Intermittent fasting : what is it, and how does it work?

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5 mins read

5:2 or 16:8? Can it help you lose weight? Or is it a health fad doomed to fade? We investigate the trend – and dig up the detail – of intermittent fasting.

Intermittent fasting is a popular dieting trend, often promoted as a great way to lose weight  . But is it just another food myth or a genuine healthy choice?
 
While many of the health and long-term benefits of intermittent fasting are still going through research, some studies have so far shown that it could play a role in managing weight1 – and it might also reduce risk factors for some diseases2 and improve heart, brain, and blood health3.
 
So, what is it? Is it safe for everyone to try? And can you still enjoy your morning flat white? We break down the facts beyond the fad.

What is intermittent fasting?

Intermittent fasting is about making a schedule for when or how much you eat, not what you eat.
 
It’s not a completely new way of eating or dieting: different religions have incorporated some form of intermittent fasting for centuries. Our hunter-gatherer ancestors did it out of necessity long before delivery apps brought meals to our doors at any time of day or night.
 
There are many ways to try intermittent fasting, but generally it’s a pattern of alternating between periods of eating and fasting. Some methods restrict eating time per day or per week, while others restrict kilojoules or the number of meals.
 
Unlike some diets, intermittent fasting doesn’t ban any type of food. So you can still try yummy new things while making sure you support your body’s defence system by filling up with all the right nutrients.

How does intermittent fasting affect your body?

When you give your body a break from digesting food, it has the chance to focus on a few other processes.
 
Fasting triggers ketosis, or ‘metabolic switching’4. Basically, when you fast, your body runs out of energy from the last thing you ate and starts to burn through its fat stores instead. If weight management is your goal, research has shown that this metabolic switch can lead to weight loss for some people – and may even improve blood pressure and cholesterol levels5.
 
According to a 2023 study, prolonged breaks from eating might also be an efficient way to kickstart a process called ‘autophagy’6, which is basically the body’s way of throwing out old and broken cells. And although more research is ongoing, the same study shows that autophagy might promote a longer life.

Different methods

You’ve probably heard of the ‘16:8’ diet, but here’s a breakdown of the most common intermittent fasting plans.
 
  • 16:8. This is a daily time-restricted fasting plan. It restricts eating to eight hours each day, followed by a fasting period over the next 16 hours. For example, you might eat between 11am and 7pm and then fast from 7pm to 11am the next morning, meaning you could time a lot of your fasting hours for when you’re asleep.
  • 5:2. This is a weekly kilojoule-restricted fasting plan. You eat regularly for five days a week, and then on two non-consecutive days a week – say, Mondays and Thursdays – you limit your kilojoule intake to around 25% of your normal daily energy needs7.
  • Alternate-day fasting (ADF). This is another kilojoule-restricted fasting plan, and it does what it says on the tin: you fast for 24 hours every other day. On your normal days you eat regularly, and on your fasting days you consume only 25% of your energy needs.
No matter which intermittent fasting plan you choose, it’s important to still get a variety of nutritious foods recommended by the Australian guide to healthy eating8. In fact, a 2021 study of optimal diet strategies for weight loss concludes that increasing fruits, vegetables, protein, and healthy fats – like in a Mediterranean diet – can help with weight loss maintenance9, if that’s your goal.
 
It’s also essential to stay hydrated when you’re fasting. Drink plenty of water. Black coffee, tea and other naturally flavoured kilojoule-free beverages are fine too, but avoid sweetened drinks, alcohol, and milk.
 
Yep, that means your flat white is a fasting no-no.

Health benefits of intermittent fasting

Early studies into intermittent fasting suggest several potential health benefits for your body and brain.
 
  • Weight loss. By restricting when you eat and how much you eat, you reduce your kilojoule intake – and we know that the chances of losing weight are higher when we burn off more kilojoules, through exercise, than we consume10. Research into the long-term weight loss benefits of intermittent fasting continues, but some studies suggest that reducing the eating window can help shed kilos11. And compared with calorie-counting diets, intermittent fasting has been shown to be just as effective at promoting weight loss for people who are overweight or obese12. It may even do a better job at reducing fat mass13. But as with any weight loss diet, results can vary14.
  • Lower blood sugar. Researchers have found that intermittent fasting can reduce blood sugar and, with guidance from a GP, may protect against diabetes15. Time plays a part here, too: eating earlier in the day improves insulin sensitivity16, which helps the body control blood sugar levels. For similar reasons, try to avoid eating before bed.
  • Brain health. Early studies show that intermittent fasting can improve learning and memory17 and potentially protect against Alzheimer’s disease18. A great reminder to keep showing the gut-brain connection some love.

Keep in mind

Intermittent fasting can be a safe diet with great health benefits, but we all have different health goals, experiences, and routines. Setting realistic goals and taking small steps are as important to our wellbeing as moving and eating well.
 
Speak to a health professional first if you’re pregnant or breastfeeding, are diabetic, have a history of eating disorders, or have any health conditions.
It’s also worth getting professional health and fitness advice before you try any intermittent fasting plan so you can keep up a balanced diet and tweak your workout routine (and workout playlist) to suit your fasting windows.
 
Then start gradually, and stay flexible. Instead of jumping headfirst into a strict fasting schedule, ease into it with small adjustments over time, such as gently extending your fasting window each day or week, or reducing your kilojoule intake bit by bit. This should help your body adjust to fasting and minimise any typical side effects, such as hunger, fatigue, or headaches. It might take time to notice the potential health benefits, so be patient.
Footnotes

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