What is resistant starch (RS)?
Resistant starch is still one of the new kids on the block when it comes to nutrition. Until the early 1980s, no-one knew what it was – it was assumed that all starches were digestible.(1) Yet it’s name should make it’s defining characteristic obvious: resistant starch resists digestion in the small intestine. Instead, it makes its way to the colon, where it undergoes bacterial fermentation, producing short-chain fatty acids (1–4). There are four categorisations of RS. RS1 is found in whole grains or pulses, and is “entrapped in a non-digestible matrix”.(5,6) RS2 is found in foods such as raw potato and green bananas; RS2 is indigestible due to ungelatinised starch granules.(6) RS3 is found in starchy foods that have been cooked and then cooled – a process called “retrogradation”.(6) Examples of this are cooked and cooled rice, pasta, potato and other root vegetables, and even sourdough bread (particularly whole grain).(7) Finally, RS4 is an industrial form of resistant starch, made by altering the chemical bonds in the starches to make them indigestible.(6) Although research into RS is in relatively early stages as compared to other aspects of nutrition, there is clear and mounting evidence that its ingestion has wide-ranging health benefits.
These benefits include:
- improved gut and colonic health (and consequently stronger immunity and better mood) (1,2,4,8–11)
- increased satiety (with potential decrease in short-term food intake) (12,13)
- greater insulin sensitivity (with implications in prevention and/or management of metabolic disorders and Type 2 Diabetes) (3,14,7,15,16)
- reduced blood glucose response after eating (3,14,17)
Suffice to say, this nutrient is worth talking about!
Resistant Starch and Gut/Colonic Health
RS is inextricably linked to the health of the gut. Gut health is a topic of central concern and has garnered a lot of recent attention, for good reason. The gut-brain axis – the complex, bidirectional interplay between the gut and the brain – has been more accurately termed the microbiome-gut-brain axis in recent studies, due to the mounting evidence of the communication between specific microbiota and the brain.(9) The reciprocal relationship between the gut and the brain has far-reaching implications for mood, modulation of behaviour, hormone regulation... ultimately, the gut-brain connection impacts upon our holistic health and wellbeing.
There are many ways in which RS serves your gut health. Let’s look at three of these ways, below.
1/ Resistant starch is a “prebiotic”(1,18)
As I explained earlier, resistant starch escapes digestion by enzymes in the small intestine, traveling instead to the colon where it undergoes bacterial fermentation.(1–4) It is by this mechanism that the beneficial bacteria of the gut, namely bifidobacterium and lactobacillus, are stimulated and other health benefits result.(19)
Note: Confused between pre- and pro-biotic? Basically, a prebiotic food or supplement feeds the good bacteria in the gut, making the right bacteria happy and healthy. A probiotic food or supplement contains live cultures of beneficial bacteria(18) – such as kombucha, sauerkraut, or yoghurt. Therefore, it can be said that eating prebiotic foods (such as those containing resistant starch) provides food for the bacteria, whereas probiotic foods donate the bacteria to the gut.
2/ Bacterial fermentation of RS produces SCFA, in particular, butyrate
Beneficial bacteria such as bifidobacteria and lactobacilli consume RS during fermentation, and produce a number of short-chain fatty acids (SCFA), such as butyrate and acetate (1,19,20) SCFA are the primary energy source for the cells that line the colon wall (colonic epithelial cells) (21) and are also absorbed into the blood stream to be used as energy by the brain, muscles and tissues. (19)
SCFA, especially butyrate, are thought to protect colonic health in humans.(2) Butyrate has the ability to “kill off” cancerous cells and protect colonic cells from transforming into potentially harmful variants.(1,2) In fact, the injection of butyrate into the colon has been shown to cause ulcerative colitis to enter remission.(1) Butyrate is also thought to play an important role in suppressing tumor cells, and repairing cell damage.(1) Therefore, it is said to be protective against colorectal cancer and other colonic disease.
3/ SCFAs lower the pH level of the colon
The fermentation of RS lowers the pH level of the colonic environment.(2,4,22) In an acidic environment, pathogenic bacteria are inhibited at the same time as beneficial bacteria proliferate.(19,23,24) So not only does RS feed beneficial bacteria, but it’s end-products (SCFAs) create a healthier gut environment and encourage a healthier microbiota.
