I’ve gotten a lot of questions about resistant starch lately. For one reason or another, some people seem to believe that resistant starch is a cure-all for gut health. Other people are on a restricted diet to manage symptoms, and think they are “deficient” in this type of prebiotic fiber, leading them to consider high-dose supplementation.
One source of supplemental resistant starch that has gained particular popularity across the blogosphere is high dose raw potato starch. Even many functional practitioners now recommend this to all patients with gut dysbiosis. But is this really a good idea?
In this article, I’ll break down the basics of resistant starch, the evidence for the various types, and how to best incorporate resistant starch into your diet and into a gut-healing protocol.
What is resistant starch?
A lot of the carbohydrates we eat are in the form of starches. Starches are long chains of glucose that are found in grains, potatoes, sweet potatoes, and other vegetables.
But not all of the starch that we eat gets digested. Some of it passes through the GI tract unchanged – that is, it’s resistant to digestion by the host. This form of starch is called resistant starch.
Resistant starch passes through the small intestine, unencumbered by small intestinal digestive enzymes. It then reaches the colon, where it can be fermented by specific bacteria into short-chain fatty acids and other metabolites.
Resistant starch is also therefore a prebiotic fiber, meaning it can selectively promote the growth of certain bacteria in the colon and confer benefits to the host.
Types of resistance starch
There are four primary types of resistant starch (RS):
Type 1 (RS1): found in whole or partially intact grains, seeds, and legumes. This type resists digestion because it is physically inaccessible, bound up within fibrous cell walls.
Type 2 (RS2): found in raw potatoes, green (unripe) bananas, raw plantains, and raw tigernuts. This is also the most common supplemental form of RS, available as potato starch, green banana flour, or high amylose corn starch. Cooking these foods renders the starch digestible and eliminates the resistant starch.
Type 3 (RS3): formed when starchy foods, like potatoes, rice, and legumes, are cooked and then cooled. This turns some of the previously digestible amylose and amylopectin into resistant starches. It is also called retrograde resistant starch. Reheating the food at a low temperature (less than 130 degrees F) maintains the RS content.
Type 4 (RS4): synthetic, man-made resistant starch produced by chemical modification.
The potential health benefits of resistant starch
Numerous studies have demonstrated the potential health benefits of resistant starch. In the gut, these are similar to the benefits of soluble fiber. They include: increased fermentation and laxation, increased uptake of minerals from the gut, and changes in the composition of the gut microbiota, including increased production of short-chain fatty acids like butyrate (more on butyrate later).
Other reported health benefits of RS include improved insulin sensitivity, lower blood glucose after meals, reduced blood cholesterol levels, and increased satiety.1 However, results from different studies have been mixed. As I’ll explain in the next few sections, it likely depends on the individual, their microbiome, the dose, and the type of RS.
Key microbes that degrade resistant starch
Only certain microbes can degrade RS. The two species in the human gut that are known to be capable of initiating RS breakdown are Ruminococcus bromii and Bifidobacterium adolescentis.
Of these two, Ruminococcus bromii has received the most attention as a “keystone” species in RS metabolism.2 By beginning the breakdown process of resistant starch, R. bromii releases sugars of various lengths and the short-chain fatty acid acetate that are then available to other species. B. adolescentis plays a similar role, releasing sugars and lactate.
Notably, neither of these two primary degrading bacteria produce butyrate directly. It is only through cross-feeding interactions that other bacteria pick up the smaller sugars, lactate, or acetate and produce butyrate.
Resistant starch response is highly individual and depends on the baseline gut microbiota
I’ve written previously about how response to prebiotic fibers is highly variable and depends on baseline gut microbiota composition. The same is true for RS, where the effects reported are often the average across a population:
“Positive responses to RS supplementation are not universal, and there are interindividual differences after consumption, as is likely to be true for most dietary fibers.” 3
For instance, one study reported a 50 percent increase in fecal butyrate levels after treatment with potato starch (RS2). The individual responses, however, were quite diverse.4 This figure illustrates the individual butyrate responses of each of the 20 subjects in the study:
Figure 1. Median butyrate concentrations for each individual before (triangles) and during consumption of RS (circles). Dotted and dashed lines denote the median values for butyrate before and during RS, respectively, for the entire study population. Reprinted from Venkataraman A, Sieber JR, Schmidt AW et al. Variable responses of human microbiomes to dietary supplementation with resistant starch. Microbiome 4, 33 (2016). https://doi.org/10.1186/s40168-016-0178-x [CC BY]
Notably, most people did see an increase in fecal butyrate. However, six of the 20 individuals had very little change, and the three individuals with the highest baseline butyrate levels (on the far-right side of the graph) actually saw a significant decrease in fecal butyrate levels.
