Michael Mosley: The Fast Diet

Michael MosleyMichael Mosley joined us for a webchat and to answer questions on The Fast Diet (also known as intermittent fasting or the 5:2 diet).

A medical journalist, Michael's BBC Horizon Programme in August 2012 first introduced the UK to intermittent fasting. His book, The Fast Diet, presents the science behind the diet, whilst his co-author, Mimi Spencer, explains the practicalities and how to go about it. The idea behind the fast diet is that you eat normally for five days and then then consume just 500 calories (women) and 600 calories (men) on two non-consecutive fasting days each week.

Breastfeeding | What to eat | Medical issues | Benefits | Best methods | Other questions


Breastfeeding and fasting

Q. LouiseMarie: How many extra calories would be recommended on fast days for breastfeeding mums? I have completed two days over the past week but am really hungry and feel I make up for it by eating loads the next day! Is that normal?

A. MichaelMosley: I am conservative about these things and would not actually recommend breastfeeding mums do fasting. I think you have enough to cope with anyway and the energy demands are high. Not aware of anyone who has studied this in humans.

What to eat on fast days

Q. TravelinColour: My question is whether this diet is more compatible with certain types of food. I'm thinking particularly of whether it is more beneficial to eat low GI / low carb, particularly on the fast days. I wonder if eating all your 500 calories as high GI carbs would make this way of eating very difficult to stick to.

A. MichaelMosley: I have a big section in my book, The Fast Diet, about GI and carbs, particularly on fasting days. I recommend relatively high protein, low carbs, low GI on fasting. The recipes in the book are based on that principle.

Q. KittyMem: I was wondering if you could outline your eating plans for a typical fast and a typical non-fast day? I love the 5:2 diet, but I am conscious about making the most of my 500 calories and getting everything I need.

A. MichaelMosley: General principles are you should eat more protein on fasting days, lots of veg, drink lots of tea and water. I largely avoid carbs. The importance of eating protein is that it is not well stored and after 24 hours without you might be running short. If you want recipes then buy The Fast Diet or go to our website, thefastdiet.co.uk, or one of the zillions of others out there that have recipes on them (though I cannot vouch for trustworthiness of content).

"General principles are you should eat more protein on fasting days, lots of veg, drink lots of tea and water."

Medical issues

Q. Salbertina: I started trying the Genesis IF diet, on which I believe your version was based. Which would you recommend for women with a family history of breast cancer? Ditto question re this diet for women of childbearing age.

A. MichaelMosley: My diet is not directly based on the Genesis IF diet, but I am certainly aware of Dr Harvie's work and she has a book coming out soon which I am looking forward to reading. As you may know, women on her 2 Day Diet not only lost weight but showed increased insulin sensitivity and reduced inflammatory markers, all of which reduce cancer risk.

I asked Dr Mattson, who is the guiding genius behind this work, about women of childbearing age and he said there was no known risk. If you starve women then their fertility will be threatened, but intermittent fasting is not anything like starvation.

Q. happynewmind: OK, I am starting today. I have put 4 stone on following an emergency complete hysterectomy and the following early menopause as a result. I cannot really exercise much at the moment as I am in pain a lot and waiting for more surgery. I have a very high rate of cancer in my family, so am interested in the benefits of this.

What I would like to know about are any side effects, both long and short term, and how long before I can expect decent weight loss.

A. MichaelMosley: The only known side effect is that you will get hungry and won't be able to stick to it. The first few weeks can be tough, so it's good to join a support group. To be properly evaluated the diet needs long-term trials, but I would add that all the great religions advocate fasting and have done so for centuries without any serious suggestion that it is harmful. I personally never go for more than 12 hours without food. Water, lots of it, is essential.

Q. tomorrowweeat: Why is this way of eating unsuitable for type2 diabetics and should you reduce Metformin dosage on fast days?

A. MichaelMosley: I think that people are rightly cautious about type2 diabetes and fear of going hypo. If you are thinking of trying it, you should discuss it with your doctor.

The benefits of fasting

Q. Issy: Could you summarise the benefits of the 5:2 diet, other than for weight loss?

A. MichaelMosley: Losing weight if you are overweight is itself a good thing (the recent report that being overweight is good for you is flawed for reasons I go into on thefastdiet.co.uk).

Benefits include improved insulin sensitivity, reduced inflammatory markers (leading to reduced risk of cancer and possibly improvements in asthma) and possibly reduced risk of dementia. They know that rats put on a 5:2 diet (they didn't volunteer) were less likely to develop dementia and their brains produced higher levels of a substance called BDNF (brain derived neurotrophic factor) which leads to production of new brain cells and may explain why people on 5:2 frequently report feeling sharper. Human trials are about to start.

Q. GreenEggsAndNichts: Are there any studies regarding differences in the way male and female bodies react to this diet?

Also, are we to assume that on non-fast days, we should be eating approximately our TDEE (total daily energy expenditure) allowance? Obviously we all have different definitions of 'eat what you want', as that's how we gained excess weight in the first place!

A. MichaelMosley: Most of the studies so far have been done with women, overweight but premenopausal. The benefits seen in women (compared to a standard low -alorie diet) include losing more weight, being able to stick to it and reduced inflammatory markers (important for things like breast cancer and asthma).

