Diet Debate: Not So Fast? Intermittent Fasting Update

It has been three years since I wrote my last post on Intermittent Fasting, so I thought it was about time for an update. In this follow up post I want to address:

  • Dr Michael Mosley’s Horizon Documentary and book “The Fast Diet”
  • Controversy over research on women and fasting
  • An update on my experiences after 4 years of experimentation
  • New research on protein fasting as an alternative
Are you Still Hungry for Success?

Michael Mosley and The Fast Diet

In August last year, Intermittent Fasting exploded into the mainstream thanks to Michael Mosley’s Horizon documentary Eat, Fast, and Live Longer, which he later followed up with a book, The Fast Diet.

Dr Mosley did a great job in helping to dispel the myths of the need to eat little and often, and never skip breakfast, and I was extremely pleased to see the concept of IF introduced to a wider audience.

I do have a few criticisms to add however.

  1. No attention to food quality

The TV message was: IF is all you need to reduce your gut, and live longer. When you’re not fasting, you can eat any rubbish you like.

Going from 21 junk meals per week, to only 17 is probably an improvement, but it can hardly be classed as a panacea.

Yes, reducing calorie intake and body-fat are two of the most important things you can do for your health, but there is still a lot more to health than simply being slim.

Personally, I’d rather see people switch to three meals per day of low energy, nutrient dense foods, than simply fasting for a couple of days per week while continuing to guzzle down empty calories, transfats and who knows what else the other five days per week.

On the flip side, however, I can see that fasting is a quick, easy and accessible change for people to make, and it is certainly better than making no change.

  1. Over playing the difficulty

In the documentary, Dr Mosely starts with a fast of several days, which, as you can imagine, is not fun.

Even when he does get into the “5:2” diet, he decides to fast on a Saturday when he is meeting his friends and family for a pub lunch. Brilliant idea…

Again, this is TV, so everything needs to be over egged for the camera, I get it. But do we really need to presume the average viewer has a sub 50 IQ? (Though as Horizon’s latest “science” program involves following pet cats with secret cameras it seems they are now shooting well below even this, but I digress).

I personally don’t find fasting to be a struggle, and never have – just start off small, gradually build up the duration, and do it during the week when you’re busy. “The Devil Makes Work for Idle Gums” as I like to say.

I guess that’s just too boring and sensible for TV though!

  1. He doesn’t actually fast

Mosley’s version of “Fasting” is actually limiting calories to 600 (500 for women) which can be eaten at any time, and spread throughout the day if necessary. Really, I think this should be termed “Intermittent Calorie Restriction”, as eating, even if it’s only a small amount, can’t really be classed as fasting.

This is not to say that this approach is necessarily wrong – as we’ll see later in this post there are many ways to skin a cat (not sure if that will be featured in Horizon’s new series), and there’s still a lot we don’t know about fasting – how it works, or which, if any, way is best?

  1. His diet appears to replicate Eat Stop Eat without giving credit

Now perhaps he came up with the idea for “The Fast Diet” completely independently, it’s not impossible. There is absolutely no way, however, one could have done all the research necessary to produce the documentary and subsequent book, without coming across Brad Pilon and Eat Stop Eat, which is essentially the same as the Fast Diet, but published in 2007.

I am certainly not saying that Mosley shouldn’t have written his book – Eat Stop Eat is an E-Book aimed at the US market, so I think there was a market for a printed book for the UK – but it just seems a little disingenuous to make absolutely no reference to it whatsoever.

I may have been more likely to purchase a copy of Dr Mosley’s book, if he’d acknowledged the preexisting version of the diet, given credit where it was due, and perhaps outlined where his version differed/improved on the original.

That he didn’t do this, actually makes me think that he probably hasn’t added anything or improved upon Eat Stop Eat, and that in all likelihood, the fact that ESE is now into it’s 5th Edition and Brad has had many more years experience with the diet, will make it a much more comprehensive guide.