There has been evidence in recent studies that resistant starch increases fat oxidation, reduces protein oxidation, decreases abdominal fat-storage, reduces the energy intake of a meal, and increases satiety following a meal (12,13,16,25,26). This has implications for weight management if consumed regularly and in appropriate doses.
The evidence is not completely uniform regarding satiety and short-term food intake; few studies found these factors to be unchanged following an RS meal in comparison to a control meal (12,5,27). However, these discrepancies may be explained by a variety of factors, such as: the gender, age, or health of the participants, the form of meal used (i.e. liquid versus solid which can impact gastric emptying rate) or the nutrient profile of the meal (i.e. the levels of macronutrients and total calories). Furthermore, in the studies in which satiety and food intake were unaffected, abdominal fat storage decreased (6,25), fat oxidation still increased, and protein oxidation decreased (thereby sparing lean body mass). (13,16,25,27) The sparing of lean body mass during weight loss or maintenance allows the basal metabolic rate to remain the same, along with the energy output.(26)
It is worth noting that in one of the studies in which RS intake did not cause an increase in satiety markers, when RS was taken in combination with protein, satiety did increase, along with the other benefits of RS ingestion. (13) So enjoying a food high in RS such as a piece of sourdough, for example, may be most effective in terms of weight management if paired with a food high in protein – such as peanut butter, or fried eggs.
Regardless of satiety markers, incorporating RS into a meal reduces the available digestible energy that could potentially be stored as body fat.(26,27)
In these ways, regular RS intake has implications for weight management.
Increased Insulin Sensitivity
RS intake has been shown to increase insulin sensitivity in healthy and pre-diabetic subjects.(3,14,15,17) Although the mechanisms by which this sensitivity increases are not yet clear, and more research is required.(15)
Insulin is the hormone in charge of shuttling our blood glucose into cells to be used as fuel, or stored as fat. Insulin is released into the blood in response to a rise in blood glucose – the more glucose in the blood, the more insulin is released to take care of it. The problem with having high levels of blood glucose (hyperglycaemia) regularly is that the cells become resistant to the effect of insulin. This means that insulin can no longer efficiently do it’s job, so increasing levels of insulin must be released to maintain glycaemic homeostasis in the blood.(15,17)
Insulin resistance is one of the first signs that a person is at risk of becoming diabetic. So by improving insulin sensitivity and glycaemic efficiency, the risk of developing Type 2 Diabetes (or another metabolic syndrome) is reduced.(17)
Reduced blood glucose response after eating
RS intake has been shown to reduce blood glucose levels (glycaemia) after eating, too. (3,14,17) Therefore RS intake has implications both as a preventative strategy against metabolic disorder and in managing those already with a metabolic syndrome such as Type 2 Diabetes.
The mechanisms by which this reduction in blood glucose occur seem to be two-fold: a reduction of digestible carbohydrate, and a reduction in the time it takes for the stomach to be emptied.
Firstly, by replacing a portion of the carbohydrate of a meal with RS, less digestible carbohydrate is available, and subsequently less glucose is released into the blood stream.(3,17) This means that the glycaemic load is lower, which is more appropriate food for those who are insulin resistant or diabetic. For example, in one study (3) subjects with Type 2 Diabetes were given four similar meals with varying levels of RS. The meal containing the most RS resulted in a much lower glycaemic response; because there was less digestible carbohydrate available.
RS may also work to reduce the glycaemic (blood glucose) response of a meal by reducing the rate at which the stomach is emptied.(19) When the stomach is emptied more slowly, glucose is released into the bloodstream more gradually. Consequently, there is a lower rise in blood glucose levels over a longer period of time. This is significant because it has been shown that even brief spikes in blood sugar are harmful to the metabolic health of the individual.(17) However, there has been some suggestion that RS only results in a slower rate of gastric emptying when eaten in the presence of enough dietary fat, which “can have potent effects on the accessibility of starch to digestive enzymes”, thereby decreasing the rate at which digestion occurs.(26) So perhaps the take-home point here is: make sure you eat enough healthy fat with your resistant starch foods!
How can you include more RS in your diet?
My favourite way to ensure we eat enough RS is to roast starchy vegetables on a Sunday, before storing them in the fridge and adding to salads during the week. When we eat bread, we opt for whole grain sourdough, which is higher in RS than any other type of bread and has proven lower glucose responses after eating.(7) Of course, resistant starch comes bound in high-carbohydrate foods. So if you have specific carbohydrate requirements, or simply for convenience, you may wish to take a resistant starch supplement – there are many available on the market.
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