the time?

Researchers looked at a number of different factors, such as the amount of short-chain fatty acids produced. Short-chain fatty acids, like acetate and butyrate, function to suppress inflammation and cancer, and our gut flora, when on plant-based diets, produce more of these than when on animal-based diets.

Other microbial metabolites, such as secondary bile acids, do the opposite, promoting the development of cancer. With a significant increase in bacterial enzyme activity to create these secondary bile acids on an animal-based diet, it’s no surprise there’s a significant increase in carcinogens like DCA, a secondary bile acid known to promote DNA damage and liver cancer. Microbial enzyme activity producing the rotten egg gas, hydrogen sulfide, also shoots up on an animal-based diet, which stinks because it’s stinky and also because it damages DNA and has been implicated in the development of inflammatory bowel diseases like ulcerative colitis. Hydrogen sulfide is made by pathogens such as Bilophila wadsworthia and is increased on the animal-based diet, again within just days of adopting it, supporting the link between diet and the outgrowth of microorganisms capable of triggering inflammatory bowel disease. Conversely, the only pathogen you see more of on a plant-based diet is just a virus that infects spinach.


Do you know What’s Your Gut Microbiome Enterotype?

See more about gut microbiomes:

I’ve produced a series about the epic fermentation battle in the gut between protein and carbs that offers lots of insight on why it matters who we have living down there:

And check out some other videos on inflammatory bowel disease:

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Why Intact Grains are Even Better than Whole Grains

Fruits and vegetables are the most nutrient-dense foods on the planet, and dark green leafy vegetables lead the pack. Each of the top five so-called powerhouse fruits and vegetables were greens. If we blend them up in a smoothie (or soup or sauce), we’re taking the food with the most nutrition and breaking all the cells to dump that nutrition into our bloodstream. Chewing is good, but blending is better in terms of digestive efficiency and nutrient absorption.

But if we take in all that nutrition and none makes it down to our colon might we be starving our microbial selves? The reason intact grains, beans, and nuts are better than bread, hummus, and nut butters is that no matter how well we chew, intact food particles make it down to your colon where they can offer a smorgasbord for our good bacteria. If our grains, beans, and nuts are finely ground up into flour or paste before we eat it, we may be leaving our gut flora high and dry. Would the same be true for fruits and vegetables?

There are special classes of phytonutrients in fruits and vegetables that appear to protect against colon cancer. They can escape digestion and absorption in our stomach and small intestine, and end up in our colon to act as prebiotics. No matter how much we chew, they stay attached to the fiber. But if we use a blender, might we prematurely detach these nutrients? No. Even if you blend in a high-speed blender for five minutes, the phytonutrients remain bound to the fiber for transport down to your colon bacteria. You can do smoothie experiments on people with ileostomy bags that drain the contents of the small intestine and show that most of the polyphenol phytonutrients make it out intact, so we don’t have to worry we may be robbing Peter to pay Paul when we blend fruits and vegetables. Is there any downside to smoothies, then?

Just as smaller particle size may improve digestive efficiency and gastrointestinal absorption of nutrients from fruits and vegetables, the same may be true for grains. There is, however, a concern that this could boost starch availability and cause a blood sugar spike. In my video Are Green Smoothies Bad for You?, I show you the rise and fall of blood sugar and insulin over four hours after eating a half-cup of brown rice compared with ground brown rice flour (kind of like a cream of brown rice hot cereal). Consuming brown rice flour gives you twice the blood sugar and twice the insulin spike compared to eating the rice intact. Same amount of food, just in a different form. This is why intact whole grains are better than even whole grain flour products.

Simply chewing really well can boost the glycemic and insulin response. If you chew rice really well compared to chewing it normally, the smaller rice particles empty out of your stomach faster, producing greater blood sugar and insulin responses. It’s ironic that there were health crusaders pushing people to chew more to digest their food better, but if what you’re chewing is a five cheese pizza, maybe it’s better not to digest so well. Believe it or not, some have even suggested that diabetics and obese persons should not chew their food so much. But, swallowing diced food without chewing would not only reduce the pleasure of eating—people could choke! Despite this, they suggest it could be a simple way to “allow patients to reduce blood glucose [sugar] levels without fundamentally altering their diets and may thus prove more acceptable” than having to do the unthinkable—just eat high fiber foods like beans, which have been shown to blunt blood sugar spikes.

What about blended beans like hummus? Unlike grains, blending legumes doesn’t affect their glycemic response. So, let’s circle back to the smoothie question: Is fruit more like grains or more like beans? If you liquefy fruit in a blender to make a smoothie, do you risk spiking your blood sugar too high? To find out, watch my Green Smoothies: What Does the Science Say? video.


My general take on beans is simple: the more beans, the better—however you get them. For more information, watch my videos Beans and the Second Meal Effect, The Hispanic Paradox: Why Do Latinos Live Longer?, and Canned Beans or Cooked Beans?.

Want more on smoothies? Here it is:

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: