I asked Dr. Amy Nett, MD, to contribute this guest post. Amy initially completed her medical training in radiology at Stanford University Hospital, but wanted to work more directly with patients, helping them to prevent and reverse chronic disease and truly transform their health. Combined with her passion for nutrition she decided to pursue a career in Functional Medicine. You’ll be hearing more from Amy in the future!

Over the past several years there has been an exponential increase in the number of studies linking imbalances or disturbances of the gut microbiota to a wide range of diseases including obesity, inflammatory bowel diseases, depression and anxiety (1,2,3,4,5). One of the best ways to establish and support a healthy gut microbiome is by providing the right “foods” for your gut bacteria. These “foods” are called prebiotics.

Why you should add resistant starch to your diet.

Prebiotics are indigestible carbohydrates, or at least indigestible to us, that reach the colon intact and selectively feed many strains of beneficial bacteria. Prebiotics are generally classified into three different types: non-starch polysaccharides (such as inulin and fructooligosaccharide), soluble fiber (including psyllium husk and acacia fibers), and resistant starch (RS). Each of these types of prebiotics feeds different species of gut bacteria, but among these, RS is emerging as uniquely beneficial.

The distinctive benefits of RS seem to be unequivocally recognized, even amongst advocates of a low carbohydrate diet.

What Is Resistant Starch?

Resistant starch is a type of starch that is not digested in the stomach or small intestine, reaching the colon intact. Thus, it “resists” digestion. This explains why we do not see spikes in either blood glucose or insulin after eating RS, and why we do not obtain significant calories from RS.

There are four types of resistant starch:

RS Type 1: Starch is physically inaccessible, bound within the fibrous cell walls of plants. This is found in grains, seeds, and legumes.

RS Type 2: Starch with a high amylose content, which is indigestible in the raw state. This is found in potatoes, green (unripe) bananas, and plantains. Cooking these foods causes changes in the starch making it digestible to us, and removing the resistant starch.

RS Type 3: Also called retrograde RS since this type of RS forms after Type 1 or Type 2 RS is cooked and then cooled. These cooked and cooled foods can be reheated at low temperatures, less than 130 degrees and maintain the benefits of RS (6). Heating at higher temperatures will again convert the starch into a form that is digestible to us rather than “feeding” our gut bacteria. Examples include cooked and cooled parboiled rice, cooked and cooled potatoes, and cooked and cooled properly prepared (soaked or sprouted) legumes.

RS Type 4: This is a synthetic form of RS that I’m including for completeness, but would not recommend. A common example is “hi-maize resistant starch.”

Once RS reaches the large intestine, bacteria attach to and digest, or ferment, the starch. This is when we receive the benefits of RS.

How Resistant Starch Impacts Our Health

The normal human gut has hundreds of bacterial species, some good and some not so good. The overall number and relative quantity of each type has a profound effect on our health and well being. Resistant starch selectively stimulates the good bacteria in our intestines, helping to maintain a healthy balance of bacteria (7).

These good bacteria “feed” on RS and produce short chain fatty acids (through fermentation), the most significant of which are acetate, butyrate, and propionate. Of these three short chain fatty acids (SCFA), butyrate is of particular importance due to its beneficial effects on the colon and overall health, and RS appears to increase butyrate production more when compared with other soluble fibers (8).

Butyrate is the preferred energy source of the cells lining the colon, and it also plays a number of roles in increasing metabolism, decreasing inflammation and improving stress resistance, as described in more detail below and previously in this great article by Stephan Guyenet.

Resistant Starch Helps to Lower Blood Glucose Levels and Improve Insulin Sensitivity

Insulin resistance and chronically elevated blood glucose are associated with a host of chronic diseases, including metabolic syndrome. Several studies have shown that RS may improve insulin sensitivity, and decrease blood glucose levels in response to meals (10, 11, 12). In one study, consumption of 15 and 30 grams per day of resistant starch showed improved insulin sensitivity in overweight and obese men, equivalent to the improvement that would be expected with weight loss equal to approximately 10% of body weight (13).

