We all know how depressing it is to put on weight. There can be few of us who don't despair when we see the needle on the scales creep upwards.
Now imagine what it's like if you're an identical twin - and while you get fatter and fatter, your twin remains resolutely svelte.
This was exactly the position I found myself in just a few years ago. I ballooned to 19st (120kg) while my identical twin, Chris, stayed at my previous weight of 12½ st (80kg). It was unbelievably soul-destroying.
But what made my weight gain even more unbearable was that I, more than anyone, should have known better. Because I'm a doctor. And not just any doctor. With my twin brother, who is also a doctor, I have made numerous television programmes on all things to do with health.
And while I was piling on the pounds, I was also studying for a Masters degree in public health at Harvard University in America, where I was a prestigious Fulbright Scholar - and this after studying for my medical degree at Oxford.
In short, if anyone should have known how important it was to keep a trim waist - and how to do it - it was me.
But the fact that even I managed to put on such a dangerous amount - my feet would ache from all the excess weight I was carrying - does underline why so many of us find slimming so difficult.
We are bombarded with confusing, contradictory and sometimes dangerous messages from the endless books, websites, DVDs, magazines, corporations, scientists and gurus who make a living out of weight loss.
Most wannabe dieters are plagued by doubts and questions. Is breakfast important? Do calories matter or should I concentrate on cutting carbs? What about gluten? Did I waste my money on this blender?
Despite all my medical training and practical experience of nutrition, I was as much at sea in the world of weight loss as anyone else - and just as easily seduced by a fad diet.
But it was painfully obvious from my ever-expanding girth and embarrassing man-boobs that I needed to do something - and fast.
So I decided to use my medical knowledge to research the science behind practically every diet out there - from the Dukan to the bone broth diet via Weight Watchers and the baby food diet - and tried a fair few of them myself. Many were impractical, unsustainable and boring. Surely there had to be a better way to lose weight?
That's when I created my own diet, inspired by the best bits I'd found from all those other diets, but instead made simple, healthy and effective.
The principles behind my Definitive Diet are simple. If you want to lose weight fast, eat just one healthy, delicious meal a day (I recommend eating at dinner time) and stick below 800 calories a day.
You could lose up to six kilos in two weeks this way, and experience all the health benefits that come with intermittent periods of fasting while you're at it. This version of my plan is a bit like a more concentrated type of 5:2 diet - where you eat normally for five days a week and fast for two - and is a great kick-start for rapid weight loss.
The second version of my diet plan is equally straightforward: to lose weight at a slower but still effective pace, skip either breakfast or lunch and enjoy two healthy meals, totalling around 1,200 calories a day. And the third way? You guessed it: eat three healthy meals a day, totalling around 1,500 calories, that are high in fibre, low in carbs and rich in healthy fats.
That's it: super-simple, while also being healthy and tasty (of course, if you have any health worries, you should always check with your GP before embarking on any diet).
It is thanks to this plan that I've lost that six stone (38kg) of excess weight and haven't strayed above 13st (82kg) for the past four years.
If you had seen me as a teenager, you would find it hard to believe that I managed to get myself into such a gargantuan state.
Both Chris and I were fit, slim, athletic teens. We rowed and swam for Oxford University when we were studying for our medical degrees. And we involuntarily burned more calories than we could consume walking miles and miles along hospital corridors in the years we spent as junior doctors. But in 2009, when we reached 30, our lives went in different directions.
I moved to the U.S. and started a family - and my weight went through the roof. In the space of 12 months, I put on 6½ st, with my 6ft frame reaching a peak of 19st. Pictures of Chris and me together at the time reveal a pretty shocking contrast, which people couldn't help but comment on.
When we were both working with genetics expert Professor Tim Spector on a programme investigating the dynamics of twins, he took one look at me, checked my health statistics and declared: "You are a disgrace to your genes."
He was right. But when I look back at my weight-gain years, it isn't easy to pinpoint precisely what went wrong. I'm sure many of you will have found this, too.
I suppose I could blame stress. Not only had I moved to a different country to study at one of the world's most high-pressure universities, but I was also dealing with new fatherhood (my son was born in 2008). No wonder my anxiety levels were creeping up.
