Its Time To Gain Knowledge On Resistant Starch By Ivan Nikolov
In this article I will make an attempt to present what is available as knowledge about resistant starch – a topic that is still little known about.
Scientists have been studying resistant starch for over 20 years now but there is still need for more precise methods of research to bring a lot more knowledge that will clarify some very important points in relation to the effects of resistant starch on the human body and especially those in relation to proper sports nutrition.
The topics that I’ll touch on are those, most closely related to the nutrition part in the sports of bodybuilding such as: physiological effects of resistant starch; lipid metabolism; resistant starch and insulin and glucose response, GI and GL. But let’s first take a quick overview to help us get an idea what a resistant starch is and where it comes from:
Resistant starch (RS) has recently been defined as “… the sum of starch and products of starch degradation not absorbed in the small intestine of healthy individuals”. In other words RS is that portion of starch (roughly about 15 per cent) that is not readily digested and thus surpasses the small intestine. To date several categories of RS are identified:
RS1 - Physically inaccessible starch due to the presence of intact cell walls in grains, seeds or tubers. Mainly found in partly milled grains, seeds and legumes.
RS2 – This is the group of the raw ungelatinized (uncooked) starches. They are classified in 3 main types: A (ex. most cereal starches, cassava starch), B (ex. Green banana and raw potato starches) and C (most legume starches). RS2 is called high-amylose maize starch.
RS3 – Retrograded starch is a starch that has been cooked and then cooled and stored for periods of time, ranging from several hours to several months (cooked and cooled potatoes, bread, cornflakes, food products with prolonged and/or repeated moist heat treatment).
RS4 – Chemically modified starches. These are derived due to cross bonding with chemical reagents, ethers, esters, etc. (certain modified breads, pastas and cakes).
The first three groups (RS1, RS2 and RS3) can actually coexist in starchy foods. For ex. RS1 and RS2 are both present in bananas, and RS1 and RS3 can be found in beans.
The American Association of Cereal Chemists and the Food Nutrition Board of the Institute of Medicine of the National Academies currently defines RS as a type of dietary fiber.
How was it found?
Englyst and coworkers accidentally found RS in 1982 while performing in vitro experiments. They found that after enzymatic hydrolysis some starch remained not hydrolyzed.
Consequent in vivo studies showed that RS resist digestion in the stomach and the small intestine and are then delivered in the large intestine where RS act as a substrate for microbial fermentation. The end products of this process are short chain fatty acids (SCFA), hydrogen, carbon dioxide and methane.
Structure and energy value
The two main components of starch are amylose and amylopectin. As a general rule starches that contain more amylose (the linear fraction of starch) produce more RS after retrogradation than those with mainly amylopectin content (the crystalline fraction of starch).
In other words starches rich in amylose are generally more resistant to digestion and also more susceptible to retrogradation.
Retrogradation occurs when starch is heated in water above its gelatinization temperature and then cooled. When cooked beyond certain temperature starch granules gelatinize (melt) thus becoming more readily digestible.
However, these starch gels are unstable and upon cooling re-form crystals that are resistant to hydrolysis by amylases (digestive enzymes). Reheating of starch reduces the RS content while continuous cycles of reheating and cooling have shown to increase RS.
Studies suggest that the energy value of RS is approximately 2 kcal/g (8 kJ/g) as opposed to the energy value for completely digestible starch 4.2 kcal/g (15 kJ/g) (Liversey 1994).
Physiological effects of RS
As already mentioned above RS escape digestion in the upper intestinal tract and once in the large intestine they are subject to fermentation by the microflora with end-products SCFA (short chain fatty acids), hydrogen, carbon dioxide and methane.
As being of most importance to the human health we will now focus our attention mainly on SCFA. SCFA are a product of bacterial fermentation. The main components are butyrate, propionate and acetate with the first one being produced in significantly higher levels in comparison with the other two.
SCFA are the main energy source of the colonocytes (the colon mucosa). They help lower the pH of the intestine, increase colonic blood flow, help reduce the presence of toxic ammonia, and help prevent the development of abnormal colonic cell populations. SCFA are known as a bio-marker for colonic health.
It’s worth mentioning also that RS serve as a physical protection for the probiotics. These are cultures of live microorganisms, which are shown to regulate the flora in the intestinal tract. Therefore, resistant starch is prebiotic, based on its probiotic protecting and stimulating properties.
