Intermittent Fasting and the Fast Diet

The question that I am being asked more than any other at the moment is ‘what do you think about the fast diet?’ For anyone not in the know, the fast diet (also know as the 5:2 diet) is where you fast for two days out of every week. On your fast days women are allowed 500 calories and men are allowed 600 calories. For the rest of the week you can eat and drink as you please.

The diet rose to popularity based on this BBC panorama episode called Eat, Fast and Live Longer. The presenter has then taken this further by creating a diet book, a cookbook and website. Its popularity is now huge and fully deserves the title of ‘the new Atkins diet’. Through this article I want to give my take on it and why I advise people to avoid it.

Throughout the show the presenter meets with lots of different scientists and laboratories. He talks about technical sounding hormones like insulin-like growth factor 1 (IGF-1) and it makes the results look like they are grounded in good science. For a diet to really take off in the mainstream people want to know it is backed up by smart and credible people in lab coats. You can rest easy knowing that it has gone through the rigour of scientific investigation; you aren’t some body-obsessed twenty something doing every celebrity diet that appears in Heat or Cosmo.

The Fast Diet 7 HealthThe problem is, the diet isn’t very scientific. It is very narrow in it’s focus when looking at whether it is improving health. One of the most significant results that the presenter sees through following the diet is a reduction in his IGF-1. This hormone is a marker for diabetes and a reduction is seen as a good thing. But hormones are like an orchestra and a change in one is not important in isolation. You have to look at all the other players in relation to one another.

With diabetes in recent decades becoming so much more of an issue, the diet appears like a good fix.  Blood sugar issues are normally reduced to one word, insulin, so at least this diet adds another hormone into the mix, IGF-1. But blood sugar isn’t just influenced by one or two hormones, rather by a whole host of them. Below is a list of hormones that have an influence on blood sugar (the list isn’t exhaustive, there are probably some I have missed). These are just the hormones and doesn’t include other vitamins, minerals, enzymes, etc:


Somatostatin (including IGF-1 and IGF-2)





Adrenocorticotropic hormone (ACTH)

Dehydroepiandrosterone (DHEA)

Growth hormone releasing hormone (GHRH)

Growth hormone

Thyroid stimulating hormone (TSH)

Thyroxine (T4)

Triiodothyronine (T3)






Corticotropin-releasing hormone (CRH)

I am always interested in the why behind any body changes. If IGF-1 ‘improves’ then why did this happen? We tend to think of things in black and white terms, like an increase in cholesterol is bad or a reduction in IGF-1 is good but without understanding the mechanism for why it is happening it is impossible to make this call. Without investigating the other hormones in the body that affect blood sugar regulation a decrease in IGF-1 doesn’t tell us too much.

With most new diets people go on there is a honeymoon period. It doesn’t matter if it is vegan, Dukkan, raw food or the fast diet, most people start to feel better in the beginning. I want to explain why I think this happens.

Whether intentionally or unintentionally, most diets lower calorie intake. While calories are like a dirty word, they are the energy you need to power your body. From my experience with working with clients, most people are not meeting their energy demands. Whether they are thin or overweight, they are not taking in enough (or the right types of foods) to support them.

When the body is in a calorie deficit it calls on stress hormones to help out. Stress hormones in a very simple sense are all about bringing energy to your cells. Stress, whether it is from being too hot, too cold, from exercising, from feeling overwhelmed, etc, causes an increased demand for energy. The stress hormones help to find this energy.

Hormones have various functions in the body and it is these other functions that help create the honeymoon effect of dieting. So let’s look at two stress hormones, cortisol and adrenaline.

Cortisol is a natural anti-inflammatory. When you break your ankle your body naturally produces more cortisol. When you get to the doctors office they will probably give you a cortisone injection, which is the same thing. It helps to reduce the pain and inflammation. It is not uncommon for people on new diets to report that their previous aches and pains have now gone away.

Cortisol suppresses the immune system. Stress from an evolutionary perspective should be short lived, something to get you out of life and death danger. It takes a lot of resources to run your immune system so during a stressful time, when energy demands are increased, it makes sense to put it on hold. It is why cortisone is also used in lots of autoimmune diseases as it suppresses the immune system from attacking itself. It is not uncommon for people on a new diet to have immune issues clear up and for them not to get the common coughs and colds they used to.

Cortisol is made from cholesterol, just like all steroid hormones. Because of the new demand for cortisol, cholesterol is being used at a faster rate than it is being created. When following a new diet people often notice that their cholesterol levels improve. (Incidentally, a new diet can also cause cholesterol to increase if the body isn’t able to make the conversion into the other hormones).

