Although it was more than 30 years ago, I still vividly remember my first day at the Royal Free Hospital Medical School in London. There were a hundred of us gathered in a lecture theatre to be greeted by the dean. He talked for a while about our future medical careers, but there were only two things he said that really stood out. The first was that, based on previous experience, four of us in that room would marry. He was spot on; I met my future wife that day.
The other thing he said was that while we would learn an enormous amount over the next five years, within 10 years of graduating a lot of what we had learnt would be out of date. This was disheartening, but the point he was making is that science is constantly changing and that keeping up with those changes represents a huge opportunity, as well as a challenge.
I mention this story because although I trained as a doctor, I have spent most of my working life as a BBC science journalist, constantly exploring new medical claims. Recently, I've become the subject of my own documentaries, a guinea pig happily trying out different, science-based, ways to achieve better health.
These approaches, though based on good scientific research, often challenge what is routinely believed.
Two years ago, I went to my doctor with a minor complaint and as part of the examination she suggested I have a routine blood test. When the results came back she told me that I was a type 2 diabetic, with a fasting glucose of around 7.2 millimoles/litre. She also told me that my cholesterol was way too high and suggested I should start on medication.
This was a nasty shock, as my father had passed away at the relatively early age of 74. When he died he was on a dozen different medicines and suffering from a range of diseases, including type 2 diabetes, heart failure, prostate cancer and what I suspect was early dementia.
When I had that medical examination I was slightly oveweight, being around 85kgs (187 lbs). I wasn't carrying a lot of fat but the fat I was carrying was abdominal fat, the sort that wraps itself around your internal organs. Having a waist that is more than half your height is a bad sign. My waist was 36 inches (I am 5' 11").
Rather than start on drugs I began researching alternative approaches and came across researchers studying something called intermittent fasting. I decided to make a documentary for the BBC science strand Horizon, with myself as the subject.
In the course of making the documentary, Eat, Fast, Live Longer, I tried different forms of intermittent fasting, ranging from doing five days of almost total calorie restriction to alternate day fasting, where you cut your calories every other day. None of them was easy. Eventually I settled on a pattern I could manage to stick to, which I called the 5:2 diet.
On a Monday and a Thursday, I ate a quarter of my normal calorie intake, going down from around 2400 to 600 calories a day.
On this diet I lost 9kgs (nearly 20lbs), four inches off my waist and my body fat went down from 28 per cent to 21 percent. My blood glucose and cholesterol levels went down to healthy levels. Soon I began to sleep better (I stopped snoring) and felt more energetic, particularly on my fasting days.
The Fast Diet
After the documentary went out on the BBC in August 2012 people began to stop me on the street and tell me they had tried my 5:2 approach and done really well on it. They also emailed details of their experiences.
Among those emails, a surprisingly large number were from doctors. Like me, they had initially been sceptical, but they had tried it for themselves, found that it worked and had begun suggesting it to their patients. They wanted information, menus, details of the scientific research to scrutinise.
So, 18 months ago I wrote a book, The Fast Diet, which soon became an international phenomenon. It has been embraced by celebrities like Beyonce and Benedict Cumberbatch ("you have to, for Sherlock"), Liv Tyler, Ben Affleck and Christy Turlington. One Foot in the Grave star Richard Wilson told a newspaper that he had lost 12lbs on the Fast Diet in just five weeks. "The great thing is that the fasting days are tough but you know that the next day you can eat."
I have had grateful messages of thanks from doctors, surgeons, parish priests, politicians and a Nobel prize-winner. I have read, via our website (thefastdiet.co.uk) and through other websites, thousands of success stories. These are a small sample:
"I heard the author on a radio show and he made so much sense I tried the diet. I have never stayed on a diet before. I lost 40 pounds in a few months. It is six months later and the weight is still gone."
"I've now lost about 19lbs in five weeks, my body fat is down from 37 per cent to 33 per cent and I can take my jeans off without undoing them and I'm happy to do so if anyone will watch!"
"My body shape has changed beyond recognition. My muffin top has gone and I have gained a waist instead. I have been doing this for 21 weeks and have lost 19lb, but also three inches off my waist, three inches off my hips, two inches off each thigh. My psoriasis has gone too. I am 42 ... and looking the best I have for 20 years."
Even more impressively, a website, which has been tracking 5,000 people who have been trying the 5:2 approach, found that they have so far lost an average of 8kgs at a rate of 0.4kgs a week. Between them, they have lost 20,000kgs, the equivalent of four large elephants
So why does it work?
I think one reason why the Fast Diet has been so successful is because you aren't on a constant treadmill, dieting all the time. I certainly find it easier to resist the temptation to eat a bar of chocolate by saying to myself, "I will have it tomorrow". Then tomorrow comes and maybe I eat it. But sometimes I don't.
