This is an update of a January 2013 NHS Choices article - see here.
Monday January 14 2013
Is intermittent fasting right for you?
Note – this article, originally written in January 2013, was updated in May 2013.
The 5:2 diet is an increasingly popular diet plan with a flurry of newspaper articles and books being published on it in the run up to Christmas 2012 and in January 2013.
The diet first reached the mainstream via a BBC Horizon documentary called Eat, Fast and Live Longer, broadcast in August 2012.
The 5:2 diet is based on a principle known as intermittent fasting (IF) – where you eat normally at certain times and then fast during other times.
The 5:2 diet is relatively straightforward – you eat normally five days a week, and fast on the other two days.
What does a daily 600-calorie diet look like?
A 600-calorie diet could consist of a slice of ham and two scrambled eggs for breakfast and then some grilled fish and vegetables for your evening meal. And of course nothing but water, black coffee and/or green tea to drink.
Champions of the 5:2 diet claim that other than helping people lose weight, 5:2 diet can bring other significant health benefits, including:
improved cognitive function and protection against conditions such as dementia and Alzheimer’s disease
protection from disease
However the body of evidence about 5:2 diet and intermittent fasting is limited when compared to other types of weight loss techniques.
What we don’t know about intermittent fasting
Despite its increasing popularity, there is a great deal of uncertainty about IF with significant gaps in the evidence.
For example, it is unclear:
what pattern of IF is the most effective in improving health outcomes – 5:2, alternative day fasting, or something else entirely different
what is the optimal calorie consumption during the fasting days – the 5:2 diet recommends 500 calories for women and 600 for men, but these recommendations seem arbitrary without clear evidence to support them
how sustainable is IF in the long-term – would most people be willing to stick with the plan for the rest of their lives?
Are there any side effects from intermittent fasting?
Little is known about possible side effects as no systematic attempt has been made to study this issue. Anecdotal reports of effects include:
bad breath (a known problem with low carbohydrate diets)
However, more research would be needed to confirm these side effects and their severity.
If you are fasting, you may want to think about how fasting will impact on your life during your fasting days. You are likely to be very hungry and have less energy and this could affect your ability to function (such as at work), in particular it may affect your ability to exercise which is an important part of maintaining a healthy weight.
Also, IF may not be suitable for pregnant women and people with specific health conditions, such as diabetes, or a history of eating disorders.
Because it is a fairly radical approach to weight loss, if you are considering trying IF for yourself, it is wise to speak to your GP first to see if it is safe to do so.
Evidence about the 5:2 diet
Despite its popularity evidence directly assessing the 5:2 model of intermittent fasting is limited.
But since this article was originally written in January 2013 we have been alerted to research, led by Dr Michelle Harvie, which did look at the 5:2 model.
In one study carried out in 2010 the researchers did find that women placed on a 5:2 diet achieved similar levels of weight loss as women placed on a calorie-controlled diet.
They also experienced reductions in a number of biological indicators (biomarkers) that suggest a reduction in the risk of developing chronic diseases such as type 2 diabetes.
A further study in 2012 suggested that the 5:2 model may help lower the risk of certain obesity-related cancers, such as breast cancer.
The increasing popularity of the 5:2 diet should lead to further research of this kind.
Evidence about other forms of IF
There is some degree of evidence about the potential benefits of other forms of IF – albeit with some limitations.
It should be stressed that our assessment of the evidence was confined to entering a number of keywords into Google Scholar and then looking at a small number of studies which we felt would be useful to explore further.
We did not carry out a systematic review (though arguably, it would be useful for researchers to do so). So the information provided below should be taken in the spirit of us trying to provide an introduction to some of the evidence and science of IF – not an exhaustive "last word" on the topic.
Is there any evidence that intermittent fasting aids weight loss?
One of the most recent pieces of research on intermittent fasting is a2012 study (PDF, 291kb) that recruited 30 obese women known to have pre-existing risk factors for heart disease.
After an initial two-week period they were then given a combination diet of low-calorie liquid meals for six days of a week (similar to Slim Fast diet products) and then asked to fast for one day a week (comsuming no more than 120 calories).
After eight weeks, on average, the women lost around 4kg (8.8lb) in weight and around 6cm (2.3 inches) off their waist circumference.
However, there are a number of limitations to consider when looking at this as evidence that it might be a generally beneficial thing to do for most ordinary people, including that:
These women may have had increased motivation to stick with the diet because they knew their weight would be monitored (this is a psychological effect that slimming clubs make use of).
The women had been told that they were at risk of heart disease. It is uncertain how well most of us would cope with such an extreme diet.
The follow-up period was short – just two months. It is not clear whether this diet would be sustainable in the long term or whether it could cause any side effects.
30 people is quite a small sample size. A much larger sample – including men – is required to see if intermittent fasting would be effective in most overweight or obese people.
Is there any evidence intermittent fasting increases lifespan?
There is quite a wide range of work on the effects of IF on combating the effects of ageing, but almost all of these studies involved either rats, mice or monkeys. One big problem with studies in animals – particularly rodents – is that they are only expected to live for a few years. While this makes them ideal subjects for longevity studies, carrying out similar, more useful experiments in humans, requires decades-long research to gain credible results.
In an unsystematic look at the evidence, we find only one study involving humans: a 2006 review (PDF, 65kb) of an experiment actually carried out in 1957 in Spain.
In this 1957 study, 120 residents of an old people’s home were split into two groups (it is unclear from the study whether this was done at random). The first group (the control group) ate a normal diet. The second group (the IF group) ate a normal diet one day and then a restricted diet (estimated to be around 900 calories) the next.
After three years there were 13 deaths in the control group and only six deaths in the IF group.
This study is again limited by the small sample size meaning that the differences in death are more likely to be the results of a statistical fluke. Also, many experts would feel uneasy about issuing dietary guidelines based on a study over half a century old with unclear methods. It is unlikely that this experiment could be repeated today – denying food to elderly people in residential care is unlikely to be looked at kindly by an ethics committee.
Is there any evidence intermittent fasting prevents cognitive decline?
It seems that all of the studies on the supposed protective effects of IF against conditions that can cause a decline in cognitive function (such as dementia or Alzheimer’s disease) involve animals.
For example, a 2006 study (PDF, 843kb) involved mice that had been genetically engineered to develop changes in brain tissue similar to those seen in people with Alzheimer’s disease.
Mice on an IF diet appeared to experience a slower rate of cognitive decline than mice on a normal diet (cognitive function was assessed using a water maze test).
While the results of these animals tests are certainly intriguing, animal studies have inherent limitations. We can never be sure that the results will be applicable in humans.
Is there any evidence that intermittent fasting prevent diseases?
Much of the published research into the potential preventative effects of IF involve measuring biological markers associated with chronic disease, such as insulin-like growth factor-I (IGF-I) – known to be associated with cancer.
Using these kinds of biological surrogates is a legitimate way to carry out research, but they do not guarantee successful real-world outcomes.
For example, some medications that were found to lower blood pressure readings taken in laboratory conditions failed to prevent strokes once they had been introduced for use in the healthcare of patients in the world.
A 2007 clinical review (PDF, 119kb) looking at the effects of IF in humans in terms of real-world health outcomes concluded that IF (specifically, alternative day fasting) may have a protective effect against heart disease, type 2 diabetes and cancer. However, it concluded "more research is required to establish definitively the consequences of ADF (alternative day fasting)".
Compared to other types of weight loss programmes the evidence base of the safety and effectiveness of the 5:2 diet is limited.
If you are considering it then you should first talk to your GP to see if it is suitable for you. Not everyone can safely fast.