In response, our pancreas releases insulin, a hormone that prompts the liver and muscles to take sugar out of the blood and turn it into glycogen. Glycogen is a complex sugar the body can quickly break down when needed. Think of it as your short-term glucose battery, available for discharge when you need more energy, says Professor Garron Dodd, head of the Metabolic Neuroscience Research Laboratory at the University of Melbourne.
The stomach slowly empties food into the intestine (at a rate of about one to four calories a minute, faster for liquids), and sugar slowly seeps into the blood. Eventually, all the sugar is absorbed from the meal and the insulin has turned much of it into stored glycogen (you can see this rise and fall in the graphic below).
As our blood sugar levels fall, we need to slowly release that glycogen to keep blood sugar in balance. This is accomplished by another hormone produced in the pancreas, glucagon, which increases blood sugar levels by triggering the breakdown of stored glycogen in the liver (it provides us with sugar overnight, hence why we don’t starve to death while sleeping).
“The whole idea is to keep blood sugar in a very narrow range. These things are opposing forces, and they have to be balanced very carefully,” says Associate Professor Neale Cohen, head of the Diabetes Clinical Research laboratory at the Baker Heart and Diabetes Institute.
Eat a nice, healthy meal – some protein, some carbs, some fibre – and blood sugar and insulin levels rise and then fall slowly. As they drop, glucagon comes online to trigger the release of stored energy, so there is no crash landing.
A spanner in the gears
You can see how blood sugar is kept in tight control. Some modern, processed foods – like chocolate bars – throw a spanner in the gears.
Protein (chicken, beef, fish) and vegetables have a low glycaemic index – they are complex and take a long time for our digestive systems to break down.
Highly refined carbohydrates, such as chocolate bars, have a very high glycaemic index: the sugar hits the bloodstream faster and all at once.
Blood sugar rises quickly and peaks higher.
In response, insulin rises quickly and glucagon drops (insulin is a suppressor of glucagon). The sugar is quickly cleared from the bloodstream.
But the system is not designed to move this fast. After the sugar goes, your blood remains awash in insulin.
And there is no glucagon to encourage the release of stored sugar. Blood sugar, previously high, now crashes precipitously.
The study below is small, but gives us fascinating real-world data. Researchers tracked glucose, insulin and glucagon in the hours after eating low, medium and high-GI meals (fried eggs and vegetables, porridge, or instant oatmeal; click through the results).
The instant oatmeal generated the highest, fastest spikes in blood sugar and insulin. Four hours after the meal, they had the lowest blood sugar.
And the instant-oatmeal eaters were hungrier sooner. When they were given the chance to order more food, they ended up eating 81 per cent more total energy compared with the group who had a low-GI vegetable omelette.
Researchers call this rapid up-and-down the “glucose dip”. In a study of 1070 people given standardised breakfasts (high carbohydrate, high protein, high fat), researchers found those with the biggest glucose dips were hungrier sooner, ate their next meal sooner, and ate more calories overall (you can see that dip in the graphic below, between the red and green sections).
What meal caused the sharpest glucose dip? The one that caused the sharpest glucose rise – in this study, drinking straight sugar. Not dissimilar to eating a chocolate bar, I suspect (I should note here this idea is controversial; at least one large randomised controlled trial failed to find a relationship between fast-digesting sugars and hunger).
Let’s say I want to avoid all this and eat something that keeps me fuller for longer. What’s a better afternoon snack? A few years ago, Australian researchers tried to answer this question by developing a “satiety index”.
Bottom of the list? Mars bars. Top scorers? Fruits and porridge.
The Examine newsletter explains and analyses science with a rigorous focus on the evidence. Sign up to get it each week.
