Resistant starch

Resistant starch could explain the better health and low cancer rates in traditional cultures where carbohydrate-rich foods form the basis of the diet. However, some people also overdo it with resistant starch and experience unpleasant side effects.

What is resistant starch?

Resistant starches are starches that are resistant to digestion in the small intestine. For this reason, they survive digestion intact and are not absorbed in the small intestine. Instead, they are fermented by the intestinal bacteria in the large intestine (97, 98, 99). In contrast to digestible, non-resistant starches, resistant starches do not increase blood sugar levels.

The undigested starch is used as food in the small intestine by good gut bacteria, which in turn produce butyrate and vitamin K2, both of which have numerous health benefits in the body. In addition, resistant starch supports the proliferation of friendly bacteria in the gut, which has benefits for gut and immune health.

Types of starch

There are two types of starch based on the branched nature of their chemical structure: amylose and amylopectin.

Digestible vs. resistant starch

Starches are also categorized based on their digestibility and glycemic index (the rate at which starch raises blood sugar levels):

Fast digesting starch - causes a rapid rise in blood sugar levels after consumption. Fast-digesting starches are found in white bread and sugary breakfast cereals.
Slowly digestible starch - is fully digested in the small intestine, but at a slower rate than rapidly digestible starch. Pasta, brown rice, rye, oats and wholemeal bread contain slowly digestible starch.
Resistant starch - is not absorbed in the small intestine and is fermented in the large intestine.

Sources of resistant starch

Resistant starch has a low glycemic index, which means that it has little effect on blood glucose levels (100, 101). Some food sources of resistant starch contain a combination of resistant and non-resistant starch, which means that the actual glycemic index of these foods can vary. There are 5 different types of resistant starch, categorized by their sources:

Type 1 resistant starch - starch that is physically unavailable for digestion because it is trapped in the fibrous walls of plants. Type 1 resistant starch is found in coarsely ground cereals or whole grains, whole seeds and whole legumes (beans, nuts, peas and lentils).
Resistant starch type 2 - non-gelatinized starch has a high amylose content. Amylose is a type of starch with a linear structure that can be more easily packed tightly and crystallized into a structure that prevents digestion. Type 2 resistant starch is indigestible when raw. Type 2 resistant starch is found in starchy fruit (green bananas), raw vegetables (potatoes) and amylose-rich starch (corn starch).
Type 3 resistant starch - is starch produced when type 1 and type 2 are cooked and then cooled. Type 3 resistant starch can be reheated at low temperatures to prevent the starch from becoming digestible again. Type 3 resistant starch is found in bread, pasta, rice and potatoes. Cooked potatoes and banana starch lose their resistance to digestion, but cooked amylose-rich corn starch maintains its resistance to digestion (102).
Type 4 resistant starch - is starch that has been chemically modified (esterified starch) to resist digestion. Type 4 resistant starch is chemically modified starch found in processed foods such as bread and crackers. Examples include Hi-Maize® starch, cross-linked starch, starch esters and ethers and cyclodextrins (103, 102, 101). - Type 5 resistant starch - is starch with amylopectin (a type of starch with a non-linear branching structure) that has been heated with oil to form a helical structure that makes it resistant to digestion (104).

All types of resistant starch have health benefits, but they have different effects on the body (105). For example, types 1, 2 and 4 are good for lowering blood sugar levels, while types 2 and 3 are good for weight and fat loss (106, 102, 107).

Cooking methods that can increase type 3 resistant starch content

1. cooking and cooling carbohydrates

When foods containing resistant starch are cooked, the starch loses its resistance to digestion. However, there are cooking methods that maintain the resistant starch content. When starch rich in amylose is heated in water, the starch particles absorb water and swell. After the cooked starch has cooled down again, the starch (amylose) molecules rearrange their structures (they crystallize) and become less digestible. This process is known as retrogradation.

Cooked and cooled starchy foods with resistant starch can be reheated at low temperatures (below 80 degrees) to maintain their resistant starch content (108).

2. baked and cooled potatoes vs. boiled potatoes

Baking does not degrade the starch as much as boiling does. Chilled potatoes (4 degrees) contain more resistant starch than hot potatoes (65 degrees) or reheated potatoes (stored for 6 days at 4 degrees and reheated to 65 degrees). Chilled potatoes contain retrograded starch, which is less digestible than cooked starch (109).

