Fasting has been practised across civilisations, throughout history, and in various religions and cultures.
Millions of Muslims across the world are currently observing Ramadan, a month of fasting each day from before sunrise until sunset.
In Australia, this equates to fasting for roughly 14 hours.
But restricting eating for specific time periods isn’t just a religious practice, it’s also an approach used in intermittent fasting.
So what exactly happens in our body when we fast?
We asked neuroscientist Mark Mattson, an intermittent fasting expert from John Hopkins University in the United States who has published his research in several medical journals, to break down the process.
According to Professor Mattson when you finish a meal and begin fasting, blood glucose or blood sugar levels will be higher than normal for about two hours because of the carbohydrates in the food.
Your body turns carbohydrates — nutrients that exist in food as fibres, starches or sugars — into glucose or blood sugar that cells in the human body use for energy.
Most of that blood sugar moves into the liver where it is stored in liver cells and used as an energy source in the ensuing hours.
“The glucose that’s in the liver will last a typical person about 10 to 12 hours and then it’s depleted,” says Professor Mattson.
When those stores become depleted, what happens next?
After glucose stores are used, cells in the body turn to a different source for energy: fat.
Fat cells release fat or fatty acids into the blood, which are converted in the liver to acids, known as ketones.
“Ketones are then used as an energy source for cells,” says Professor Mattson.
There are two ketones that cells can use for energy.
One is called acetoacetate, and the other is called beta-hydroxybutyrate (BHB), explains Professor Mattson.
“These ketones are particularly important during long-term fasting for … your muscle cells, your heart cells, and the nerve cells in your brain and elsewhere,” he says.
At 14 hours after fasting begins, “good things” start to happen in the body as ketone levels rise, says Professor Mattson.
In response to the metabolic switch to ketones, at least a dozen different proteins increase in the body.
“Those proteins do a lot of good things for cells that make them function better and resist stress and disease,” he says.
“Ketones can reduce appetite, and they reduce oxidative stress and inflammation levels throughout the body and brain.”
The human body’s natural recycling system also switches on at this point, he says, and a process called autophagy begins.
Autophagy is when your body cleans out damaged cell parts.
The BHB ketone does this work, as it “turns on” certain genes in cells that make cells more stress resistant, says Professor Mattson.
Those genes also “remove dysfunctional and damaged molecule(s)” in the cells, he explains.
Fasting not for everyone
Professor Mattson, author of The Intermittent Fasting Revolution: The Science of Optimizing Health and Enhancing Performance, told the ABC that intermittent fasting reduced levels of abdominal fat, suppressed inflammation, and protected against cardiovascular disease, cancers, and neurodegenerative disorders.
Intermittent fasting has gained popularity due to these claims, but some experts say further research is needed.
Many experts say intermittent fasting is not a diet. Rather it is focusing on when to eat — more of an eating pattern.
It’s also generally agreed that fasting isn’t for everyone.
Australia’s peak body for dieticians, Dietitians Australia, says on its website that people with a history of disordered eating, pregnant women, young people, athletes and those with certain medical conditions should not fast.
See a dietitian if you need advice and support, says the website.