Of Mice and Men Of Rats and Women

Around the same time as the Horizon documentary came out last year, there was also a bit of a furore around women and fasting, largely due to this article on Paleo for Women by Stefani Ruper.

The article highlights a few studies that might raise a red flag for women and fasting – Two rat studies1,2 and one study in humans3.

Let’s take the rat studies first – Essentially, the female rats appear to go into a kind of “Starvation Mode”, during which Stefani notes “They stop ovulating and menstruating. They become hyper-alert, have better memories, and are more energetic during the periods in which they are supposed to be asleep

Could Intermittent Fasting cause women to stop ovulating and trigger sleep problems?

Before we get too alarmed, let’s look at the actual protocol to which the rats were subjected.

The first key point, is that the rats were actually doing ADF – Alternate Day Fasting – rather than IF – Intermittent Fasting. The majority of popular IF protocols involve fasting for periods of 16 to 24 hours maximum. The rats, however, were given their food on the fed day at 10.00am, then not provided with any more food until 10.00am, 48 hours later.

This is a very long fast by anyone’s standards – a lot longer than recommended in any of the popular IF regimens.

It should also be noted, that the food was given at 10.00am, which for the rats, who are normally nocturnal, is pretty similar to giving humans food at 10.00pm.

Picture this scenario – you eat at 10pm and go to bed. The next day you get no food whatsoever. Come 10pm you are pretty damn hungry, but have to go to bed without food. You then wake up the next day, still famished, but have to go until 10pm again without eating anything. When you do get the food, you’re given a whole days worth in one sitting.

Do you think that perhaps you might be a little stressed? That perhaps your sleep patterns might be a bit disrupted? (It has previously been suggested that the timing of meals when breaking fasts can have a critical effect on circadian rhythms4).

Let’s add into this that you’ve not opted to do this, but this fasting regime has been forced upon you, you’ve no idea why, or how long it’s going to last, you’re being made to run through mazes to test your memory, oh and don’t forget you’ve been genetically engineered for metabolic derangement.

I suspect that being forced to fast for 36-48 hours, has a very different stress effect than choosing to fast for up to 24 hours, as may the timing of the meals.

The study in humans looked at the effect of alternate day fasting (36 hour fasts) on 16 non-obese subjects, 8 men and 8 women over 3 weeks. The downside for the women was that after the 3 weeks their glucose tolerance was found to be slightly impaired3.

A follow up study by the same researchers however, did see a potential benefit for the women – an increase in HDL cholesterol. Conversely, men didn’t see this result, but did see a reduction in triglycerides which the women did not5.

These studies certainly show that men and women react differently to fasting, but I’m still reluctant to draw any conclusions as A) The study group is very small, B) The study was only 3 weeks long C) The study again looks at 36 hr Alternate Day Fasts rather than the more popular Intermittent Fasting protocols.

What we need is a study looking at a large group of women, following a protocol such as the Eat Stop Eat/Fast Diet, (where calories are restricted just two days per week, and no one goes without food in excess of 24hrs) over a longer time period.

Fortunately, Harvie et al have done just that in this study here6 where a large group (107) of obese women, were randomly assigned to either a calorie restriction group (1500kCal per day) or an intermittent fasting group (just 540kCal 2 days per week) and the results tracked over 6 months.

Actual women, following the actual diet. Now we’re getting towards the type of research one can actually start to draw some meaningful conclusions from.

Results: Last observation carried forward analysis showed IER and CER are equally effective for weight loss, mean (95% confidence interval [CI]) weight change for IER was −6.4 (−7.9 to −4.8) kg vs.−5.6 (−6.9 to −4.4) kg for CER (P value for difference between groups = 0.4). Both groups experienced comparable reductions in leptin, free androgen index, high sensitivity C-reactive protein, total and LDL cholesterol, triglycerides, blood pressure and increases in sex hormone binding globulin, IGF binding proteins 1 and 2. Reductions in fasting insulin and insulin resistance were modest in both groups, but greater with IER than CER; difference between groups for fasting insulin −1.2 [−1.4 to −1.0] μU/ml, and insulin resistance −1.2 [−1.5 to −1.0] μU/mmol/L (both P=0.04).

