• Intermittent fasting is an approach to eating that alternates between periods of fasting and eating.
• Fasting periods can be variable and there is no one ‘right’ way to do this.
• If weight loss is a person’s goal, then intermittent fasting does not show better results than any other approach to eating less. Some people do well with intermittent fasting, some not so well – like any weight loss diet.
• Intermittent fasting is not suitable for people who are pregnant or breastfeeding, have a history of eating disorders or are taking medications for diabetes.
• For people with conditions such as diabetes or cancer, it is advised to seek professional medical advice to determine if intermittent fasting is appropriate for you.

5 minute read

Intermittent fasting is a recent popular dietary trend. Peoples of some cultures have been fasting for thousands of years for health or spiritual reasons. And each day we ‘break the fast’ with the first meal of the day no matter what time it is eaten.

What has made intermittent fasting so popular is its claims about weight loss and other health benefits. There are also many different ways to undertake fasting, making its flexibility appealing to many people.

There is nothing complex about intermittent fasting. It is simply the practice of alternating periods of normal eating with extended periods of low to no food intake.

Most diets only focus on what to eat. Intermittent fasting is different. It isn’t dogmatic about what foods you should or shouldn’t eat, only when you eat them.

Types of fasting

There are several different ways to do intermittent fasting (also known as intermittent energy restriction). They all share the common theme of splitting the day or week into eating and fasting periods.

During the eating period you should eat normally with a focus on enjoying food and opting for more nutritious choices. Drink water during fasting periods to avoid dehydration.

Some of the common fasting schedules that people use are following, but the list is by no-means exhaustive.

• The 16/8 plan involves eating only during an 8-hour period followed by a 16-hour fasting window. So, if the last meal is at 8pm, the next meal isn't until midday the next day. This way of fasting is known as time-restricted eating.
• The 5:2 diet is another form of intermittent fasting. It involves choosing two days each week where only 2000 to 3000 kilojoules are consumed. The other 5 days of the week are spent eating normally.
• The Eat-Stop-Eat method of fasting involves fasting for a full 24 hours one or two days per week.
• Some advocate for extended fasts that may go for several days but these should only be done under medical and/or dietetic supervision.

There are endless variations to fasting. Such as 12/12 (12 hours of fasting followed by a 12-hour eating window). This is the simplest method as it can mean having the last meal at 7pm and then eating again at 7am the next morning.

There is also 14/10 fasting (14 hours fasting followed by a 10-hour eating window). Or even 18/6 or up to 20/4 (20 hours fasting followed by a 4-hour eating window).

Which way of fasting is best? There isn’t a clear answer to this. Many people find the 16/8 method to be the simplest and most sustainable method to stick to.

Fasting and weight loss

The idea behind fasting for weight loss is that when we eat after an extended fast, we don’t fully compensate for the food we’ve gone without. That creates an energy deficit and results in weight loss.

There have been many human weight-loss trials comparing intermittent fasting against traditional energy restricted diets. Most studies find minor differences in weight loss between approaches.

Intermittent fasting ‘works’ for weight loss simply because you eat less food. Not everyone gets the same results. That means intermittent fasting doesn’t appear to be any better or worse than any other type of energy restrictive diet if weight loss is the goal.

Most behavioural weight loss methods, including intermittent fasting, result in short-term weight loss followed by weight regain. Remember that a healthy lifestyle has benefits regardless of weight. This is where an Accredited Practising Dietitian (APD) can help to ensure you're eating well no matter what your nutrition goals.

Other benefits of fasting

What is novel about the explosion in interest in intermittent fasting is the small but growing clinical research base behind it. Several studies have found improvements in various health markers.

These include:

• decreasing blood pressure
• lowering inflammatory markers
• improving blood cholesterol and lipid levels
• lowering resting heart rate.

However, it is not clear whether changes to these health markers are due to the fasting pattern itself or the overall energy deficit.

Chronic inflammation occurs alongside the development of many chronic conditions. This includes heart disease, type 2 diabetes, some forms of cancer, inflammatory bowel disease and even depression.

Several studies have found intermittent fasting can decrease blood markers of inflammation. This means in the future it may be found useful in treating inflammatory conditions.

Outside of weight loss, intermittent fasting may have an extra advantage on insulin sensitivity and abdominal obesity compared to a traditional kilojoule-reduced diet.

There have been claims that fasting helps you to live longer. But these are harder to prove with most of the evidence coming from animal studies.

While some of the benefits of intermittent fasting are flow-on effects from any weight loss, there could be some extra beneficial changes happening. And that introduces the term 'autophagy'.

Autophagy is a natural cellular process by which cells degrade unnecessary or damaged components. This allows the body to regenerate newer, healthier cells. In times of stress and when nutrients are scarce, autophagy is ramped up to provide an alternative source of energy from recycled cellular material.

The autophagy story reads great on paper. But most of the research on it is from animals, rather than humans. So, scientists still do not know how long people must fast to see a significant uptick in autophagy.

Autophagy seems to decline as we age, so the benefits of fasting could be greater in older people. That makes sense because there has been a lot more time for cellular waste by-products to build up.

Not for everyone

Intermittent fasting is not for everyone.

• If you’re underweight or have a history of disordered eating, intermittent fasting is not for you unless under the supervision of a health professional. Intermittent fasting could be a dangerous triggering factor for someone vulnerable to an eating disorder. Eating disorders can develop in people of any age, gender and body type. Learn more about eating disorders. 
• Intermittent fasting is not appropriate for adolescents.
• If you are taking medications to treat diabetes, you should not try intermittent fasting without talking to your doctor first. These medications remove glucose from the blood and without any food, they can do their job too well and put a person at risk of hypoglycaemia.
• For people with cancer undergoing active treatment, they should speak to their medical team before trying fasting.
• You should also not do intermittent fasting if you are pregnant or breastfeeding,

Hunger is the main side effect of intermittent fasting. You may also feel weak, and your brain may not perform as well as you're used to. This may only be temporary as your body adapts. If you experience ongoing adverse effects of intermittent fasting that disrupt your day-to-day life, stop and seek advice from a health professional.

For athletes, there appears no great sports performance benefit while fasting. This is separate to using intermittent fasting for weight loss to help improve the power-to-weight ratio where a performance benefit would be likely. But intermittent fasting is just one in a long-line of ways to achieve weight loss.

Intermittent fasting can work well for some people, but it is not a magic bullet to improved health. It is simply one of many lifestyle habits that a person can choose to adopt. For lasting, long-term health benefits, it is best to find a nutritious eating pattern that you enjoy and can stick with.