By Dr Malcolm Mackay
Resistant starch may be a key dietary component for gut health. While most starch is broken down and absorbed in the small intestine, resistant starch is in a physical form that resists enzymatic digestion in the small intestine and travels on into the colon where it acts as a “prebiotic” for healthy gut microbes in the large bowel (colon and rectum). Resistant starch is considered to be one of the three classes of dietary fibre along with insoluble fibre and soluble fibre. Resistant starch is found in legumes, whole grains and potatoes. Cooking and cooling starches such as rice, pasta and potatoes raises the resistant starch content, even for refined grains. Under-ripe bananas are very high in resistant starch.
There are four categories of resistant starch:
- RS1 The starch is physically inaccessible to digestion due to intact cell walls. It is found in whole grains, seeds and legumes.
- RS2 The starch granules are protected from digestion by the structure of the starch granule. Found in uncooked potatoes, green bananas and high amylose maize.
- RS3 The starch has been cooked but then undergoes a process called retrogradation as it cools. It is found in cooled cooked potatoes, rice, pasta, bread, legumes and even cornflakes.
- RS4 The starch has been chemically modified to produce a resistant starch not found in nature.
It has been suggested that the type of resistant starch is not strongly correlated to its physiological function or health benefits . RS2 high amylose maize starch (HAMS) has the potential to be used as food additive and has been the focus of much of the research.
Resistant starch bypasses the small intestine and acts as a “prebiotic” for healthy gut microbes in the large bowel (colon and rectum). Higher intakes result in a greater numbers of the bacteria that feed on resistant starch (F.prausnitzii and R. Bromii). Yoghurt and probiotics do not contain the microbes that feed on resistant starch. The gut microbiome changes quickly in response to changes in diet – a study published in the journal Nature (David et. al 2014) found that it only took 3 days of a plant-based diet to build a healthy starch digesting set of microbes and only 3 days of animal products for a more inflammatory, cancer promoting set of microbes. Presumably, the gut microbiome begins to change quite early in the 3-day period – a warning to those who regularly go a day or two with little resistant starch or other dietary fibre.
The CSIRO Hungry Microbiome video uses computer animation to explain how microbes digest starch and produce butyrate and other short chain fatty acids (acetate, propionate). Butyrate is the preferred fuel of the cells lining the colon (colonocytes). Butyrate not only maintains healthy colonocytes and but also encourages mutated, potentially cancerous cells to self-destruct.
A lot of research on resistant starch is centred on colon cancer, the most common internal malignancy in Australia. Comparison of colon cancer rates between countries shows a stronger negative correlation between bowel cancer and starch (and thus resistant starch) intake than other forms of dietary fibre. Therefore even a traditional post-industrial revolution Asian diet base on white rice is associated with lower rates of colon cancer. However these people also had a low intake of animal protein foods, particularly red meat which is strongly associated with colon cancer. Current research into resistant starch and colon cancer reminds us of the metaphor of mopping up around an overflowing sink without turning off the tap – with resistant starch supplements mopping up the cancer risk caused by a diet centred on red meat and other animal protein foods. One study observed that a high red meat diet of 300g per day increased RNA cancer risk markers in rectal mucosa cells and that adding a resistant starch supplement to this diet prevented some of these changes. We note that the association between red meat and colon cancer is not just based on observational studies but is also supported by interventional studies such as this one.
The bacterial digestion of resistant starch has many potential health benefits. There is a simple bulking effect, with resistant starch producing more than its own weight in bacteria and hence stool mass. This is said to dilute the potentially toxic products such as ammonia, phenols and hydrogen sulphide that can result from protein fermentation. However, we say cut off the supply line for toxic protein fermentation by not eating animal protein-rich foods.
The fermentation of resistant starch into short chain fatty acids lowers the pH of the colon contents which encourages the growth of other beneficial bacteria. Might resistant starch be a key ingredient for an anti-candida diet?
Butyrate produced in the colon may improve blood sugar metabolism, reducing the response to a glycaemic load. Resistant starch fermentation continues for many hours after food leaves the small intestine, effectively “drip feeding” the liver with butyrate during periods of fasting between meals. Recent studies indicate resistant starch may have a role in enhancing both short term and long-term satiety. Perhaps “eat resistant starch for satiety” should replace the “eat protein for satiety” mantra. Resistant starch only yields 2-3cal per gram (vs 4cal/g for other starch) but the math does not suggest that this is important for weight control.