RS and lipid metabolism
RS were continuously shown to cause a significant reduction in a number of measures of lipid metabolism (total lipids, total cholesterol, LDL, HDL, VLDL, triglycerides, triglyceride-rich lipoproteins) in laboratory rats.
Studies with humans have shown controversial results.
J. A. Higgins and co. performed a study on12 subjects who were given meals containing 0%, 2.7%, 5.4%, and 10.7% RS as a percentage of total carbohydrate intake.
The conclusion of their study: “The data indicate that replacement of 5.4% of total dietary carbohydrate with RS significantly increased post-prandial lipid oxidation and therefore could decrease fat accumulation in the long-term.
However, some other studies indicate that RS consumption does not affect the measures of lipid metabolism in humans (Nugent, A. P). Therefore it is evident that more research is needed to help us better understand the effects of RS on lipid metabolism in humans.
RS and insulin and glucose metabolism, GL and GI
Insulin is a hormone directly responsible for the transportation of the blood glucose from the blood stream to the muscle and adipose (fat) cells.
Avoiding insulin spikes is considered the main factor in body weight management as this hormone not only stores the blood glucose in the adipose cells but also inhibits the release of fat and its subsequent use for energy.
Foods rich in RS cause slow and steady insulin release, which enables for a greater percentage of the stored fat to be transported to the liver and eventually used for energy. Therefore, RS play a very important role in the insulin response moderation, this way regulating the glucose metabolism.
As a consequence RS-rich foods are important aid in the body weight management, treatment of obesity, decreasing the risk of developing diabetes as well as other major diseases.
In a recent study Y. Yamada and co. examined the inhibitory effect of a single ingestion of bread containing 6 g of resistant starch (test food) on the postprandial blood glucose levels in male and female adults.
Bread not containing resistant starch (placebo) was used as control food. They concluded: “…Postprandial increases in both blood glucose and blood insulin were significantly inhibited in subjects in the borderline group who took the test food in comparison with the placebo group.
Although consumption of foods, rich in RS generally results in reduced glycemic response it isn’t still clear whether this is due to the lack of digestible carbohydrates or RS poses specific physiological properties resulting in decreased glycemic response.
However, due to the same low glycemic response, RS display an interesting and beneficial relation to the glycemic index (GI- an index that ranks foods oh how they affect the blood glucose level) and glycemic load (GL- it measures how much of carbohydrate is in a serving with a certain GI).
If some of the digestible carbohydrates are replaced with RS that will help lower the overall GL this way decrease the negative effect on health that some carbohydrate foods with high GI and GL might have.
How much RS is required for good health?
Currently an official analytical method for measuring the resistant content of foods doesn’t exist. It has been estimated that resistant starch intake in Australia is around 5-7 grams/person/day; 3 – 6 in EU; 10 – 18 in India and China and 2.76 in the UK.
Approximately 20 grams a day is recommended to obtain the beneficial health benefits of resistant starch.
Natural and commercial sources of RS
A short list of natural sources of RS includes cereal starches, green banana, raw potato, cooked and cooled potato, legume starches and other food products with repeated moist heat treatment.
In addition to the natural food sources of RS, some commercially manufactured forms of RS such as Hi-maize, a commercial form of RS2, and NOVELOSE 330, a form of RS3 are also available.
As mentioned by A. P. Nugent in his extensive report on RS “there are number of advantages to using commercially manufactured sources of RS in food products. Unlike natural sources of RS (e.g. legumes, potatoes, bananas), commercially manufactured resistant starches are not affected by processing and storage conditions”.
For example, he states, the amount of RS2 in green bananas decreases with increasing ripeness, however, a commercial form of RS2, Hi-maize, does not experience these difficulties.
Although more long term research on humans is needed, it is clear that RS posses many health benefits with the greatest and most well researched one being the protective benefit that RS has on the colon.
It is of a big importance also the fact that RS have shown to have prebiotic (probiotic promotion and protection) functions thus helping for the regulation of the intestinal flora.
However, in relation to bodybuilding applied nutrition, more long-term studies are needed to determine to what extend RS may be eventually beneficial for better management of the lipid and glucose metabolism.