A follow-up study revealed the individuals that these low butyrate responders tended to have a slight increase in Bifidobacteria with RS, but no changes in R. bromii.5 High butyrate responders, on the other hand, tended to have a significant increase in the abundance of R. bromii or Clostridium chartabidum. This was especially true when they were well-stocked with the butyrate producer Eubacterium rectale at baseline.
Another study using “humanized” mice supported these findings.6 Scientists transplanted four human gut microbiotas into germ-free mice. They then tracked the composition of the microbiota before and after feeding them SymbioIntest, an RS3 supplement manufactured in Germany, at 10 mg per day for four weeks. What did they find? Two of the donor microbiomes underwent substantial rearrangement upon RS3 feeding, while the other two did not. Only three of the four donor microbiomes had a substantial increase in butyrate production with RS3 feeding, with the fourth already having high butyrate production at baseline before RS3 supplementation.
Altogether, these data highlight the role of baseline gut microbiota composition in determining an individual’s response to resistant starch. In the future, advanced diagnostic tests may allow us to characterize an individual microbiome from a fecal sample and determine whether RS is likely to yield positive effects. Until then, we really can’t know whether any given individual will respond positively to RS supplementation, other than conducting an n=1 experiment.
Raw RS2: not so good for the gut?
That being said, I do think there are certain types of resistant starch that warrant caution when experimenting. One example is resistant starch type 2 (RS2). This is one of the most widely promoted “healthy” sources of resistant starch, often consumed as raw or unmodified potato starch.
Anecdotally, some people have reported benefits from using RS2, and some animal studies support potential benefits for gut health. However, I think that in most cases, the potential benefits of RS2 do not outweigh the potential risks.
Studies suggest caution with RS2
A number of studies suggest caution with RS2 may be warranted, especially in susceptible populations without robust gut health. In 2015, one research group looked at the effects of supplementing the diet of rural Malawian preschool-aged children with 8.5 grams/day of high amylose corn starch for four weeks. They found that RS2 increased Lactobacillus but decreased a number of prominent butyrate-producing taxa, including Roseburia, Blautia, Lachnospiraceae, Clostridium cluster XIVa, and Butyricoccus. It also significantly reduced microbial diversity and increased fecal calprotectin, a clinical marker of gut inflammation.7
Another study published in 2019 looked at the ability of the fecal microbiota from adults with cystic fibrosis (CF) to produce short-chain fatty acids from high amylose maize starch (HAMS).8 They found that the gut microbiota retained its ability to ferment these fibers, but that opportunistic bacteria filled the role normally played by beneficial bacteria. The authors write:
“In a subset of CF patients, the presence of HAMS leads to enterococcal overgrowth and the accumulation of lactate, suggesting that a prebiotic strategy would not be appropriate in all instances.” 8
Moreover, several studies in animal models of colorectal cancer have shown that isolated raw potato starch could promote tumor formation, especially in the absence of dietary soluble fiber.9,10 And even in healthy human adults, RS2 has also been shown to increase DNA adducts, a biomarker of colorectal cancer risk, in the colonic mucosa.11
Starch digestibility shapes the gut microbiota
Perhaps the most robust evidence, however, comes from a study published by Dr. Peter Turnbaugh’s laboratory in December 2019.12 In a series of experiments, they fed mice and humans raw or cooked meat and raw or cooked vegetables. They then looked at the effects on the gut microbiota composition and indices of gut health.
Surprisingly, they found no significant differences in the effects of cooked vs. raw meat on the gut microbiota. However, raw tuber consumption led to lower gut bacterial diversity than cooked tubers. The tuber they used? Sweet potatoes!
Manipulating the amount of isolated indigestible starch (RS2) in the diet recapitulated the effects of the raw or cooked tubers. The authors concluded that starch digestibility is a key factor in shaping the gut microbiota.
If this was true, we would expect to see stronger effects of cooking in starch-rich foods, especially those with a high quantity of low digestible starch, than in starch-poor foods. The researchers next tested an assortment of starch-rich vegetables with low digestibility (sweet potato and white potato) and higher digestibility (corn and pea) when raw, in addition to a few non-starchy vegetables (carrots and beets).
Indeed, the foods with the high quantity of low-digestibility starch (sweet potato and white potato) led to the most divergent gut microbiota signatures when consumed cooked vs. raw. Moreover, raw tubers increased markers of gut microbial cell damage and reduced bacterial activity to a similar degree as that seen with the antibiotic ampicillin! This effect was not seen with cooked tubers, or with any of the non-starchy vegetables, regardless of whether they were raw or cooked.
