“Resistant starch?” I hear you ask, “what’s that?”
If you just happened to be watching that rather interesting programme the other night – “The Truth about Carbs” on BBC1 – you would have discovered that there is a little-known type of carbohydrate called resistant starch. And, just like fibre, resistant starch can help keep bowel cancer at bay.
Most of us love starchy carbohydrate foods like bread, pasta, rice and potatoes but they’re not always good for our health. Although starch is an important part of a healthy diet, it’s easily broken down. As soon as we consume starch the body very quickly starts to digest it, releasing sugars into the blood which in turn causes our bodies to release the hormone insulin.
WHAT IS IT?
Resistant starch is a form of starch that cannot be digested in the small intestine. Most starch is usually quickly broken down into glucose (which is used for energy) and if we eat too much, the glucose can then be stored as fat. Due to its molecular make up however, resistant starch doesn’t get broken down like normal starch, and instead it passes through the stomach and the small intestine intact until it reaches the large intestine. Resistant starch is then fermented in the large intestine, producing short chain fatty acids which serve as an energy source for colonic cells and our gut microflora. Foods that increase the amount of short chain fatty acids in the colon are thought to be beneficial to health by helping to prevent the development of abnormal cells in the gut.
Resistant starch is naturally present in some foods such as bananas, potatoes, grains, pulses and seeds and is also produced or modified commercially and incorporated into some food products.
There are several different types of resistant starch:
- RS1: This is inaccessible to digestive enzymes due to the physical barriers formed by cell walls and protein matrices. Present in bread, seeds and pulses.
- RS2: This type of resistant starch is protected from digestion due to its crystalline structure. Present in potatoes (higher amounts in raw versus cooked), bananas (higher amounts in unripe fruits).
- RS3: This is retrograded starch which is formed when starchy foods such as potatoes and pasta are cooked and then cooled.
- RS4: This is a chemically modified starch. Present in foods containing modified starches such as some bread and cakes.
Note: In addition, two different components have been proposed as RS5. Either amylose–lipid complexes, which either form during processing or are created artificially; or resistant maltodextrin which is processed to purposefully rearrange starch molecules.
Researchers have known about resistant starch for over twenty years now, so why the sudden excitement?
RS3 – Why we are getting excited!
Back in 2014, the BBC’s “Trust me I’m a doctor” programme conducted an experiment with Dr Denise Robertson, from the University of Surrey and ten volunteers. The team were well aware that if you cook and cool down starchy foods, your body will treat it much more like fibre, creating a smaller glucose peak and helping feed the good bacteria that reside down in your gut. You will also absorb fewer calories. However what no-one knew was what would happen to the resistant starch if you reheated the cold food up again.
The volunteers had to undergo three days of testing in all, spread out over several weeks. On each occasion they had to eat their pasta on an empty stomach. The volunteers were randomised to eating either hot, cold or reheated pasta on different days.
On one day they got to eat the pasta, freshly cooked, nice and hot with a plain but delicious sauce of tomatoes and garlic. On another day they had to eat it cold, with the same sauce, but after it had been chilled overnight. And on a third day they got to eat the pasta with sauce after it had been chilled and then reheated.
On each of the days they also had to give blood samples every 15 minutes for two hours, to see what happened to their blood glucose as the pasta was slowly digested.
Just as expected, eating cold pasta led to a smaller spike in blood glucose and insulin than eating the freshly boiled pasta had. But unexpectedly, the team found that cooking, cooling and then reheating the pasta had an even more dramatic effect. Or, to be precise, an even smaller effect on blood glucose, reducing the rise in blood glucose by 50%. This certainly suggests that reheating the pasta made it into an even more “resistant starch”. It’s an extraordinary result and one never measured before.
The health benefits of resistant starch
- When regular starch becomes resistant starch, most of the sugars it contains aren’t released in your gut and so your body will take in fewer calories from the same food.
- Because less sugar is released into the blood stream from this resistance starch, there’s less of a blood sugar spike. In turn, this reduces the levels of insulin in the blood.
- Resistant starch is indigestible and so shares many properties with fibre, helping food pass through the gut and generally improving digestion.
- Once it reaches the lower gut, resistant starch feeds our beneficial bacteria, which in turn produce chemicals which can help our immune systems, cardiovascular health and many other benefits.
Freeze first then toast: make your bread work for you!
We love our bread. And it can be hard to give that up. But maybe we don’t need to. If you can’t stop eating bread, you can make the bread healthier for you by simply freezing it then toasting it. This turns some of the easily digestible starch into resistant starch. The act of freezing and then toasting means that your body gets far fewer calories from the bread. In effect, the resistant starch feeds your gut bacteria rather than feeding you. It really is that simple!
BBC Trust me I’m a Doctor
BBC The truth about carbs
British Nutrition Foundation