Will resistant starch change the way we think about carbs? – by Dr Denise Robertson
Carbs in the diet, especially if you have diabetes, seems to split opinion. Although there is no consistent evidence to recommend the ideal amount of carbs for everyone with diabetes, new healthy eating guidelines for the general public recommend that 50% of our daily calories come from carbs. Of course, that isn’t a license to eat lots of sweets and sugar, most of your carbs should be in the form of wholegrains, fruit and vegetables, beans and pulses.
Starch vs sugar
Like other animals, we can store extra glucose for later in a molecule called glycogen in our liver and muscles. This allows us to maintain our blood glucose overnight when we aren’t eating. Plants aren’t so different, except that they store their glucose as starch.
Glycogen and starch are very similar, both made up of chains of glucose molecules. So if starch is made up of glucose, how does it differ from sugar? Well, sugars are only made up of one or two glucose molecules. That’s why they can be absorbed very quickly after eating, increasing blood glucose.
By comparison, starch molecules are massive. They can be chains of several million glucose units in length and require two sets of enzymes to break down the chains. Then the glucose can be absorbed and appear in the blood. So the crucial aspect here is time. If all the glucose is absorbed quickly, it will have a greater effect on blood glucose levels.
There are also different types of starches. Refined versions such as white bread tend to be absorbed more quickly compared to wholegrain versions such as brown rice and wholegrain bread.
OK, so what is resistant starch?
Although once said by a famous TV presenter that the name was boring, it does exactly as it says on the tin. Resistant starch is resistant to the enzymes in the gut. So the absorption of glucose is either slowed down, or in some cases stopped altogether.
What makes starch resistant? Well, this can be a number of things. The starch could be trapped inside of a grain (like in wholegrain foods), the starch granule could be raw (as in a green banana) or there could have been selective breeding of the plants for increased resistant starch content (such as in some types of corn and wheat). These are all natural forms of resistant starch.
But we now know that the way we cook, store and prepare our foods can also make the starch more resistant.
How can we make starch more resistant?
Simple things like freezing and refrigeration of foods after they’ve been cooked can make the starch resistant. And food combinations we chose might also have a significant impact.
Our group did a simple demonstration where we looked at the effect of chilling and reheating pasta in the presence of a small amount of oil (featured on the BBC’s “Trust me I’m a Doctor”). We saw that refrigeration of the pasta led to a smaller rise in blood glucose levels, compared to the freshly cooked pasta.
We know that the cooling of starch that has been cooked in water allows new bonds to form between the glucose and water molecules. This means the enzymes of our gut no longer recognise them.
But there was a new interesting effect of the oil. Reheating the pasta, simply in the microwave, caused even more new bonds to form between the glucose and the fat molecules. It made it even harder for the enzymes to digest.
So the same meal prepared in a different way could change the overall effect on blood glucose. Although this process of repeated cooling and reheating is used by the food industry, showing that we as consumers could get a similar effect in our home kitchen was something new and exciting.
But there are various ways of reheating food, at different temperatures and for different lengths of time. This is a detailed part of the research that still needs to be carried out.
But what happens to the glucose if we don’t absorb it?
Resistant starches act the same way as dietary fibre – they go to feed our gut bacteria. We know a lot more about these bacteria now. And carbohydrates, such as starch, are their preferred food source, needed to keep them healthy.
The bacteria take the glucose and convert it into different molecules, which we use. Those molecules may have beneficial effects on the health of the gut, but also in the long-term, in improving how we respond to insulin.
In our Diabetes UK funded research, we’ve shown the important effects of resistant starch for improving the way our body fat and muscles respond to insulin and take up glucose and fat from the blood.
So, by eating more resistant starch, we may be able to reduce blood glucose spike following a meal and in the long-term improve how our body deals with both carbs and fats. And we want to investigate this area of research further.
Clinical advisors at Diabetes UK say…
“It’s an exciting avenue of research, but until we understand more, we recommend a healthy, balanced diet as the best way for people with Type 2 to maintain a healthy weight and help to manage their diabetes, and for those with Type 1 to lead a healthy lifestyle.
“And just a note of caution that it’s very important to follow food safety guidelines when cooling, storing and reheating your food to avoid food poisoning. Go to the home hygiene section of the NHS website for more information.”