In recent years, fasting diets for weight loss have become increasingly popular. And it’s easy to see their appeal. They don’t necessarily omit any particular food groups, or even require much research, and their relatively simple central concept - restricting calorie intake during certain hours or on certain days - arguably makes them easier to grasp and maintain than other diets.

But, other than weight loss, can fasting have other health benefits?

Some experts think so; preliminary studies have suggested that following a ‘fasting-mimicking diet’ (or FMD) could lower the risk of certain diseases and contribute toward longevity.

However, research into this area is still relatively young, so it is perhaps too early to draw any definitive conclusions.

It’s also important to note that for some people with certain medical conditions, fasting may present more risks than benefits; so before embarking on any diet or weight loss programme, it’s crucial to speak to a GP first.

To discuss the subject of fasting further, we spoke to Chloe Hall, dietitian and spokesperson for the British Dietetic Association (BDA).

What is a fasting diet?

It’s important firstly to define what a fasting diet is, and also note that there are several different types.

Fasting diets are based on a concept called ‘intermittent fasting’, and revolve around having cyclical periods of fasting and non-fasting.

Fasting doesn’t necessarily mean not eating at all. In some contexts, it may just mean placing significant restrictions on calorie intake.

One such example is the 5:2 diet.

‘The 5:2 diet has been particularly prominent over the last few years, because of numerous books and coverage in mainstream and social media.’ Chloe notes.

Michael Mosley's 2012 BBC documentary Eat, Fast and Live Longer is widely held as the catalyst behind the 5:2 diet (or ‘Fast Diet’) gaining mainstream popularity.

In short, the 5:2 diet involves eating normally (2,000 calories for women, 2,500 for men) on five days per week, and having two days per week where calorie intake is restricted to a quarter of that (500 calories for women, 600 for men).

As Chloe explains, the plan may seem more attractive to some, due to the uncomplicated nature of it: ‘I think that it appeals because you don’t have to follow a ‘diet’ every day of the week, and this may fit into people’s lifestyles better than a traditional calorie controlled diet.’

The diet has become vastly popular, with proponents claiming that it could improve insulin sensitivity, blood pressure and cholesterol levels.

But support for it wasn’t wholly unanimous. Writing in 2013, NHS Choices pointed out that there was limited evidence to support the safety and effectiveness of the 5:2 diet when compared to other weight loss regimes.

Other diets based on the concept of intermittent fasting include:

  • the 6:1 diet, which applies the same rules as the 5:2 diet, but involves just one day of calorie restriction per week instead of two;
  • the 16:8 (for men) or 15:9 (for women) diet, which involves only consuming calories during an eight or nine hour window each day, and fasting for the other 16 or 15. During the fast period, only calorie-free drinks are allowed;
  • the ‘eat-stop-eat’ diet, which involves a full 24-hour fast, observed once or twice a week;
  • and ‘alternative-day fasting’, which applies the rules of calorie restricted days similar to the 5:2 diet, but instead these days are observed every other day (instead of twice a week), in effect making it a ‘4:3 diet’.

The benefits of fasting

Fasting diets have been written about extensively in mainstream media. A quick Google search will bring up a number of articles extolling their benefits, with references to celebrity names who reportedly follow them.

While the number of scientific studies is still relatively thin, the rising popularity of fasting diets has led to more research into their efficacy, not just as a means of weight loss, but also in promoting wider health benefits.

Initial studies have yielded some quite promising results; however, for a number of reasons, the extent to which fasting can benefit health isn’t yet clear.

The evidence is not conclusive and we are still not completely sure if fasting is beneficial to health.’ Chloe tells us. ‘A lot of the studies have been done in animals and, therefore, aren’t necessarily translatable to humans.’

For example, studies by researchers at the University of South California have tested the effects of periodic low calorie fasting in mice. These found that applying fasting-mimicking diets in mice with multiple sclerosis decreased cell destruction and promoted cell regeneration, and helped to stabilise blood sugar in mice with both type 1 and type 2 diabetes.