Evolutionary perspectives on raw starch consumption
Evolutionarily, this makes sense. Our microbes have evolved with us, throughout history and through shifts in hominid diets. The advent of cooking significantly increased the digestibility of starches and allowed for increased energy extraction of calories from food. (Some anthropologists have even proposed that the cooking of starchy tubers may have led to the evolution of the modern human brain.13)
It is entirely likely that cooking and the associated changes in our gut microbiome shaped us into the species we are today. Carmody & Turnbaugh et al. concluded:
“Because cooking is human-specific, ubiquitous and ancient, our results prompt the hypothesis that humans and our microbiomes co-evolved under unique cooking-related pressures.” 12
Once humans had the advent of cooking, they likely did not eat large quantities of uncooked or unprocessed tubers. Likewise, traditional agricultural societies in the last 10,000 years went to great lengths to soak, sprout, or ferment starchy grains, legumes, nuts, and seeds to make them more digestible.
Salivary amylase copy number and resistant starch degradation
Interestingly, a recent study published in Cell Host & Microbe suggests that copy number for salivary amylase is associated with the number of RS degraders in the gut microbiome.14 Salivary amylase is responsible for beginning the process of starch digestion in the mouth (digestible starches, that is, not resistant starch). A dietary shift in the Neolithic period is thought to have selected for duplications of the salivary amylase gene, leading to a greater capacity for starch digestion among populations with an agrarian background.15
The researchers recruited participants that varied in salivary amylase copy number and assessed their fecal microbiota when consuming a prescribed diet. Those who had a high number of amylase copies had higher abundance of the Ruminococcaceae family, which contains the primary RS degrader Ruminococcus bromii. Meanwhile, those with low salivary amylase copy number had a higher abundance of gut microbes involved in the degradation of digestible starch and other complex carbohydrates. This data further supports the idea of diet-driven changes in the gut microbiome in the last 10,000 years, and also may partially explain the individuality in responses to RS supplementation.
Getting resistant starch from whole, cooked foods (RS3)
Overall, I’d suggest caution with RS2. Sure, eat a slightly green banana now and then. But guzzling a daily drink of high-dose raw potato starch? We just don’t know enough to say it’s universally a good idea for everyone. The safer, and more evolutionarily familiar, option is to get resistant starch from whole, cooked foods. Cooked and cooled sweet potatoes, white potatoes, and white rice are simple and easy choices.
What about whole, cooked grains and legumes? I find it funny how many Paleo diet folks won’t hesitate to take high doses of raw potato starch but will turn their noses up at properly prepared whole legumes. I’ll have much more to say on the role of grains and legumes in the modern diet in a future article, but the short answer is that – when properly prepared – I think these can have a place in a healthy diet for many people.
If despite all of this, you still choose to supplement, an RS3 supplement is probably the best choice. RS supplements of any kind should always be taken with a meal containing other fibers, so as to spread fermentation along the entire length of the colon.
Go easy on the inflamed gut
If you can’t tolerate whole food sources of resistant starch and other fibers, your gut is probably not in a place where it can handle large quantities of isolated prebiotic fiber. In my experience, it’s usually best to slowly work on bringing down inflammation, supporting gut epithelial metabolism, addressing nutrient deficiencies, and then begin to diversify your diet to slowly include more whole food sources of prebiotic fibers, including resistant starch.
Pay attention to symptoms. Regardless of what is happening to the gut microbiota, it’s important to pay attention to how any dietary change impacts your system overall. The gut is a complex system, and we can’t rely on gut testing alone to determine whether things are moving in the right direction. If you introduce resistant starch into your diet and are doubled over in pain, that’s a definite sign to reduce your dose or back off.
Summary & takeaways
1) Resistant starch is a prebiotic fiber with many potential benefits to health. These include improved insulin sensitivity, reduced blood glucose response, greater satiety, and enhanced butyrate production.
2) However, responses to resistant starch supplementation are highly individualized. While resistant starch supplements tend to increase butyrate levels, responses are quite variable across the population. Currently, an n=1 experiment is the only way to know if resistant starch is right for you.
3) It’s probably best to avoid raw RS2, including isolated forms like raw potato starch and green banana flour. Several studies have suggested that raw RS2 may be damaging to the gut microbiota, even in healthy individuals, and may exacerbate dysbiosis or mucosal inflammation in diseased states.
4) Emphasize resistant starch and other fibers from whole, cooked plant foods. Cooked and cooled RS3 in the form of whole foods is much more evolutionarily familiar than raw, isolated RS2. If you choose to incorporate cooked grains and legumes for their resistant starch content, be sure to prepare them properly.
5) Take it slow if you’re dealing with ongoing inflammation, bloating, or abdominal pain. You may need to bring down inflammation and actually reduce fermentation to support the gut barrier before introducing significant amounts of resistant starch and other prebiotic fibers.
What do you make of this research? How do you incorporate resistant starch into your diet? Be sure to share in the comments below!