3. cooked and cooled rice

Steaming, pressure cooking and sautéing rice produces higher amounts of resistant starch than boiling. Cooling the rice after cooking increases the resistant starch content.

Supplement sources for resistant starch

In addition to consuming starchy fruits and vegetables and the foods mentioned above, an easy way to include resistant starch in the diet is to add resistant starch to foods.

Figure 1: Source: (112)

1. raw potato starch

Raw potato starch contains by far the highest amount of resistant starch and has the lowest glycemic index. Maize has the next highest proportion of resistant starch, followed by tapioca, wheat and rice (100).

2. hi-maize®

Hi-Maize is an amylose-rich resistant starch that has been treated with heat and moisture to significantly increase the resistant starch content (102). For people who are sensitive to plant-based starches, Hi-Maize is a good hypoallergenic alternative. It is not derived from a nightshade plant and the processing process destroys most plant-based immunostimulants. However, Hi-Maize should be avoided if you have tested allergic to corn.

Hi-Maize has a uniquely high resistant starch and fiber content. Hi-Maize starch consists of 50% resistant starch. One serving (11 grams) contains 7 grams of dietary fiber. Unlike other natural sources of resistant starch, Hi-Maize has a high gelling temperature, which ensures that this starch can withstand most normal food processing steps (113, 114). Hi-Maize is used in a range of processed foods that include low-fat snacks, high-fiber bread, pasta, breakfast cereals and gluten-free products (114).

Health benefits of resistant starch and the underlying mechanisms

The health benefits of resistant starch come from many properties of this particular type of starch, including the following:

By acting as a dietary fiber, resistant starch slows the digestion and absorption of food and nutrients in the small intestine and increases stool volume in the large intestine (1). - Resistant starch serves as food for healthy intestinal bacteria such as bifidobacteria in the large intestine (1).
Resistant starch can reduce insulin resistance caused by inflammation (2)
Resistant starch provides the raw material for the production of short-chain fatty acids (butyrate, acetate, propionate) and other beneficial metabolites in the colon (3).
Resistant starch stimulates fat burning and glycogen storage instead of fat storage (2).
Short-chain fatty acids support the function of the intestinal barrier (i.e. they help to repair a permeable intestinal wall) and the release of hormones and enzymes in the digestive tract (3).

Resistant starch and metabolic health

Resistant starch can lower blood sugar levels after meals

There are many ways in which resistant starch helps to normalize blood sugar levels. These include:

By acting like a dietary fiber, resistant starch slows down carbohydrate digestion and absorption (1).
By activating glycogen synthesis genes, resistant starch causes the body to store more carbohydrates in the muscles and liver (in rats) (4).
Resistant starch can reduce insulin resistance (5).

Supplementation with resistant corn starch has been shown in studies to help control blood glucose levels in overweight but otherwise healthy individuals (6). One study found that daily consumption of amylose-rich resistant corn starch over a 6-week period improved glucose balance in 18 overweight adults. Glucose balance is the process of maintaining normal blood glucose levels (7).

Resistant starch can improve insulin resistance

Insulin resistance occurs when cells no longer respond to insulin, resulting in high blood glucose levels. Insulin resistance is associated with a higher risk of developing type 2 diabetes and heart disease. Consumption of resistant starch improves insulin sensitivity and reduces the amount of insulin needed to control blood glucose levels in animals and humans. Resistant starch may improve insulin sensitivity in several ways:

An increase in the secretion of certain bile acids into the digestive tract, which helps improve insulin resistance via GLP-1 (8, 9).
A reduction in adipose tissue macrophages - immune cells that drive the development of insulin resistance (5, 3, 10, 11, 12).
Short-chain fatty acids (fermentation products of resistant starch) signal the brain and liver to reduce glucose production, which can improve insulin sensitivity (13).
An increase in adiponectin levels, which improves insulin sensitivity by increasing fatty acid oxidation and inhibiting glucose production in the liver (14).
An increase in ghrelin levels, which inhibits glucose-stimulated insulin secretion from the pancreas (15).

