Jessica Irvine releases new book on losing weight titled The Bottom Line Diet

IN her highly confessional new book, economics editor Jessica Irvine reveals her binge-eating past and how she overcame her battle with weight.

How I lost weight, and kept it off

IN her highly confessional new book, economics editor Jessica Irvine reveals her guilty binge-eating past and how she has overcome a lifelong battle with her weight.

Read an extract of it below:

THIS is written for the 20-year-old version of myself.

It is the tough love talk I wish someone had given me all those years ago about my body and how I was supposed to look after it.

Before I started buttering my toast on both sides because, hey, it tastes better that way. Before I started driving to McDonald's on my lunch break, parking away from work and secretly devouring an entire Big Mac meal, with fries and coke, plus an extra cheeseburger because one burger never quite filled me up.

Before I started ordering takeaway Thai curries for dinner every night. (They've got vegetables in them so they must be healthy, right?) Before I decided that exercise wasn't for me. (It was too time-consuming, or work was too stressful.)

Before I started going to nice restaurants and eating entree, main and dessert. Oh, and a bottle of wine, please. Make that two. Before I put on 20kg and had to work hard - am still working hard - to get it off and keep it off.

I have never been obese. But throughout my 20s I consistently gained one or two kilograms a year until, six months before my 30th birthday, I weighed in at 87kg, which, for my height of 179cm, put me officially in the overweight category.

I was a ''creeper''. Left to my own eating and exercise habits, I have no doubt I would have made my way into the obese category - too stunned to even begin figuring out how to stop myself.

At 87 kilograms ... Jessica Irvine is pictured here while on holiday in China in June, 2010.

I'm a university-educated journalist. I'm supposed to be able to figure this stuff out. But I couldn't. Every year I'd write a New Year's resolution column telling readers my latest idea about how I would lose weight that year. I'd go to the gym three times a week. (I didn't.) I'd do one minute of exercise for every dollar I spent. (The plan was derailed when my 15-year-old Corolla required an expensive timing belt change.)

In 2010 I found out I had polycystic ovarian syndrome, a hormonal imbalance that affects one in 10 women and is associated with weight gain. There you go then, I thought. It's not that I'm eating too many pies; I have a terrible disease.

Everyone has a moment when they realise they're overweight. For me, it was seeing a photo of myself taken at a friend's party on New Year's Eve 2010. The camera never lies.

I still remember the agony of getting ready that evening. It was summer - too hot to wear my usual uniform of stretchy denim jeans and a black T-shirt. So, reluctantly, I pulled on some shapeless knee-length black shorts and a billowy top that could have hidden any pregnancy.

But, I reasoned, the top was low cut, and cleavage - as any woman who has been overweight can attest - is the refuge of the larger woman, the ultimate accessory to distract from, well, the rest.

Before losing weight .... Jessica Irvine reveals what motiated her to change her diet.

I didn't feel fashionable. I didn't care. I had numbed myself to the possibility of ever looking particularly cute. If I could just pass for bland, sink into the background, that'd be great.

A few months later, with the image from that photo seared into my retinas, I signed up to an online body transformation program.

I had never tried dieting before. Women ate 1200 calories a day, men a bit more, and everyone exercised for about an hour six times a week. And guess what? It worked. The results were immediate.

I dropped a kilogram a week. By the end of the year I was 20kg lighter and boasting about my weight loss in newspaper columns.

A year later, however, I was 10kg heavier again. I had regained half my weight loss and was well on my way to putting it all back on. Little by little, mouthful by mouthful, I undid all my hard work. I had fallen into the same trap as 95 per cent of people who lose weight.

But I was determined to be part of the other 5 per cent. Of all the things I had learnt, one fact stood out, a fact I had never known before.

It was a rule of thumb commonly advocated in the United States: that it took a calorie deficit of roughly 7700 calories (or 32,000 kilojoules) to lose 1kg. Could it really be as simple as that? Calories in and calories out?

I decided to go it alone. I made a New Year's resolution to drop the 10kg I had regained in time for the federal budget in May 2013. (I'm an economics journalist, so the date meant a lot to me.)

I've always been a numbers geek, so I did it the only way I knew how: I started a spreadsheet. In one column: calories in. In another: calories out. I crunched the numbers for almost five months.

And it worked. I dropped only 5kg rather than my goal of 10kg, but I could see exactly why. I had eaten more calories than I had set out to eat and had done slightly less exercise.

Three years after embarking on my weight-loss journey, I'm still 15kg down from my initial weight. As I write this, I weigh 72kg, putting me smack-bang in the middle of my healthy-body weight range for my height. I have beaten the odds. I am a successful loser.

And so I want to share with you the secret to enduring weight loss. Here it is. Weight loss comes down to one simple equation: energy in minus energy out. Weight loss, for all its emotional and mental struggle, is a straightforward numbers game. Get the numbers right, and anyone can lose weight.

The new Jessica ... on the front cover of her new book, The Bottom Line Diet.

Of course, if it were really that simple no one would be fat, right? Well, no. As it turns out, weight loss, while uncomplicated in theory, is difficult to achieve in real life. It's a struggle to make good food and exercise choices in our obesogenic environment. There are measurement issues, too. Our bodies are not walking calorie calculators. It can be hard to work out the calories in your food. It's harder still to measure exactly how many calories you burn in a day.

I'm not saying it's easy, or that what you eat isn't important too. What I am saying is that reducing your energy intake so that it is below your energy expenditure each day is the only proven way to lose weight in the long term.

The only weight-loss secret you can hope for is knowledge of how your body works. Knowledge is power. I now know that it is possible to both lose weight and keep it off. I want to share that knowledge with you.

I have always been a details person. Give me the specifics. There are facts about food that, once you know, you cannot un-know. Like there 1600 calories in a Meatlover's pizza, 980 calories in a packet of Shapes biscuits and 1195 calories in a large packet of Doritos. And did you know it would take a middle aged woman about four hours of gentle walking to burn the same amount of calories?

These facts will change the way you look at food and exercise forever.

I need to tell you what I am and what I am not. What I am not: a scientist, a nutritionist or an exercise physiologist. What I am: a professional journalist and a person who has lost a substantial amount of weight.

Looking great and feeling better ... Jessica Irvine.

I have interviewed many of Australia's and, indeed, the world's leading experts on weight-loss science. I have spoken to endocrinologists, nutritionists, obesity experts, biologists and exercise physiologists. The each have an interesting story to tell about how the body works - about the impact of hunger hormones, gut bacteria and genes. But on the basic science, they are unanimous: it is impossible to lose weight without eating fewer calories than you expend in a day.

I'm not going to lie to you. To lose weight you will have to change your life, your habits. But that's okay; chances are your current lifestyle is making you not only fat, but unhappy. Am I right? Perhaps you suffer energy slumps after eating, or a racing heart after climbing the stairs. Perhaps you just can't find anything to wear. Losing weight is hard work, believe me. But I know being overweight is even harder.

You might think losing weight is impossible. But I'm here to tell you it is possible. Anyone can do it if they know how.

"The Bottom Line Diet: How I lost weight, kept it off and you can too" by Jessica Irvine is published by Allen & Unwin. On sale now. RRP $22.99.