Further, RS has been shown to exert a “second meal effect.” This means that not only does RS beneficially decrease the blood glucose response at the time it’s consumed, but, somewhat surprisingly, blood glucose and insulin levels also rise less than would otherwise be expected with the subsequent meal (14).

Why the Popular Press Has Touted Resistant Starch as a “Weight Loss Wonder Food”

RS appears to have several beneficial effects that may contribute to weight loss, including decreased blood insulin spikes after meals (as discussed above), decreased appetite, and decreased fat storage in fat cells. There may also be preservation of lean body mass, though further studies in humans are needed to confirm if there is a significant impact in overall body weight (15).

Further, several studies have shown alterations in the gut microbiome in association with obesity, which subsequently change towards that seen in lean individuals with weight loss (16, 17). For example, one study demonstrated that the relative composition of the gut microbiota of two predominate beneficial bacteria, Bacteroidetes and Firmicutes, varied considerably in association with body composition.

Specifically, obese individuals often have a higher proportion of Firmicutes to Bacteroidetes, which may be reversed with weight loss, gastric bypass surgery, or treatment with prebiotics (3). However, not all studies confirm a significant or measurable change in the composition of the microbiome in obese compared to lean individuals, and further studies are needed (18, 19).

Butyrate Plays an Important Role in Gut Health and Decreasing Inflammation in the Gut and Other Tissues

As mentioned above, RS intake allows for increased production of butyrate by our gut microbes. Butyrate acts as a powerful anti-inflammatory agent for the colonic cells, and functions to improve the integrity of our gut by decreasing intestinal permeability and therefore keeping toxins in the gut and out of the bloodstream. (20, 21).

The SCFAs that aren’t utilized by the colonic cells enter the bloodstream, travel to the liver, and spread throughout the body where they exert additional anti-inflammatory effects.

Resistant starch is also associated with decreased risk of colorectal cancer, thought to occur through several different mechanisms including: protection from DNA damage, favorable changes in gene expression, and increased apoptosis (programmed cell death) of cancerous or pre-cancerous cells (22, 23).

Adding Resistant Starch to Your Diet

Some common food sources of RS include green (unripe) bananas, plantains, properly prepared cooked and cooled parboiled rice or legumes, and cooked and cooled potatoes. See this link for a more complete list of RS quantities in food.

However, if you are on a low carbohydrate diet or don’t tolerate those foods well, you can add RS to your diet without adding digestible carbohydrates.

Bob’s Red Mill Unmodified Potato Starch (NOT potato flour) is one of the best sources of RS with approximately eight grams of RS in one tablespoon. Potato starch is generally well tolerated even by those who react adversely to nightshades.

Plantain flour and green banana flour are also excellent sources of RS, and there may be benefit to including all three of these sources (specifically alternating your source of RS rather than relying on a single one).

These are relatively bland in flavor and can be added to cold or room temperature water, almond milk, or mixed into smoothies. But to maintain the benefits of RS, these should not be heated above 130 degrees.

Tim Steele (Tatertot) has written about some of the research on RS supplementation, and in particular the potential further benefit of combining potato starch with psyllium husk fiber to even further increase butyrate production in the colon.

Take It Slow

If you choose to try supplementing with RS, start with small doses of about ¼ teaspoon once daily, and very gradually increase the amount as tolerated. Some increased gas and bloating is expected as your gut flora changes and adapts, but you do not want to feel uncomfortable. If you experience marked discomfort, then decrease the amount you’re taking for a few days until your symptoms resolve, and then try increasing again gradually.

Studies indicate that the benefits of resistant starch may be seen when consuming around 15 to 30 grams daily (equivalent to two to four tablespoons of potato starch). This may be too much for some people to tolerate, particularly in the setting of gut dysbiosis, and going above this amount is not necessarily beneficial.

If you experience marked GI distress with even small amounts of RS, this may be an indication of SIBO (small intestinal bacterial overgrowth) or microbial dysbiosis, and you may need to consider working with a healthcare practitioner to establish a more balanced gut microbiome through the use of herbal antimicrobials and probiotics before adding RS or other prebiotics.