Resistant starch can help treat metabolic syndrome

Metabolic syndrome - also known as prediabetes - is a group of factors that increase the risk of heart disease, diabetes and stroke. These risk factors include a large waist circumference, low HDL cholesterol levels, high blood pressure, high triglyceride levels and high blood sugar levels (16). In a study of 20 adults, resistant starch reduced the amount of insulin needed after food intake, which helped treat metabolic syndrome (17). The addition of resistant starch to the diet of patients with metabolic syndrome improved cholesterol levels, triglyceride levels and insulin sensitivity (18, 10).

Resistant starch could help in the treatment of type 2 diabetes

Type 2 diabetes occurs when a person develops insulin resistance. It is caused by genetic predisposition, obesity, high blood sugar levels and inflammation. Resistant starch can potentially reduce the risk of developing type 2 diabetes in animals and overweight adults by improving insulin sensitivity, lowering blood glucose levels and reducing blood lipid levels (19, 6, 12). Supplementation with resistant starch could also prevent complications that can result from high blood glucose levels in type 2 diabetics (20). A study conducted on 56 women with type 2 diabetes found that resistant starch improved blood glucose levels, reduced the release of toxins by bacteria and increased antioxidant levels (21). Short-chain fatty acids increase levels of glucagon-like peptide-1 (GLP-1) - a hormone that lowers blood glucose levels by stimulating insulin secretion. GLP-1 may treat diabetes by lowering blood glucose levels (22, 23).

Resistant starch helps lower blood triglyceride levels

A study conducted in animals found that resistant starch interferes with the absorption of dietary fats, which can prevent an increase in blood triglyceride levels after a meal. Resistant starch also improved the movement of food pulp in the digestive tract (24). This has been confirmed in both humans and rats, in which resistant starch reduced triglyceride levels after meals (25).

Resistant starch can improve cholesterol levels

The addition of resistant starch to bread significantly reduced total cholesterol levels in rats (1, 25).

Resistant starch reduced LDL cholesterol levels and total cholesterol levels in humans and pigs (26, 27).

Resistant starch may help prevent heart disease

Including resistant starch in the diet could improve heart health by lowering cholesterol levels (28).

In a double-blind study of 86 subjects, type 4 resistant starch reduced abnormal fat levels in the blood. Thus, intake of this starch could promote heart health (18). Hardening of the 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 (6).

Beans rich in resistant starch reduce cholesterol levels and lower the risk of heart disease and diabetes (29).

Resistant starch slows the progression of chronic kidney disease

Chronic kidney disease is often a complication of heart disease and diabetes (30). A diet rich in amylose-rich corn starch can slow the progression of chronic kidney disease in rats by reducing oxidative stress, reducing inflammation and preventing damage to the intestinal wall (31). Supplementing the diet with resistant starch reduced the levels of toxic metabolites (indoxyl sulfate and P-cresol sulfate) in 56 patients with chronic kidney disease who were on dialysis (32).

Resistant starch helps with weight loss and weight maintenance

Resistant starch promotes a healthy energy balance and prevents weight gain In obesity-prone rats, resistant starch and regular exercise prevented weight gain by reducing the energy gap between the urge to eat and reduced energy needs (33). Resistant starch reduces fat accumulation and blood glucose levels and increases the breakdown of fat through fermentation in the gut, which may help with obesity (34, 35).

Resistant starch helps burn fat

Resistant starch helps burn fat through the following mechanisms:

A reduction in fat accumulation and an increase in fat oxidation after meals (in both rats and humans) (25).
Resistant starch forces the body to burn fat for energy by inhibiting glucose utilization in humans (36).
A reduction in fat production in the body while increasing the production of phospholipids, which are the main component of cell membranes (37).

Resistant starch reduces appetite

The consumption of resistant starch increases the levels of the appetite-reducing hormone peptide YY (PYY), which promotes the feeling of satiety (38, 39, 40,41). A study conducted with 20 healthy adults found that the consumption of resistant starch over a period of 24 hours significantly reduced the amount of food eaten. Even when food intake was lower, there was no link between food consumption and the subjects' assessment of their appetite (17).

Resistant starch and the health of the digestive tract

Resistant starch acts as a prebiotic

Prebiotics stimulate the growth of beneficial intestinal bacteria (probiotics). By increasing the number of beneficial bacteria in the colon, resistant starch has several health benefits, including improved immune function, prevention of the growth of harmful bacteria, normalization of energy production and even a lower risk of cancer (42, 43, 44, 45).

