At Positano Italian restaurant in Guildford, Surrey, 10 of the staff agreed to take part in an experiment, devised by Dr Robertson. Each of them ate one bowl of white pasta a day for three days. On each day the pasta was prepared in a different way (as follows) and topped with the same simple tomato sauce.
- Day 1: Hot freshly cooked pasta
- Day 2: Cold pasta that had been chilled overnight
- Day 3: Pasta that had been chilled overnight and reheated
After eating each bowl of pasta the participants measured their blood glucose levels every 15 minutes for two hours.
Eating freshly cooked pasta caused the biggest rise in blood glucose.
Eating chilled pasta caused a slightly lower rise.
Unexpectedly, pasta that had been cooked, chilled and then reheated caused the lowest rise of all.
How it works
Starch is the most common carbohydrate in our diets and is essentially a chain of glucose molecules linked together. Raw starchy foods (e.g. raw potatoes) have a highly ordered structure and are hard to digest, but heating them in water weakens this structure, making it easier for the gut to break down each chain and absorb glucose into the blood.
Glucose from cooked starchy foods, such as white rice, pasta and potatoes, is absorbed almost as quickly as glucose from a sugary drink. We say these foods have a high ‘glycaemic index’ or GI.
However, when starchy foods are cooled their structure is reorganised again and the digestive enzymes in your gut can’t break them down as easily. The food now contains more ‘resistant starch’, which is not broken down and essentially becomes fibre.
This explains why the chilled pasta caused a smaller rise in blood glucose – but why was reheated pasta even more effective?
This is where Dr Robertson’s research is ground breaking. “The influence of cooling we’ve known about for 20 years,” she says, “but the influence of reheating after cooling was unexpected. We’re still not sure exactly why it works. All we can assume is that the process of heating, cooling and reheating pasta or other starchy foods must be creating more resistant starch.”
Dr Robertson warns that there are various ways of reheating food, at different temperatures and for different lengths of time and that this is a detailed part of the research that she is currently looking into.
What are the health benefits?
Dr Robertson is one of the world’s leading researchers of ‘resistant starch’ and, with funding from Diabetes UK, has studied its impact on people at high risk of Type 2 diabetes and people with Type 2 diabetes for the first time.
According to Dr Robertson, there are two health aspects to ‘resistant starch’.
- If you swap a meal made with normal starch for one made with ‘resistant starch’ it brings your glucose level down straight away. Over time, high glucose and high insulin levels can contribute to insulin resistance and Type 2 diabetes, so ‘resistant starch’ could be one way to help reduce the risk of Type 2 or reduce the impact of the condition in people who already have it
- ‘Resistant starch’ can dramatically increase the fibre content of your diet without affecting the appearance, taste or texture of the food and without you knowing the difference. Foods fortified with ‘resistant starch’ can give you the fibre equivalent of brown rice, brown bread or wholemeal pasta without changing your diet. One short-term effect of this is that it helps you feel fuller for longer after a meal.
Not a magic bullet...
Dr Robertson is keen to point out her work doesn’t mean we can all eat as many carbs as we like if they are cooked in this way. But these changes could help people to dramatically improve the health impact of carbs, just by the way they prepare everyday starchy foods. “We’ve never said that food with more resistant starch is lower in calories,” she says, “Because it’s not. There will be a small calorie difference because you’re getting less glucose, but it’s not a huge amount, so it’s not going to cure obesity.”
Dr Robertson is continuing her research in this area and emphasises there’s a lot of potential for ‘resistant starch’ but also a lot we still don’t know.
Diabetes UK’s clinical advisors recommend…
… that a healthy, balanced diet is the best way to maintain a healthy weight and can help people to manage Type 2 diabetes or reduce the risk of developing the condition. Foods that include resistant starch add to this balance and allow individuals to incorporate more fibre into their diets. It is very important to follow food safety guidelines when cooling, storing and reheating foods in order to minimise the risk of potential food poisoning. Go to the home hygiene section of the NHS website for more information on this.