I could also blame what you might politely term opportunism - or, more accurately, greed. For my apartment was directly above a gourmet burger bar and next door to a burrito restaurant. I also became a passionate enthusiast for Chinese takeaways.
As well as all this, though, I was in denial. Yes, all that delicious food was immensely enjoyable - and I was in a land of large people where being 19st is perfectly acceptable.
This is surely a factor behind many people's weight gain. As more of us become fatter, it is too easy to look around you and think that is the norm.
But it shouldn't be. And looking back, there's no doubt I was eating too much - but I'd never had to restrain my eating before and I refused to start, instead stubbornly ignoring my expanding figure.
All the while my health was clearly suffering.
When I finished my Masters at Harvard, I remember receiving my graduation photo and being startled to see a fat man with boobs looking back at me.
Not only did my feet ache under this new strain, but eating all that rich food meant I was suffering terrible indigestion and acid reflux, in which stomach acid leaks into the oesophagus, causing a painful burning feeling.
I even had to sleep with my head and shoulders propped up, to make it harder for the acid to travel upwards in this way.
Then, at the end of 2009, I flew back to the UK to start filming the BBC series The Secret Life Of Twins with Chris - only to find there wasn't one item of clothing among those I'd left in my wardrobe in the UK at the beginning of the year that still fitted me 12 months on.
Health checks for the TV programme revealed what I had been trying to ignore - my blood pressure had skyrocketed, and tests showed I was right on the edge of getting type 2 diabetes.
Chris was understandably worried, seeing my explosive weight gain as a sign that I must be unhappy. He begged me to do something about it.
So I tried but it was almost impossible. Even with all my knowledge, it required a superhuman effort to get the numbers on the scales to shift at all.
Because look into dieting in any depth and you will find not just a total lack of consensus about what works and what doesn't but wild, angry arguments.
This is ridiculous because the issues are not really very complicated. Compared with questions such as 'how should I raise my children?' and 'how do you make more money?', agreeing on what to eat and how to exercise to feel good, look good and live long shouldn't be hard.
Yet it seemed to be at first. I tried running, but my thighs chafed painfully. So I tried eating a bit less instead, but I was flummoxed without a structure or plan.
I had tried the Paleo diet, where you eat foods that approximate to those of Paleolithic humans - red meat and vegetables, but no farmed grains or processed foods. Yet I found it just wasn't a way of eating I could sustain.
I also knew about the low-carb diet, after reading all the medical research and interviewing experts. But when I tried extreme low-carb dieting for myself, I found existing on meat, fish, eggs and cheese extremely boring, constipating and, in the long term, unhealthy. In short, despite all my efforts, nothing seemed to be working.
Even if the numbers on the scales did dip temporarily, they would quickly creep back up.
The turning point came when I moved away from the U.S. and told myself: "Enough - I'm a doctor, I know how to do this."
It was this personal journey that led me to make the TV series How To Lose Weight Well and ultimately to devise my Definitive Diet.
While the plan is based on my personal experience of dieting and weight gain, I have also drawn on more than a decade of research, interviews, documentary-making and painstaking examination of the scientific literature.
I promise, you really don't need to try another diet because I've done all the hard graft for you.
I've examined just about all the diets out there. And I mean examined. I read almost every diet book, looked into every fad - and as well as most of the major diet plans, I tried various really weird weight-loss regimens.
I don't know any other doctors who have done this. Indeed, the world of fad dieting receives little serious medical scrutiny.
Today, I live between London and America. I'm a lecturer at Fordham University in New York and still make TV programmes with my twin Chris. And I've stayed slim for the past four years, which I'm very proud of.
It's now not hard for me to keep the weight off. Yes, I have to devote a little concentration each day to stop myself eating exactly what I please. But that's all.
Having been there, I don't believe it's your fault you're overweight. But I do believe anyone can get to the weight they want without endangering their health or suffering too much - and I hope my Definitive Diet will help you do that. After all, it's the best of everything I know about weight loss: ideas begged, borrowed, adapted... or learned the hard way.