There is some evidence that men find it easier to exercise in the fasted state, and burn more fat, but I have not seen studies that say if the same is true one way or the other with women.

"I believe one reason this approach works is because our bodies were forget in a climate of feast or famine."

I believe that one reason this approach works is because our bodies were forged in a climate of feast or famine. It would be strange if women never went hungry in the past.

Best methods for fasting

Q. justonemorepie: I keep getting told that 'breakfast is the most important meal of the day' and on a fast day I don't eat until the evening. Can you give me a quick comeback for those who don't endorse fasting re breakfast. I find myself drawn into lengthy conversations about the benefits of fasting.

A. MichaelMosley: People may say that breakfast is the most important meal of the day, but all breakfast means is when you break your fast. Some people like eating in the morning (I do), others prefer to eat later in the day. In the trials run by Dr Krista Varady, the 500 calories were eaten all in one meal, at lunch.

Q. Badvoc: I am finding it very hard to have only 500 calories on fast days. I also find it hard when I am on my period. Do you have any tips for making the fast easier to do? Timewise etc. Is there anything I can do to assuage my PMT/hormone related binging?

A. MichaelMosley: The first few weeks are the toughest, and it is a good idea to join a support group, like ones you will find here on Mumsnet or Facebook.

I have a website, www.thefastdiet.co.uk, which is also quite active. This is a very democratic movement and hugely self-supporting.

The reason for 500 calories is that that was the figure the scientists have researched. There is nothing magic about it. If 600 works better for you, fine. The main thing is those calories should be satisfying, satiating and nutrient rich. Loads of recipes around online, in our book The Fast Diet, etc etc.

Some people have asked what regime I am currently on. Well I did 5:2 until I had dropped to my target weight, 12 stone. My wife, a GP, then said I was looking too gaunt so I switched to what I call 6:1. I do my 600-calorie thing once a week and a few days a week skip lunch. That way, two to three times a week I go for 12-hour periods without food. Haven't had my bloods retested yet but feel fantastic on it.

Q. mintycake: Is it better to fast on consecutive days or space them out? Also, does it matter what makes up the 500kcal on fast days?

"I knew veg was good for me, but used it as a garnish. Now I pile my plate. Someone contacted me via my twitter account, @drmichaelmosley, the other day to say she had eaten veg for the first time in her life and enjoyed it!!"

A. MichaelMosley: I started out doing Mondays and Thursday because it suits my life. Some people prefer consecutive days. Does it matter? No-one knows. These days, as I mentioned before, I'm largely doing one day a week and skipping lunch. Will check my bloods and see how they are doing. If glucose and cholesterol are up I will revert to 5:2. In terms of dementia risk, the first human trials are due to start soon.

The biggest change I have seen in myself, apart from significant weight loss and improvements in biochemistry, is my attitude to hunger and to vegetables. I used to snack as soon as I felt a pang, but these days I just let it go. I knew veg was good for me, but used it as a garnish. Now I pile my plate. Someone contacted me via my twitter account, @drmichaelmosley, the other day to say she had eaten veg for the first time in her life and enjoyed it!!

Other questions about the Fast Diet

Q. Kirsty3333: I was wondering what your thoughts were on very low-calorie diets as a weight management tool? Looking at sensible, controlled calorie intake every day?

A. MichaelMosley: Sensible controlled calorie intake is great if you can do it. The problem is lots don't manage. That is why 5:2 was invented. Dr Michelle Harvie of the Genesis Project in Manchester did a randomised trial comparing standard low-calorie diet with 5:2 and found after four months women were twice as likely to stick to 5:2, lost more weight and had better biomarkers.

Q. LizzC: Unfortunately, I missed the Horizon programme (and didn't know there was a book out either - think I must have had my head in the sand). Is there anywhere I can watch this episode, and could you also let me know what the book is called, please?

A. MichaelMosley: The book is called The Fast Diet; I should add there are a number of others out there called some variation on The 5:2 Diet, written by people who have little or no science training, simply saw my programme and copied it. I've read a couple of these books and they were not good. I have higher hopes for Dr Michelle Harvie's new book, though I have not seen it yet

Q. Salbertina: I'm so appreciative of how you've brought this mainstream. Will public health messages catch up?

A. MichaelMosley: Public health people are rightly cautious. I think it is fair enough to say, "Wait and see". There are a couple of new studies coming out that will hopefully add support to The Fast Diet approach. I hasten to add, this is not my scientific work. I pulled together the work of leading scientists into what I hope is a doable approach. They were very pleased by the programme and by the response.

Q. AntiDiet: I wonder if you could say something about the theory that going for long periods of time without eating slows down your metabolism and disrupts your leptin and ghrelin balance so that you are more prone to putting on and holding fat. How does the 5:2 diet not fall into this trap?

A. MichaelMosley: The claim that you have to eat regularly in order to keep your metabolism revved up is a myth. Numerous studies have looked into this and found no difference in the metabolic rates of people eating their calories as either three meals a day or six. Nor is there any evidence that not eating for a couple of days triggers some metabolic slowdown. There is some evidence that it speeds it up. It certainly turns on fat burning.

The thing we know is that when you start to lose weight, on whatever diet, your metabolic rate will slow simply because you are carrying around less weight. That is why you eventually plateau.

That's all from me. I also chat and answer questions on twitter @drmichaelmosley.

Last updated: about 3 hours ago