Starch, known in some circles as a controversial carb, is often labelled as either good or bad. It turns out, the way starch is digested determines its nutritional qualities and its effect on our health.
The importance of starch to humans dates back to the Palaeolithic era. Researchers believe starchy foods from roots and tubers might have had a crucial role in the evolution of modern humans (Homo sapiens) from their early hominin ancestors.
Around one million years ago, humans began to consume starchy plant foods regularly, possibly thanks to the discovery of the cooking process. Around the same time, genetic variation equipped humans with multiple copies of the salivary amylase gene (AMY1), which initiates the digestion of starch in the mouth.
University of Sydney researchers hypothesis that the increased availability of dietary starch led to an expansion of the human brain.
Today, starch is found in many staple foods and is the main glycaemic (glucose-releasing) carbohydrate in human diets, contributing to 50 to 70% of dietary energy.
Raw starch is poorly digested.
“We need to cook our food to be able to eat it,” said Professor Emeritus Les Copeland AM, an agricultural chemist who has studied starch for 40 years at the University of Sydney.
The application of heat and absorption of water disrupts, at least partially, the starch structure, making it more digestible. But not all starch is digested the same way.
Starch is a polymeric carbohydrate consisting of numerous glucose molecules joined by glycosidic bonds. Some starch molecules are linear, and others have a more complex, tree-like structure.
The way these two are combined makes some starches less digestible than others, so they take longer to go through the body.
Rapidly digested starch (RDS) is found in highly processed foods such as many breakfast cereals and white bread. It’s digested chiefly within 20 to 30 minutes, releasing glucose and setting off rapid insulin response.
“Over time, exposing yourself to this sort of rapid release of glucose increases the risk of health issues such as diabetes and obesity,” Emeritus Professor Copeland said.
That does not mean we should be avoiding RDS altogether.
Glucose is an essential energy source for all tissues, especially the brain, kidneys, red blood cells and reproductive tissues. The brain alone uses about 25% of the total energy expenditure even though it accounts for less than 10% of body weight.
Glucose is also the primary energy source for foetal growth, and higher starch intake during pregnancy and nursing is essential.
Slowly digested starch (SDS) takes longer to break down and moves from the stomach to the small intestine, largely intact. This type of starch is found in whole grains, legumes and starchy nuts.
SDS digestion results in a slower release of glucose and consequently a moderated insulin response. Also, emerging evidence suggests that remnants of SDS that reach the ileum - the junction between the small intestine and the colon - trigger the release of hormones that make us feel fuller.
Emeritus Professor Copeland, who in 2020 received the F B Guthrie Grain Science Medal, which recognises outstanding scientific achievement and contribution to knowledge in the field of grain science, said an active area of research focuses on resistant starch.
This is found in starchy raw food like green bananas, some nuts and seeds, or that has been refrigerated after cooking like potatoes.
Resistant starch granules are often encapsulated into bulky structural material, which renders them hardly digestible.
Researchers have discovered that resistant starch has a vital role in human health because it reaches the colon - the last section of our intestine - and becomes food for gut bacteria.
“The gut has a colony made up of thousands of different species of bacteria that work in a collaborative way to break down resistant starch and draw nutriment from it,” said Emeritus Professor Copeland.
“They grow and proliferate. The health and richness of this community of bacteria are vital, and when that balance is lost, that is associated with illness.”
Emeritus Professor Copeland said what we eat has a profound effect on our health, but “if something is good for you, it doesn’t mean that more is better for you".
Instead, he said it is crucial to find balance in our diet of different foods that contribute to body health and functions.
“We eat meals, not food.”