Adrenaline is one of the first hormones to be released in a stress situation. It improves focus and concentration, perfect attributes for helping you fight or flee from a lion. People on a new diet often comment about their newfound mental clarity.

Adrenaline and cortisol increase energy and it will feel like more energy than you were getting on your old diet. Food shuts off stress hormones and moves blood to the digestive and reproductive system, this can lead to drowsiness and feeling less alert. By eating less these hormones are never turned up (or at least less affected) and therefore give the feeling of more constant energy.

All of these attributes – reduced pain, inflammation, illnesses, and cholesterol; increased alertness, clarity and energy – all make you think that this new diet is the holy grail. All along you have been searching for something and now you have found it.

Unfortunately the honeymoon starts to end. There is only so long that your body can keep producing cortisol and adrenaline before its production starts to slow. The high stress hormones suppress the thyroid gland, which is the master gland for controlling metabolism. Because the body has been so focused on cortisol production the other steroid hormones have fallen by the wayside. You get decreased amounts of pregnenolone, progesterone and DHEA – collectively known as the hormones of youth due to their youth giving properties. Libido is affected due to decreased testosterone and progesterone.

Personally this is what I see happening with the fast diet. Yes there are improvements at first but I put this down to the stress response rather than the real long-term positive changes*. In the long run (a minimum of six months, but more like one or two years) I would be interested to see how people are fairing. And not just how they ‘feel’ but what all of the body markers are saying. Unfortunately this is just speculation at this stage as I can’t find anywhere that all these different factors have been tested and analysed while doing the diet.

When the diet first started gaining exposure last year the people who brought it to my attention were a couple of clients. Both had incredibly disorganised eating patterns and food issues. They would often go all day without eating and then binge at night. They were both drawn to intermittent fasting because it legitimized what they were already doing. Despite both being in poor health, if they could make just a few adjustments they would then be able to say they were doing intermittent fasting.

Fasting asks a lot of the body and most people aren’t healthy enough to be doing it. More so when they are doing it on a weekly basis. In my mind the fast diet is just another fad. It doesn’t take into consideration the health of the person and what is going on. Unfortunately I don’t see it going away anytime soon.

*My caveat to this is that the fast diet could be helpful with making someone eat better food. If you are someone coming from eating a junk filled diet but now while doing the fast diet you start eating more whole foods then it will be of benefit. But then the benefits aren’t so much from the fast diet and more from better quality food. Sometimes people need the spark to get them to start and if the fast diet provides this then it is a good thing. But in the end they would be better taking their new food choices and ditching the fasting element altogether. 

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Chris Sandel is the founder of He is a nutritionist, working with clients on a one-on-one basis, as well as creating online trainings and products about health and nutrition. He is the author of The Health Trap: Why "Healthy" Eating Isn't Always Healthy which is available on Amazon UK and Amazon USA.

Chris has three free emails series. One is on how to quit dieting. One is on simple tests you can do at home. And the other is his take on the world's healthiest foods.


  1. Super article Chris, particularly the explanation around the honeymood period. I have heard so many people tell me how great they feel when they’re on some ludicrous diet and I’ve always wondered how that could be. Your clear, detailed, science backed yet easy to understand articles are very refreshing to read. Thanks!

  2. Vanessa says:

    Excellent article Chris! As always!

    I once went on a ridiculous raw-veggies-no fruits detox 🙂 I felt GREAT for 2 entire days hahahaha. Even my chronic rhinitis improved for those 2 days, it was amazing! Then, I felt like I was pretty much about to faint most of the the time. I guess it was the cortisol then, doing a good job initially.

    How do we know if are having the appropriate caloric intake?

  3. Chris Sandel says:

    Hi Nayef. The honeymoon period on diets is is unfortunate as it makes people feeling like what they are doing is helping them. In reality, it is really the opposite. Glad you liked the article.

  4. Chris Sandel says:

    Hi Vanessa,
    I tend to use body temperature, pulse, frequency of urination, warmth of hands and feet as well as people’s food logs to see if they are eating enough. There is no magic formula that involves age, weight or anything like that, it is about listening to the body’s feedback and making adjustments accordingly. If your body temperature is below 36.7 C (98 F), you get cold hands and feet, you urinate more than every 3-4 hours and your pulse isn’t between about 70-85 then it is a fair indication that you are not eating enough or there are things going on in your life that make it impossible to meet your energy demands. This means you either need to change what you are eating or change how you are living your life. Obviously when working with clients there are lots of other variables we can use to determine if the demands are being met but in a general sense these are some of the key markers I look at.

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