Conclusion: IER is as effective as CER in regards to weight loss, insulin sensitivity and other health biomarkers and may be offered as an alternative equivalent to CER for weight loss and reducing disease risk.6

So here we have some positive results for IF – positive at least for overweight women looking to lose weight and improve their lifestyle related disease risk over a period of 6 months. This is of course still just one study, looking at one demographic, over what is a relatively short period of time.

Based on the currently available research, here are my thoughts on Intermittent Fasting for women:

Fasting for Fat Loss

Intermittent Fasting can definitely be an effective fat loss strategy. Though there is no hard evidence that it is any more effective than any other means of calorie restriction, if it suits your personality and lifestyle, it can certainly work.

A few caveats however:

  • Don’t over do it. Don’t go more than 24hrs, and ensure you eat at least 500kCals on a fast day
  • Eat a nutrient dense, varied diet. Don’t use fasting as an excuse to binge on junk
  • Remember fasting is a stress factor – if you have lots of stress from other areas, give fasting a miss until you’ve addressed these issues

Fasting and Pregnancy

Fasting whilst pregnant or trying to conceive would most likely be a very bad idea.

That said, so would any other form of calorie or dietary restriction.

It is well documented, that being overweight or obese during pregnancy can have severe repercussions on the future health of the baby. In an ideal world, it would be best to reduce weight and improve health prior to conception, and IF could be one way to do so.

The world is not an ideal place, however, so if one did find oneself pregnant and overweight, it is probably prudent to eat a varied whole food diet comprised predominantly of pasture raised meats and organic fruits and vegetables to satiety. I’ve argued before that it’s not necessarily excess body fat which is the problem, but the diet and lifestyle which contribute to it. Aggressively trying to shed the fat is unlikely to help a growing baby, but providing it with low toxin, nutrient dense food will!

Other Women

I’ve heard it alleged that there are women out there who neither want to lose weight, nor have babies…

Intermittent Fasting has not only become popular due to its effectiveness for fat loss, but due to its purported benefits for health and longevity – IF, it is claimed, will bring all the benefits of CR, but without the negative side effects.

In these studies looking at women and female rats, however, there didn’t appear to be any significant difference between IF and CR. In the obese women, both IF and CR produced the same weight-loss, and same improvements in health. In the rats, caloric restriction resulted in the same loss of fertility and negative stress responses.

One of the theories behind the mechanism by which IF and CR promote longevity, is by triggering the body to go into conservation mode: Resources are scarce, therefore keep going until times of plenty at which point replicate via the mode of sexual reproduction. Perhaps the trade off for a longer life, could be reduced fertility?

This is pure speculation, however, and I’d have to concur with Stefani in calling for much more research on the topic.

My Fasting Protocol

Though I do still practice Intermittent Fasting, I am now much more casual about it than I was when I first started.

I initially began with the eat stop eat approach of 2 x 24 hr fasts, and then later gravitated to a LeanGains style 8 hr eating window. In both cases I was initially quite obsessional (as is my nature), with not letting one morsel of food pass my lips, and sticking precisely to the time frame.

I now tend to cycle between the two – alternating between periods of just having two meals per day, but at no particular fixed time, to generally having three meals per day, but having two very low calorie days per week.

On the later, this may still sometimes entail a total fast until the evening, or I may have a few light snacks during the day.

In either cycle, if I really want to eat – either because I’m particularly ravenous, or perhaps something unexpectedly tasty has just been put on offer, I’ll eat it. It’s a health habit, not a religion after all!

