Eating pure starch may not seem like the healthiest choice. However, resistant starch is an exception: it feeds gut probiotics, balances blood sugar, improves metabolism, and more. Read on to learn the health benefits and side effects of resistant starch. If you are interested in hacking your gut microbiome, this post is a must-read.
Potential health benefits of resistant starch stem from the following properties:
- By acting as a dietary fiber, resistant starch slows down digestion and absorption in the small intestine and bulks up the stool in the large intestine 
- By feeding good bacteria like Bifidobacteria in the large intestine 
- By reducing insulin resistance & inflammation 
- By supporting the production of short-chain fatty acids (butyrate, acetate, propionate) and other beneficial metabolites in the large intestine 
- Short-chain fatty acids support intestinal barrier function (i.e., help repair leaky gut), and healthy secretions of hormones and enzymes in the gut 
One study found that consuming high-amylose maize resistant starch daily for six weeks improved glucose balance in 18 overweight adults. Glucose balance is the process of maintaining normal blood glucose levels .
According to preliminary research, there are many ways in which resistant starch may help normalize blood glucose, including:
- Being dietary fiber, it slows down carbohydrate digestion and absorption .
- By activating glycogen synthesis genes, it causes the body to store more carbohydrates in our muscles and liver .
- By improving insulin sensitivity .
Insulin resistance occurs when cells fail to respond to insulin, leading to high blood sugar levels, and it is associated with a higher risk of developing type 2 diabetes and heart disease.
Resistant starch intake may improve insulin sensitivity and reduce the amount of insulin required to manage blood sugar in both animals and humans. It may help by:
- Increasing the excretion of certain bile acids into the gut, which helps decrease insulin resistance through GLP-1 [8, 9].
- Reducing fat tissue macrophages, which are immune cells that drive the development of insulin resistance [7, 3, 10, 11, 12].
- Short-chain fatty acids (fermentation products of resistant starch) signal to the brain and liver to reduce glucose production, which may improve insulin sensitivity .
- Increasing adiponectin, which improves insulin sensitivity by increasing fatty acid oxidation and inhibiting liver glucose production .
- Increasing ghrelin, which inhibits glucose-stimulated insulin secretion from the pancreas .
Resistant starch potentially reduces the risk of developing type 2 diabetes in animals and overweight adults by improving insulin sensitivity, reducing blood glucose, and reducing blood fat levels [16, 4, 12].
Supplementing the diet with resistant starch may prevent complications resulting from excess blood sugar in patients with type 2 diabetes. One study of 56 women with type 2 diabetes found that resistant starch improved blood glucose levels, reduced toxins released by bacteria, and increased antioxidants [17, 18].
Metabolic syndrome is a group of factors that increase the risk of heart disease, diabetes, and stroke. These include large waistline, low HDL cholesterol, high blood pressure, high triglycerides, and blood sugar levels .
In one study of 20 healthy adults, resistant starch decreased the amount of insulin released after food intake, which makes it a promising complementary approach to metabolic syndrome .
In a double-blind study of 86 individuals, resistant starch type 4 reduced abnormal fat levels in the blood .
The hardening of blood vessels is often a precursor to heart disease. Resistant starch potentially reduces the risk factors involved in the hardening of blood vessels in overweight individuals .
Supplementing the diet with resistant starch decreased toxic metabolite (indoxyl sulfate and p-cresol sulfate) levels in 56 patients on kidney dialysis .
Please note that the effects of resistant starch on weight control stem from preliminary low-quality research. Although it may theoretically support metabolism and weight control in multiple ways, solid clinical evidence is lacking.
Resistant starch may stimulate fat burning by:
- Reducing fat accumulation and increasing fat oxidation after meals .
- Forcing the body to burn fat by lowering blood glucose .
- Decreasing fat production while increasing the production of phospholipids .
A study of 20 healthy adults found that consuming resistant starch over a 24-hour period significantly reduced the amount of food eaten. Although food intake was lower, there was no association between food consumption and how subjects rated their appetite .