Changes in Bacteria phyla and their species due to the consumption of resistant starch in humans.

Figure 1: Bacteria whose numbers were affected by the consumption of resistant starch, based on stool samples from 10 volunteers (45).

Type 4 resistant starch reduced Firmicutes (p<0.001) on average by more than 10% and increased the number of Bacteroidetes (p<0.01) and Actinobacteria (p<0.05) by about 5% each, while the amount of Bifidobacteria was also significantly increased (46). The reduction of Firmicutes and the increase of Bacteroidetes is associated with thinness (47). However, this study also observed a strong individual variance in gut flora changes in response to resistant starch consumption in the 10 subjects.

Resistant starch can increase acidity in the digestive tract

In animal studies, fermentation of resistant starch produced short-chain fatty acids, which increase the acidity of the digestive tract (48, 41). Higher acidity improves the absorption of minerals, promotes the growth of beneficial bacteria and suppresses the growth of harmful bacteria (49, 50, 51).

Resistant starch improves the function of the digestive tract

The short-chain fatty acid butyrate (a fermentation product of resistant starch) provides energy to the cells of the intestine and is essential for promoting colon function. Populations at low risk for intestinal disease are characterized by high consumption of resistant starch and high butyrate levels (42). Resistant potato starch increases butyrate concentrations in the intestine, which has a positive effect on the intestinal flora. Butyrate also protects the intestinal mucosa and reduces inflammation (52).

Resistant starch helps in the treatment of diarrhea

The consumption of resistant starch helps in the treatment of infectious diarrhea in humans and animals by reducing the amount of harmful bacteria in the intestine (53). Resistant starch increases the concentration of short-chain fatty acids, which can improve the treatment of acute diarrhea in children under 5 years of age (54).

Resistant starch can help with autoimmune diseases

Resistant starch helps with a pathologically permeable intestinal wall

Permeability of the intestinal wall resulting from damage to the intestinal barrier impairs immune function and increases the risk of other diseases such as inflammatory bowel disease, celiac disease, autoimmune hepatitis, and type 1 diabetes (55). Intestinal bacteria break down resistant starch into short-chain fatty acids (which are important nutrients for the cells of the intestinal wall), which reduce disruption in the narrow passageways of the cells lining the intestine. The intestinal wall is a protective barrier that is permeable to nutrients but blocks harmful pathogens (56, 57).

Resistant starch reduces inflammation and keeps the immune system in balance

The inner lining of the gut is influenced by gut bacteria and the immune system, which has an impact on the barrier function, the composition of the gut flora and the balance of the immune system (58).

Resistant starch helps to keep the immune system in balance via different mechanisms:

Resistant starch can alter the composition of gut bacteria (59, 60).
Resistant starch can stimulate Treg, which reduces Th1, Th2 and TH17 dominance (61).
Resistant starch can suppress pro-inflammatory cytokines such as IGN-gamma by increasing butyrate levels, increase levels of the anti-inflammatory cytokine IL-10 and increase PPAR-gamma levels (61, 62).

Resistant starch can reduce oxidative stress

Inflammation and autoimmune diseases increase oxidative stress. Unmitigated oxidative stress and low levels of antioxidant enzymes can exacerbate autoimmune diseases (63). In rats with chronic kidney disease, resistant starch reduced oxidative stress by increasing the activity of antioxidant genes including Nrf2, SOD and glutathione peroxidase (64). In diabetics, consumption of resistant starch also protected blood vessels from oxidative damage due to high blood glucose levels (65). Resistant starch increases the amount of Lactobacillus bacteria in the gut, which produce antioxidants. For this reason, supplementing the diet with resistant starch could reduce oxidative stress and improve the function of the inner lining of blood vessels in patients with type 2 diabetes (66, 21).

Resistant starch may help in the treatment of inflammatory bowel disease

Resistant starch reduces the severity of inflammatory bowel disease in animals (67). Isomaltodextrin, a type of resistant starch, reduces inflammation in intestinal diseases by suppressing inflammatory proteins and increasing levels of anti-inflammatory proteins (68).

Dietary fibers such as resistant starch modify the intestinal flora and increase the production of short-chain fatty acids, which reduce inflammation in inflammatory bowel disease (69).