Protein and Fasting

There is a growing body of research which indicates that the potential health and life extension benefits of Intermittent Fasting and Calorie Restriction, may actually be a result of reduced protein intake, and the effect this has on a process called autophagy.78,9

According to Mizushima et al:

Autophagy, or cellular self-digestion, is a cellular pathway involved in protein and organelle degradation, with an astonishing number of connections to human disease and physiology. For example, autophagic dysfunction is associated with cancer, neurodegeneration, microbial infection and ageing. Paradoxically, although autophagy is primarily a protective process for the cell, it can also play a role in cell death. Understanding autophagy may ultimately allow scientists and clinicians to harness this process for the purpose of improving human health.7

Is it possible that one can get the potential health improvement, and life extension benefits of fasting, but without actually fasting?

With a protein fast, rather than consuming nothing at all, one just restricts oneself to carbohydrates and fats. One could snack on fruits and vegetables, or have a salad dressed with olive oil, drink coffee with cream (or butter?), basically have anything that is low in protein.

The fact that there is no restriction of calories, may well make this the best option for women (and perhaps men) who do not have body-fat to lose, and/or have other stressors which cannot be avoided, but still want to try and reap the potential health benefits of IF.

I mentioned earlier that I am not as concerned about abstaining from all food for the full 24 hrs when on an Eat Stop Eat type cycle – what I will do, however, if I feel the need to snack on something during a “fast day” is stick to fruits and veggies, and/or coffee with cream.

Incidentally, coffee is the faster’s friend. Caffeine both blunts appetite and increases fat burning, double win. Better yet, caffeine is fat soluble so adding some cream boosts its effects. There is even some (very preliminary) evidence that caffeine may help drive autophagy, and this might be one of the mechanisms through which it helps reduce mental deterioration and mortality rates!9

The polar opposite of protein fasting is a Protein Sparing Modified Fast which I first came across in Lyle McDonald’s Rapid Fat Loss Handbook and have had great success with over the years.

This may be the better option for the individual who is more concerned with losing a large amount of fat over the short term, but who struggles too badly with hunger cravings whilst fasting, and also has a lot of other stress to deal with.

During a Protein Sparing Modified Fast, an individual just eats foods which are high in protein, but low in carbs and fat.

The advantage of a diet such as this are:

  • The removal of carbs and fat greatly reduce the number of calories consumed
  • Protein is the most satiating of the macronutrients, so will greatly reduce hunger
  • Protein is difficult for the body to convert to fat
  • Protein has a high thermic effect of feeding, requiring more kcals to digest than carbs or fat
  • The extra protein helps spare muscle, which can often be lost during periods of extreme dieting

Example meals might be cottage cheese or tuna salads, and lean steak or chicken breast with steamed veggies. Essentially a lean animal food combined with a low calorie/low carb plant food.

This is certainly not a diet I would recommend following long term, due to its high protein content combined with low calorie and nutrient value, but it can certainly be a good short term solution for rapid fat loss, with minimal hunger cravings and muscle loss.

Another viable option could well be cycling between the two – having high protein, low fat and carb days, alternated with low protein, high fat and carb days, or perhaps sticking to carbs and/or fats throughout must of the day, and just keeping protein consumption to a specific window.

I’ve been tending to just stick to one main macronutrient during the day – i.e. one day I might just have coffees with cream, another just fruits, others I’ll have tubs of cottage cheese (usually with some carrot sticks or sliced peppers). Regardless of what I’ve eaten through the day, I’ll then have a mixed meal of animal protein with veggies in the evening, with some additional starches in the form of potatoes or rice if I’ve been training/highly active that day.


Intermittent Fasting, Protein Fasting, and Intermittent Calorie Restriction are all valid fat-loss strategies, that also have other positive effects on numerous markers for disease such as reduced blood pressure, better insulin sensitivity, better lipid profile etc.

It should be noted however, that when it comes to fat loss and short-term health improvement, there is little evidence that these approaches are any more effective than any other form of calorie restriction.

An advantage of IF over the other diets is that it is incredibly simple to understand and follow, and doesn’t require any weighing, measuring, recording or special purchasing or preparation of foods.

A downside of IF, however, is that it doesn’t necessarily pay any heed to food quality, and books and films such as Mosely’s may give out the message that it’s fine to eat junk food five days per week so long as you fast on the other two.