Prebiotics stimulate the growth of beneficial or probiotic gut bacteria.
By increasing the number of good bacteria in the large intestine, resistant starches may offer several health benefits, such as improving immune function, preventing the growth of bad bacteria, and normalizing energy production [37, 38, 39, 40].
Technical: Gut bacterial composition changes from the consumption of resistant starch in humans.
RS4 decreases Firmicutes by more than 10% on average and increases Bacteroidetes and Actinobacteria by around 5% each, while also significantly increasing Bifidobacterium .
However, this study also found a large individual variation in the changes in gut flora in response to resistant starch consumption among the 10 subjects.
The short-chain fatty acid butyrate (fermentation product of resistant starch) provides energy for colon cells and is essential for promoting large bowel function. Populations at low risk of large bowel diseases have high dietary resistant starch intake and high butyrate levels .
Consuming resistant starch helps treat infectious diarrhea in both humans and animals by reducing harmful bacteria in the gut .
Resistant starch increases short-chain fatty acid concentrations, which improves the treatment of acute diarrhea in children under 5 years old .
Intestinal permeability, which results from a damaged intestinal barrier, affects the immune function and puts individuals at risk for other diseases like inflammatory bowel disease, celiac disease, autoimmune hepatitis, and type 1 diabetes .
Intestinal microbiota breaks down resistant starch into short-chain fatty acids, which strengthen the colon lining: a protective barrier that allows nutrients into the colon while keeping harmful pathogens out [50, 51].
The intestinal lining is affected by intestinal microbiota and the immune system, affecting barrier function, the composition of the microbiota, and immune balance .
Resistant starch helps balance the immune system by:
- Altering the composition of the gut bacteria [53, 54]
- Stimulating Treg, which lowers Th1, Th2, and Th17 dominance 
- Suppressing inflammatory cytokines such as IFN-gamma, increasing anti-inflammatory cytokine IL-10, and increasing PPAR-gamma through the increase in butyrate levels [55, 56]
Isomaltodextrin, a type of resistant starch, reduces inflammation in gut diseases by suppressing pro-inflammatory and increasing anti-inflammatory proteins .
Dietary fiber like resistant starch modifies the gut microbiota and increases short-chain fatty acid production, which reduces inflammation in IBD .
Despite the potential of resistant starch to combat gut inflammation in multiple ways, we can’t make definite conclusions in the absence of clinical trials in IBD patients.
Resistant starch and other dietary fiber have a well-known protective role against colon cancer. They protect colon lining cells and suppress tumor growth by increasing short-chain fatty acids and supporting gut probiotics [60, 61, 62].
In rats with colorectal cancer, resistant starch increased butyrate concentrations and was associated with reduced tumor incidence, number, and size .
In another study on rats, it protected colon lining cells from DNA damage and reduced cancer-causing toxins in the gut .
Clinical trials are needed to investigate the potential use of resistant starch in colon cancer prevention. At this point, it can’t be recommended for cancer prevention or treatment.
Resistant starch maintains vitamin D balance in rats with type 2 diabetes by preventing urinary excretion of vitamin D-binding proteins .
Resistant starch exerts some of its health benefits through the so-called gut-brain axis. For example, butyrate is absorbed through the stomach, crosses the blood-brain barrier, and exerts beneficial effects on the brain .
Resistant starch may thus help with brain function, but most research in this area is done on animals.
Resistant starch made the brain more responsive to glucose and improved motor coordination in rats .
RS types 2 and 4 influence different parts of the microbiota. Specific bacterial populations can be targeted depending on the type of resistant starch .
One study found that RS type 4 decreased blood glucose more than resistant starch type 2 .
Another study found that RS type 4 was better at reducing oxidative stress levels than type 2 .
Foods high in RS, like beans, cannot be fully digested and can cause stomach discomfort .
Consuming too many non-digestible carbohydrates like resistant starch may cause stomach discomfort, gas, and diarrhea .