We consider it reasonably well established that a whole foods, plant-based diet prevents, treats and often reverses inflammatory bowel disease (ulcerative colitis and Crohn’s disease). There is also some evidence that the gut microbiome plays a role in these diseases and there are already some centres treating ulcerative colitis with faecal transplants. Dietary resistant starch may be an important factor in inflammatory bowel disease. It provides the right fuel for colonocytes, prevents the degradation of the mucus layer that protects colon cells, regulates the pH of the colon and encourages the growth of a health supporting microbiome.
Irritable bowel syndrome (IBS) is the diagnosis given when there is gut pain, distension and altered bowel function and conditions such as coeliac disease and inflammatory bowel disease have been excluded. Some WFPB practitioners say that IBS is a low grade inflammatory disorder. Given the key role of resistant starch in supporting gut health it would be surprising if a high intake, more typical of those on non-Western diets, were not beneficial.
The gut has its own nervous system and numerous regulatory hormones. There remains much to be learnt about how the interaction of the gut microbiome and nutrients such as resistant starch affect the rest of the body. It has been postulated that brain function may be altered, and there is some research suggesting that a more vegetarian diet improves mood.
Recent evidence suggests that different types of dietary fibres act through different mechanisms and are more effective in combination than individually for promoting bowel health, so we should not focus solely on resistant starch. Fortunately for those taking the whole foods, plant-based approach, whole grains, legumes, vegetables and fruits provide a wide range of dietary fibre types. The basic nutrition principle of consuming nutrients from whole foods rather than as isolated extracts, the “whole” principle promoted by T. Colin Campbell, suggests that adding a resistant starch supplement to our diet will not be an effective method of supporting gut health.
The optimal intake for resistant starch is not known. Australians consume only about 5g per day while the intake in China is 15g per day. The Diet and Bowel Health report recommended an intake of 20g per day which is four times the current SAD (standard Australian diet) intake. This is interesting given that a total fibre intake of only 30g per day is considered adequate for Australians. A whole foods plant-based diet provides two to three times as much total dietary fibre as this, and assuming that it is starch-based, a significant amount of this will be resistant starch. I have no estimates of typical WFPB diet resistant starch intakes. It is a reasonable assumption that when the majority of one’s calories come from whole grains, legumes and starchy vegetables, that this will provide an optimal level of resistant starch intake.
The current trend in Australia seems to be to eat less carbohydrates, and hence less resistant starch – whole grains have suffered from doubt created by the Paleo/Atkins movement, potatoes have been labelled as high GI and the misguided quest to eat more animal protein has displaced starches from the diet. Even vegetarians and vegans seem to moving away from grains and other starches, and replacing the lost starch calories with richer, high fat foods such as cheese, coconut and nut products. We suspect that this reduction in starch and resistant starch intake is driving the apparent epidemic of gut problems, particularly pain, bloating and bowel disturbances. The most popular remedy seems to be a gluten free diet which may reduce FODMAP intake but tends to further reduce the intake of starch and resistant starch. Few people are cured by this approach, which is not surprising, given the importance of resistant starch in maintaining a healthy gut. We encourage you not to get caught in the downward spiral of treating gut problems with a diet that is low in resistant starch. Resistant starch may be essential for your recovery.
RESISTANT STARCH FOOD VALUES:
Food Resistant Starch (per 100 g)
Potato sliced, boiled and cooled 0.8 g
Potato, steamed, cooled 6 g
Potato, roasted, cooled 19 g
Cashew nuts 13 g
Bananas, green 38 g
Bananas, ripe 5 g
Oats, cooked 0.2 g
Oats, rolled, uncooked 11 g
White beans, cooked/canned 4 g
Lentils, cooked 3 g
Rye bread (Burgen) 4.7g
Barley, pearl, cooked ~ 2g
- The Hungry Microbiome: why resistant starch is good for you – This CSIRO animation shows how resistant starch moves through the intestine, feeds the healthy bacteria of the gut Microbiome, nourishes the cells lining the colon and helps prevent cancer.
- Resistant Starch Facts – CSIRO
- Diet and Bowel Health: Expert Round Table Report 2011 – This report from the Gut Foundation website discusses resistant starch, gut health and bowel cancer and makes some conservative recommendations for resistant starch intake.
Page last updated 11 January 2015