Valter Longo, a professor and spokesperson for USC, has warned that while these findings may be encouraging, they are ultimately based on tests in mice; whether they would apply to humans is another matter.

At present, research into the effects of fasting diets on people is quite limited. But, Chloe adds: ‘There is some evidence, however, that intermittent fasting can improve insulin sensitivity and reduce obesity related cancers such as breast cancer.’

For instance, one study undertaken at the University of California reviewed data collected over a period of 12 years, from 2413 participants who were all women who had been diagnosed with breast cancer. It found that women who fasted for less than 13 hours on a nightly basis were more likely to have a recurrence of breast cancer, compared to those who fasted for more than 13 hours on a nightly basis.

Further research into the effects of fasting-mimicking diets on people is ongoing.

In a small study of 71 people, researchers at the University of Southern California have looked into the effects of fasting diets on reducing risk markers for age-related diseases and inflammation, with promising results.

Another study by researchers at Newcastle University has also suggested that extreme low calorie diets could help to reverse type-2 diabetes in some cases. (Once again, the authors stressed the importance of consulting with a doctor first before embarking on any diet plan, particularly in the case of diabetes.)

What is the science behind it?

Fundamentally, as Chloe points out, a fasting diet is quite simply ‘another method of reducing your calorie intake, therefore creating an energy deficit resulting in weight loss.’

As a fat-burning tool, the theory behind intermittent fasting is that it gives the body enough time to burn off glucose stores as sources of energy first, before moving on to fat stores, and subsequently burning these off. When this fat is burned off then, it aids weight loss, as well as helping to lower blood pressure and cholesterol.

This process of burning fat is also thought to help insulin sensitivity, by getting rid of fat around the liver and pancreas.

Professor Roy Taylor, who led the aforementioned study at Newcastle University, has developed the Twin Cycle Hypothesis. In this, he posits that type 2 diabetes is the result of excess fat around the liver and pancreas, and that too much fat in these areas prevents these organs from functioning as they should.

One theory behind fasting-mimicking diets and their proposed positive effects on insulin sensitivity then, is that they encourage the body to burn this fat around the liver and pancreas during the fasting period, which in turn enables both organs to work better.

Is there a right and wrong way to fast diet?

As we’ve discussed, many people may turn to fasting diets because the rules governing non-fasting periods seem relatively liberal when compared to other diets.

But, as Chloe illustrates, fasting diets aren’t a one-size-fits-all solution. As with any kind of diet, some people may take to them more readily than others: ‘Skipping meals may not work for everyone and may leave people lacking in concentration, feeling dizzy, with headaches, bad breath and dehydration.’

Food is a source of water for our bodies, so it’s vital if you are following a fasting diet to drink plenty of fluids and stay well-hydrated.

Important to remember too is that a bad diet followed by a fasting period is still a bad diet. Whether fasting or not, eating healthily and continuing to follow good dietary practices during non-fasting periods remains crucial, as Chloe explains:

Fasting can lead to nutrient deficiencies if done frequently, and if the rest of the diet on the non-fasting days are not nutritionally balanced. Some people use fasting as an excuse to eat large amounts of food on non-fasting days and this can lead to disordered eating.’

‘Eating a healthy, balanced diet is still important as otherwise you could end up with nutrient deficiencies which may leave you feeling tired and unable to do your day to day activities. What we eat is almost more important than when and how we eat, and by eating a nourishing diet rich in fruit and vegetables, we can reduce our risk of a range of diseases.’

The best person to start with if you are considering a fasting diet (or any kind of diet for that matter) is your GP. They can help you determine any potential benefits and risks depending on your individual medical history, offer guidance and, if necessary, refer you to a dietitian for further advice.

‘Fasting can pose a risk to people’s health if they have medical conditions such as diabetes or are at risk of developing an eating disorder.’ Chloe explains.  

‘If you are on medications or have a medical condition such as type 2 diabetes it is important to speak to a health professional before embarking on this sort of diet.’

You can find more information and helpful factsheets about healthy eating, dieting and weight loss on the BDA website.