Intermittent fasting also teaches you better ways of eating. If you follow our recipes and satisfy your hunger on fasting days by eating vegetables and good protein, then, over time, you'll discover that when you get hungry you are more likely to crave healthy stuff.
There is also good evidence that intermittent fasting shrinks the stomach, meaning you are less likely to want to gorge on your non-fast days.
As someone recently wrote to me: "You don't get cravings, you don't spend money on special foods or programmes. I lost more than 25 pounds and my husband lost more than 35 pounds. It was easy to do and we have maintained the weight loss, even over the holidays. I wish I had discovered this method 30 years ago."
Another reason it works is because it is, at least initially, quite challenging. When we are stressed, at a cellular level, hundreds of protective genes spring into action. Going without food for quite short periods of time allows the body to clear out old damaged cells, making way for new ones.
As Professor Longo, of the University of Southern California's Longevity Institute, told me: "Temporary periodic fasting can confer long-lasting changes that can be beneficial against ageing and diseases. Even if you take a cocktail of powerful drugs you will not get close to what fasting does."
There are lots of myths about intermittent fasting, such as the claim that you will go into starvation mode or you will feel tired all the time. This makes no sense from an evolutionary perspective. Imagine what would have happened to our remote ancestors if every time they didn't eat for a few hours they had become tired, feeble and been unable to hunt. They would have gone extinct.
Research shows that when you cut your calories for a short period of time your metabolic rate speeds up and you start to burn fat. It is only after prolonged fasting and a significant drop in body weight that your metabolic rate slows. This is mainly because you are carrying a smaller body around.
Another claim I've heard from dieticians who should know better is that intermittent fasting leads to muscle breakdown and protein deficiency.
If your protein intake is adequate, and we actually recommend an increased protein intake on fasting days, then you are not going to get muscle protein breakdown. In fact, the evidence from human studies points towards intermittent fasting being better than standard diets when it comes to muscle preservation.
Not losing weight
No diet is going to work for everyone. Although many people easily adapt and start losing weight straight away, others find it far more challenging. If you persist it will work, but there are things to bear in mind.
Firstly, I would not obsess about weight. What you really want to do is lose fat, preferably around the gut. I would always start by measuring your stomach, around the belly button, and see what happens over a period of time. On a normal diet you will lose a mix of fat and muscle, which is why it is important to up your exercise levels when you diet, to maintain muscle mass. Muscle is more metabolically active than fat; it burns calories even when you are asleep.
What is unusual about intermittent fasting is that studies have consistently shown that people on it tend to lose mostly fat and little muscle.
For example, in a study where 107 young overweight women were randomly allocated to either a standard low calorie diet or a diet where for two days a week they ate 650 calories a day, they found the intermittent fasters lost far more fat. At the end of the study the intermittent fasting group had lost an average of 6kg of fat and three inches from their waists compared to 4.9kg of fat and two inches from waist for the normal dieters.
I also think that people have to be realistic about what they eat and drink on non-fast days. You shouldn't be obsessing about calories on your non-fast days, because that defeats the whole purpose of it, but nor should you go wild.
If you are not getting slimmer then I would look first at the calories you are getting from drinks on your non-fast days. Juices, lattes, alcohol, fizzy drinks and smoothies all contain a lot of calories. If you can move to drinking more water and sugar-free tea/coffee that will help. Calories you drink do not satiate. If you eat three apples they will fill you up. Drink three apples in the form of a small fruit juice and it will not fill you up.
The other thing is you should be moving more. I always take the stairs, even up seven flights. Get a pedometer. Aim to do 10,000 steps a day. Most people do less than 5,000.
A long-term study on people who lost weight and kept it off found that those who were successful all increased the amounts they walked.
My top tips for would-be fasters:
* Make sure you are well hydrated. We get a lot of fluid from food, so if you cut your calories to a quarter during your fasting day you will be consuming less water. I recommend drinking lots of calorie-free fluid during the day. This can be black tea, black coffee (the idea that coffee makes you dehydrate is a myth), water from the tap, herbal teas, whatever. I am not a fan of diet drinks, but if it helps you get through the day, fine.
* Make sure you get good amounts of protein and vegetables on your fasting days. Protein will fill you up and you can eat a lot of vegetables for very few calories. There are lots of recipes in The Fast Diet Recipe Book and my website, thefastdiet.co.uk, has a friendly forum where people offer help and support
* Do tell your friends and family that you are doing it. You are more likely to stick to it if you make a public commitment.
* Doing it on busy days will help your fast time fly.
* Find a way to fit in some exercise during your fasting days, whether it is walking, cycling or simply taking the stairs. Research shows that people burn more fat in a fasted state. Don't, however, attempt a marathon.
* Once you have lost the weight you want to lose you should be able to keep it off with 6:1; cutting your calories just one day a week. Do weigh yourself a couple of times a week and if the weight starts to creep up, return to 5:2