Intermittent fasting – chances are, if you haven’t tried it yourself, you know someone who has, or does it. While fasting is not a new concept, in recent years it has become a big player in the diet industry, with a range of products, books and gurus claiming that fasting holds the key to longevity, weight control and disease prevention.

One of the biggest issues when it comes to reviewing the effectiveness of intermittent fasting is that there are a number of different regimes, utilised in a number of different ways.

So, in an attempt to understand intermittent fasting regimes in general, we need to take a closer look at what exactly is meant by fasting; whether people actually do it as described, and if so, are there any definitive health- and weight-related benefits?

What is ‘fasting’?

Different cultures and religions have fasted for thousands of years, for a range of reasons. More recently, fasting has infiltrated mainstream diet culture, largely off the back of the success of the 5:2 diet, the regime that suggests followers slash their calories to just 500-600 calories on two non-consecutive days per week.

Researchers investigating periods of intermittent fasting have shown that short periods of severe caloric restriction have beneficial effects on glucose levels in the body, as well as on blood pressure and cholesterol. It was also noted in these early studies that small amounts of weight loss were observed too, likely as a result of an improvement in glucose regulation.

What about the 16:8?

Off the back of the success of the 5:2 plan, the 16:8 plan quickly emerged as an alternative to the 5:2. Utilising the principles of “time-restricted feeding”, the 16:8 encourages followers to consume their daily calories within an eight-hour feeding window.

It appears that extending the overnight fast helps to reduce a number of inflammatory markers in the body, which over time also supports weight loss. The 16:8 most commonly starts after the evening meal and extends overnight so followers do not eat until late morning or lunchtime the next day.

Does fasting achieve weight loss?

Like most diets, whilst the primary benefits of fasting from a cellular perspective are improved metabolic health, which can slow the cellular ageing process, for many it is the potential weight-loss benefits that are of greatest interest.

A recent review of 27 trials found that a range of studies looking at intermittent fasting and its effect on weight loss led to average losses of 1 to 13 per cent of baseline weight and specifically improved glucose control for those participants who also had diabetes. It was noted by researchers that the results show wide ranges of outcomes for those who fast regularly, with it working much more effectively for some followers than others.

While some weight loss is positive for any overweight individual, it should be noted that 1 to 13 per cent weight losses are not significantly greater than that seen in traditional reduced-calorie diets, nor are the losses overly high for individuals.

The pros

One of the standout benefits of fasting regimes is that they are relatively easy to follow.

In the case of the 5:2, you only have to keep your calorie intake strict on two days of the week, and can basically eat normally for the rest of the time. Or, in the case of the 16:8, there is no calorie restriction – you can eat until you are satisfied each day, as long as you wait 16 hours before eating again.

When compared to the daily focus required to achieve consistent calorie deficits as required on a regular reduced-calorie plan, you can see why users may find fasting a lot easier.

The cons

It is safe to say that many people who claim to “fast” do not necessarily “fast”, at all. Rather, the 5:2 can very quickly become more like an 800-calorie diet, as an extra biscuit or coffee or glass of wine slips in. Or, in the case of the 16:8, a cheeky coffee before the 16-hour fast is achieved negates the benefits that come from not eating for this extended period of time.

It should also be said that much of the research that looks at the metabolic benefits of fasting has been conducted on severe calorie restriction, or the 5:2, a much harder regime for some to adhere to than the 16:8.

One of the standout benefits of fasting regimes is that they are relatively easy to follow.

There is also the argument that the negatives of the 16:8 outweigh the benefits metabolically. As the body is programmed according to circadian rhythm, in which we generally burn more calories in the first half of the day, there is an argument that delaying the first meal to extend the overnight fast negatively affects metabolic rate.

Here, followers will often report little to no hunger throughout the morning, before becoming increasingly hungry as the day progresses, and overeating through the afternoon and evening, ultimately negating any weight-related benefits.

Is fasting for you?

There is no doubt that there are a number of benefits for the body when we dramatically cut back our calories on a regular basis. It is better for our digestive system to have a break from constantly eating and reminds us what it actually feels like to be hungry.

It helps to reduce inflammation in the body, and specifically for those with glucose regulation issues, it aids glucose control. But its success rests largely on one’s ability to commit to the fasting protocol consistently. Like all diets, you have to follow it most of the time for it to work, not just for a day or two at a time before reverting to your usual habits.

For individuals who sit down all day for work, and who are not overly interested in food, indeed it may be relatively easy to incorporate regular fasting into their lives. On the other hand, if you are a busy mum with three kids and you go to the gym every day, chances are cutting your calories back that much when your energy demands are high may leave you feeling tired, irritable and battling extreme hunger and cravings, even if it is just for a day in total.

The bottom line

When it comes to metabolic health and moderate weight loss, there are clear benefits associated with brief periods of significant caloric restriction, ie, the 5:2 as opposed to the 16:8, for which the long-term weight loss outcomes are not as significant.

The downside of the 5:2 plan is that this more extreme fasting can be tricky to do in everyday life, especially for those with high energy demands or a long history of following restrictive diets, inducing obsessive food behaviours and binge eating. For this reason, the potential benefits of committing to the 5:2 each week need to outweigh the negatives and it is not a sustainable way to eat for many people.

In saying that, given that most of us routinely eat too much, there are benefits in having a lower calorie day a couple of times each week, even if it is as not as low as 500-600 calories, and for digestive health, to aim for at least 10 to 12 hours without food overnight. Routinely delaying breakfast though, as part of a 16:8 plan, then means you will probably overeat for the remainder of the day, simply negating any of the potential benefits associated with intermittent fasting.