The 5:2 Diet – Fad or Fiction?

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On Monday 6th August 2012 Dr Michael Mosley presented an attention grabbing addition of the BBC Horizon show, entitled “Eat, Fast and Live Longer”(2). The show delved into the science behind fasting, with Michael Mosley using himself as a human guinea pig in an attempt to prove how a diet based around fasting could provide significant health benefits beyond just weight loss.

Fast-forward 12 months and Michael Mosley has certainly done well for himself. With bestselling book releases in the UK and the US, countless television appearances,and his own website (dedicated to helping those on his “Fast Diet”achieve success(3)), he has positioned himself as the apparent guru for all things fasting.

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Mosley promotes a type of dietary change dubbed “intermittent fasting”. This is where a person drastically reduces their calorific intake on two days of the week (down to 500 calories for women and 600 for men) while leaving their intake at the recommended level (2000 calories for women and 2500 for men) for the remaining 5 days.

However, despite its rapid rise to popularity, I can’t help but feel that a real question has to be asked over whether or not there is a solid evidence base behind “The Fast Diet”, and if indeed this is the kind of lifestyle advice that GPs nationwide should offer.

Fasting itself as a concept is nothing new. From Christianity to Buddhism, fasting plays a part in the vast majority of all major world religions (4). Surprisingly, it is only recently that academics have looked into the possible clinical benefits of fasting. One early study (5), which specifically lookedat the impact of fasting during Ramadan, yielded some interesting results. Carried out in 2002 on 83 volunteers, the study showed a decrease in serum LDL-cholesterol and an increase in HDL-cholesterol due to sustained fasting. The study’s authors proposed that fasting, therefore, helped to reduce the risk of developing long-term cardiovascular pathology.


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The vast majority of studies in recent years have focused on the effect of a fasting style diet on those under the “obese” category. The results do seem very promising. Reductions in cardiovascular morbidity (6), decreased blood pressure and LDL-cholesterol levels (7) and a possible protective effect against breast cancer8 have all been shown. After reading through a variety of these studies it is poignant that the majority of them have only used animal models so far. On top of this, many of the studies admit in their conclusions that their own evidence is not conclusive enough to recommend intermittent fasting over other dietary regimes and that generally more evidence is needed.

I have to admit, on the face of it, fasting makes a huge amount of sense. Basing our diet on that of our hunter-gatherer ancestors, who would feast and fast depending on their success in finding themselves food, may possiblybethebestdietary match for our evolutionary development. But sadly the fact is that there is not yet enough evidence to propose that fasting is beneficial for the population as a whole. If you browse the Internet, good luck finding a relevant Cochrane review or even systematic review of the evidence in support of intermittent fasting.

It will be really interesting to see the conclusions drawn by more long-term studies looking at the impact of intermittent fasting. A longer, healthier, life is something we all aim for, but in an NHS heavily focused on acting under the remit of Evidence Based Medicine, I can’t imagine that clinicians will be breaking away from their current dietary and lifestyle advice any time soon.

One thing that cannot be denied in all of this is that Michael Mosley, using the power of mass media, has successfully managed to get vast swathes of the UK and US population to at least try his “Fast Diet” on the basis of, largely, his own anecdotal evidence. So at least until more research is carried out, perhaps the most poignant message to take away from this case is a reminder of the responsibility and influence that the ‘Doctor’ title brings.


1. BBC Horizon “Eat, Fast and Live Longer” <>st (Accessed 31 July 2013)

2. <; (Accessed 31st July 2013)

3. Fasting Chart [Online] Available at: <http:// Chart.aspx> (Accessed 31st July 2013)

4. Effects of Ramadan Fasting on serum low- density and high-density lipoprotein cholesterol concetrations [Online] Available at: <http:// 225_226/01-247.pdf>

5. Intermittent fasting: a dietary intervention for prevention of diabetes and cardiovascular disease? [Online] Available at: <http://>

6. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women [Online] Available at: < PMC3017674/pdf/nihms224118.pdf>

7. Energy restriction and the prevention of breast cancer [Online] Available at: <http:// %2FPNS %2FPNS71_02%2FS0029665112000195a.pdf &code=1c9a4ba83281e2533e3ad43a422129d3 >

8. (Image) Intermittent Fasting [Online] Available at health/52shutterstock_113201323copy.JPG

9. (Image) The 5:2 Diet Book [Online] Available at kate-harrison/prod9781409146698.html



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