OPINION: Intermittent fasting diets involve periods of fasting cycled with periods of feeding. Fasting involves a zero or reduced calorie intake from foods and drinks. Feeding can involve food and drink consumption under strict rules – or not – and can be ad libitum (eating based on your hunger and fullness) – or not.

The term intermittent refers to the fact that the fasting periods are not continuous. Continuous “fasting” diets also exist and, conversely to intermittent fasting diets, involve energy restriction to below-“normal” requirements for continuous, prolonged periods: weeks, months, or more. Of course, you couldn’t fast continuously on zero calories, because you’d starve to death.

Intermittent fasting has been part of some health and religious practices, such as Islamic Ramadan, for thousands of years. It has been – and currently is – linked to living a longer life. More recently, intermittent fasting has gained popularity in weight-loss circles, partly due to both obesity and society’s “thin ideal”.

Intermittent fasting methods

There are many ways to fast intermittently. Each way differs in when and how fasting and feeding are recommended. I list three examples from the various schemes that seem to have gained popularity recently.

The 5:2 Fast Diet™ was created by British medical doctor Michael Mosley and British journalist Mimi Spencer in 2012-3. It involves five days of “normal” eating and two days of a reduced calorie intake of approximately one-quarter of a person’s usual requirement.

This is about 500 calories for women and 600 calories for men. A 500-calorie day might include: a black coffee and a small-medium boiled egg with three asparagus spears for breakfast; one piece of bread with no butter, a slice of ham and some lettuce for lunch; a herbal tea or low-calorie hot chocolate for an afternoon “snack”; and a small piece of fish with 100g of boiled potatoes and 100g of peas for dinner. Which doesn’t actually sound that bad!

There is an online calculator on the Fast Diet website, which can more accurately estimate what one-quarter of energy intake means for you (a heavier body weight and higher levels of physical activity mean a higher fasting day calorie intake). The two fasting days can be one after the other, although they don’t have to be, and you can eat what you like on the five non-fasting days.

Another example is the 16:8 diet, which was popularised as Leangains by US “nutritional consultant” and personal trainer Martin Berkhan. It involves repetitions of the schedule of 16 hours of fasting overnight and in the morning, and then eight hours of feeding in the afternoon and evening.

No calories are ingested during the fasting phase, although foods and drinks with minimal calories are allowed, such as coffee with a splash of milk and “calorie-free” sweeteners or gum. The feeding phase can also involve calorie intake reductions, depending on weight-loss goal, starting body weight and exercise habits.

Leangains – as the name suggests (lean refers to muscle tissue) – targets fitness enthusiasts by providing specific guidelines on pre- and post-exercise nutrition. There are suggestions as to the amount of calories and protein consumed before and after a workout. Apparently, Hugh Jackman followed this way of eating in preparation for his role in Wolverine.

The final example is Canadian Brad Pilon’s Eat Stop Eat, where the fasting period lasts for 24 hours and is carried out once or twice per week. Similar to Leangains and 5:2, it involves calorie reductions tailored to your weight loss aim, body weight and physical activity.

Similar to Leangains, you are required to do resistance training to build muscle mass. In contrast to the fasting periods in Leangains, but similar to 5:2, you still actually eat – albeit a very much reduced calorie intake.

The evidence on intermittent fasting for weight loss

It is well known that prolonged and severe dietary energy restriction (such as not eating anything or eating very little for many days/weeks) can result in changes to your body’s physiology that will make it more efficient at using any calories that you do give it. That is, it goes into “starvation mode” (also called the “famine reaction”), where post-energy restriction re-feeding results in more weight and fat being recovered in feeding than was lost during starvation.

This is because the human body is a beautiful product of evolution, having adapted to periods of famine. The problem is, those of us who are lucky enough to have plenty to eat – at all times – need to avoid this adaptation if trying to lose weight.

Several 2015 reviews outlined below report that intermittent fasting can result in successful weight loss. This means that, overall, intermittent fasters apparently don’t compensate during feeding periods for the reduced calorie intake of the fasting periods.

One 2015 review summarised studies that investigated the effect of intermittent fasting on body weight and other health markers over a maximum of about six months.

It reported that both alternate-day and whole-day (5:2 and Eat Stop Eat) fasting reduced body weight and body fat in normal-weight, overweight and obese people; but that research on the less-than-24-hour, time-restricted feeding schedule (like 16:8 Leangains) “is limited, and clear conclusions cannot be made”. However, remember that weight loss can occur on a number of dietary regimes, including those that don’t advocate intermittent fasting.

One 2015 systematic review of 40 clinical trials involving intermittent fasting periods of one to seven days over about three months (but up to about two years) concluded that intermittent fasting methods were valid – but not superior – ways to lose weight compared to continuous energy restriction. That is, consuming 500 calories on Tuesday and Thursday and then “normal” calories the rest of the week may not mean that you lose more weight compared to if you ingested 1,400 calories every day that week.

Another 2015 systematic review outlined how three higher-quality randomised controlled trials on intermittent fasting over three months all reported that their participants lost weight.

So, it seems that the evidence suggests intermittent fasting is effective for weight loss in the short term – just like other popular diets. However:

substantial further research in humans is needed before the use of fasting as a health intervention can be recommended.

This is partly due to the lack of clinical studies of more robust design that look at long-term weight loss (over many years), and partly due to the negative aspects of intermittent fasting.

Intermittent fasting may make you feel very hungry, or “hangry”; and may be quite impractical for some people. You wouldn’t want to be in a fasting period during a wedding, for example.

Intermittent fasting may also lead to malnourishment if someone already has a poor diet or takes it to the extreme. Additionally, it may not be suitable for pregnant women and people with specific health conditions, such as diabetes or a history of eating disorders. I know I would struggle being so strict with my eating pattern with my history of bulimia.

The verdict?

The evidence suggests that intermittent fasting can be an effective way to lose weight over months, and may work for some. However, one size does not fit all, and it could be another’s idea of hell. It may not be superior to other methods of weight loss and may not work in the longer term. More research is needed, including on the long-term effects of intermittent fasting and the effects of combining intermittent fasting with exercise.

Rebecca Charlotte Reynolds is a nutrition lecturer at UNSW.

This opinion piece was first published in The Conversation.