Of course, the same can be said of all the other diets – You can restrict your calories while eating only cakes, live a low carb life at McDonalds, and of course you can scoff jaffa cakes by the bucket load on a low fat routine.

Another potential downside for some, is that the stress of fasting may be too much, leading to a deterioration of health. The same however, could be true for any form of restrictive fat loss strategy. It is for this reason that when fat-loss and/or health improvement is the goal, a well rounded approach that takes into account all lifestyle factors should be adopted.

Intermittent Fasting and Calorie Restriction both significantly increase the lifespans of worms and mice, however results for larger mammals, including monkeys have not been so spectacular.

The research is still really in its infancy, and the effects are far from proven, despite what may be claimed in the popular media and diet books. Though there are numerous signs and indications that there may be long term benefits to IF, until there have been long term human studies, it’s really little more than an educated guess!.

If there are longevity benefits, we also need to find whether these come from the reduction in calories, increased time in the fasted state regardless of total calorie consumption, or from the reduction of protein or any other macronutrient.

Personally, I find the IF lifestyle liberating and enjoyable. It enables me to eat large, high calorie meals when I want to, and frees me up from the worry and hassle of finding and preparing healthy food when out and about or on the go.

It enables me to maintain a physique I’m happy with now, without making any undesirable sacrifices. If it turns out to extend my life, then that’s an added bonus, but it’s not a deal breaker.

If you find IF a struggle; you hate the feeling of hunger and spend your days fantasising about your evening meal, don’t do it! Even if it does extend your life, what’s the point if most of it has been spent miserable and hungry?

When it comes to long term well-being and longevity, eating nutrient dense whole foods, managing stress, getting good sleep, having a good social support network, and spending plenty of time being active in nature are the key factors in being able to Live Now, and Thrive Later

Image courtesy of David Castillo Dominici /

1) Sex-Dependent Metabolic, Neuroendocrine, and Cognitive Responses to Dietary Energy Restriction and Excess Bronwen Martin et al

2) Conserved and Differential Effects of Dietary Energy Intake on the Hippocampal Transcriptomes of Females and Males Bronwen Martin et al

3) Glucose tolerance and skeletal muscle gene expression in response to alternate day fasting Heilbronn LK

4) Effect of feeding regimens on circadian rhythms: Implications for aging and longevity Oren Froy and Ruth Miskin

5) Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism Leonie K Heilbronn et al

6) The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomised trial in young overweight women Michelle N. Harvie et al

7) Autophagy fights disease through cellular self-digestion Noboru Mizushima et al

8) Mitochondrial oxidative stress, aging and caloric restriction: the protein and methionine connection Pamplona R, Barja G

9) Macroautophagy: The key ingredient to a healthy diet? Adrienne M. Hannigan and Sharon M. Gorski

6 thoughts on “Diet Debate: Not So Fast? Intermittent Fasting Update”

  1. Hi – I have just read this and I am wondering why tuna/meat together with salad/steamed veggies are low in nutrients and not suitable for a long term diet? If they aren’t suitable – what else needs to be added?

    1. Hi Barbara,

      Thanks for the comment – firstly, no restrictive diet is good long term. You don’t want to keep losing weight forever, or you’ll waste away!

      Many of the nutrients in animal products are stored in the fats – most notably vitamins A,D and K. Restricting oneself to low fat versions/egg whites/skimmed/trimmed etc can remove a lot of the goodness.

      Also, getting too high a % of your total calories from protein does not seem to be prudent in the long term.

      Eating a mix of muscle meats, along with fattier cuts, organs and bones, and/or full fat dairy, whole eggs, and whole, oily fish/seafood is the best way to ensure you get all the nutrients you need. Equally, adding in starchier veg such as tubers, potatoes, squash etc to get not only a wide range of vits and mins, but also to ensure you get plenty of calories is also good in the long term.

      A low calorie, low fat, low carb, high protein, diet should be a short term measure only.

  2. Pingback: Cyclical Cyclical Dieting? – Live Now, Thrive Later

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