When we eat foods that have a lot of easily digestible carbohydrates in them, our blood sugar levels can rise in a sharp spike, just to come crashing down again within an hour of the meal. High blood sugar, or hyperglycaemia, is not good for your health. It is one of the diagnostic features of Type II Diabetes, which can often remain unrecognised and untreated for a long time.
This has motivated food scientists to rank different types of carbohydrates, for example starch or various sugars, by their glycaemic potency, or their potential to raise blood glucose levels.
At Plant and Food Research, John Munro and Suman Mishra have come up with a way of testing whole foods, and combinations such as breakfast cereals and kiwifruit, for their impact on blood sugar, and they found that kiwifruit, combined with a starchy food, can slow down the sugar loading.
From our point of view, because we’re food scientists, we’re interested in identifying the factors in foods that prevent the food from leading to a high blood glucose peak.
The glycaemic index (GI) was introduced to provide a guide for the glycaemic potency of carbohydrate components in a food relative to that of glucose itself. But John Monro says the index is a measure of available carbohydrates and doesn't take into account the effects of mixing whole foods together in a meal. He sees several problems with it, which prompted him to look for a better measure of the blood-sugar effect of whole foods.
High concentrations of glucose in the blood lead to a rapid throughput of metabolic products into mitochondria, the energy-producing organelles in each of our cells, which in turn produce highly reactive free radicals. They inhibit an enzyme that is part of the pathway that gets rid of glucose and because of that, glucose is shunted off into subsidiary pathways.
A number of those produce harmful products that interact with proteins throughout the body - and that leads to a systemic dysregulation that ultimately causes the wide range of complications that emerge with long-term untreated Diabetes.
His team’s focus is on finding foods that help lower blood sugar levels, and there are several aspects they are investigating. Particle size can make a difference by slowing down the processes in the gut, including the mixing of food and the rate at which glucose is transported through the intestinal wall. Certain food ingredients, such as phytochemicals and extracts from seeds, can also react with digestive enzymes and partly neutralise them.
The team uses an in-vitro digester which simulates the processes in the stomach and small intestine to track when and how starch is digested into glucose, how quickly certain foods such as breakfast cereals are digested, and what difference the addition of fruit can make to the rate of sugar uptake. This information is then calculated against other processes, such as muscle activity, that normally remove glucose from the blood and tissues.
You get the build-up of blood sugar when the disposal effort by your liver and muscles cannot keep up with what’s being loaded up.
John Monro says the tests show that kiwifruit can lower the rate of glucose uptake from starchy foods such as breakfast cereals, rice and noodles, as long as the overall carbohydrate intake remains constant, i.e. the kiwifruit replaces some of the other foods, rather than adding to the overall carbohydrate content of the meal. On top of the benefits in the glycaemic response, adding kiwifruit also improves the overall balance and vitamin uptake.
Fruit contains fructose and sucrose, two sugars that are chemically different from glucose, but also play a role in the body’s blood sugar balance. John Monro says fructose has been linked with Diabetes because high-fructose syrups are used in many processed foods, but “the kind of levels we get with normal, healthy intakes of fruit are not harmful at all”.
Fructose actually helps glucose disposal in the body. We’ve evolved to eat more fruit than cereals, and our physiology is more suited to that.