Resistant starch promotes nutrient absorption and availability

Resistant starch helps in the treatment of vitamin D deficiency

In rats with type 1 diabetes, consumption of resistant starch helped treat vitamin D imbalance by improving nutrient and vitamin reabsorption in the kidneys (megalin-mediated endocytosis) (70, 71).

Resistant starch helped to achieve vitamin D balance in rats with type 2 diabetes by preventing urinary excretion of vitamin D-binding proteins(71).

Resistant starch can increase mineral absorption

Raw resistant starch (but not retrogarded types) has been shown to aid magnesium absorption (72, 73).

In pigs, a diet containing 16% resistant starch significantly increased intestinal absorption of calcium and iron (74).

Resistant starch could protect against cancer

Resistant starch can reduce the negative effects of a meat-rich diet Red and processed meats contain large amounts of heme iron, which can increase levels of nitrogen compounds that can damage DNA in the gut, thin the intestinal lining and cause cell death (75). In addition, high-protein diets can result in higher IFG-1 levels, which can promote tumor growth (76).

Resistant starch can mitigate these effects via the following mechanisms (77):

Resistant starch can protect DNA from damage caused by nitrogen compounds.
Resistant starch can reduce MMP activity by increasing TIMP enzyme levels (an enzyme that inhibits MMP), thereby preventing cancer growth and metastasis.
Resistant starch can reduce inflammation.
Resistant starch can reduce insulin and IGF-1 levels (although these effects were not statistically significant).

The health benefits of resistant starch appeared to be based on butyrate production and an increase in Lactobacillus levels (78).

Resistant starch could prevent and help treat colorectal cancer

Resistant starch may reduce the risk of colorectal cancer by protecting the cells of the inner lining of the colon and reducing the risk of tumor growth (79, 80, 81). Resistant starch can prevent the development and progression of colorectal cancer (77). In rats with colorectal cancer, resistant starch increased butyrate concentrations and was associated with a reduction in tumor incidence, number and size (82). In rats, resistant starch protected the cells of the inner lining of the intestine from DNA damage and reduced the levels of carcinogenic toxins in the intestine (83). In addition, a combination of resistant starch and the bacterium Bifidobacterium lactis protected rats from developing colorectal cancer (84).

Resistant starch could support the treatment of pancreatic cancer

A diet rich in resistant starch has been associated with a change in gut flora and a slowdown in the growth of pancreatic cancer tumors in mice (85).

Resistant starch can affect brain function

Resistant starch exerts some of its health benefits through the gut-brain axis, as the brain influences gut bacteria and vice versa. In addition, butyrate is a substance that is absorbed through the digestive tract and crosses the blood-brain barrier (86). For this reason, resistant starch could support brain function. However, most research on this topic has been conducted with animals.

Resistant starch could increase dopamine levels and protect dopaminergic neurons

Since resistant starch increases butyrate concentrations, it could indirectly increase dopamine levels by reducing the degeneration of dopamine-producing neurons (87, 88). This suggests that resistant starch and butyrate may help with diseases such as Parkinson's that affect dopaminergic neurons.

Other effects of resistant starch on brain function

Mice fed HA-7 resistant starch showed increases in anxious behavior (89). However, these mice had significantly lower cortisol levels compared to mice fed normal corn starch. Resistant starch also made the brain more reactive to glucose and improved motor control in rats (90).

Limitations and negative effects of resistant starch

Effects may vary from person to person

The effects of resistant starch depend on the composition of the intestinal flora, which varies from person to person. Treatment with resistant starch should be personalized to maximize the positive effects (91, 92, 93).

The effects depend on the type of resistant starch

Type 2 and type 4 resistant starch affect different parts of the gut flora. Specific bacterial populations can be targeted depending on the type of resistant starch (45). One study found that type 4 resistant starch lowered blood glucose levels more than type 2 resistant starch (94). Another study found that type 4 resistant starch was better than type 2 resistant starch at reducing oxidative stress levels (95).

Resistant starch can cause stomach discomfort

Foods such as beans, which are rich in resistant starch, cannot be fully digested and can cause digestive discomfort (29). Consuming excessive amounts of indigestible carbohydrates such as resistant starch can cause stomach discomfort, bloating and diarrhea (96).