With everyone from fashion models to cricket stars worrying about their waistlines, battling the bulge is a national obsession. Open the pages of any women's magazine, and you're sure to be bombarded with suggestions on how to lose a few kilos. And yet, researchers still disagree over the most basic question - why are Australians getting fatter?
The two contending explanations are energy intake and energy use. While some argue that the biggest change of the past few decades has been in diets, others claim that the major shift has been in how we exercise.
Now, a study from the United States - the world's podgiest country - provides an answer. Three Harvard economists - David Cutler, Ed Glaeser and Jesse Shapiro - say most of the rise in obesity is due to higher food consumption, largely caused by changes in food technology.
Over the past few decades, obesity (defined as having a body mass index over 30, as compared with the healthy range of 20 to 25) has become as American as apple pie. In the 1970s, 16 per cent of Americans were classified as obese. Today, the figure has risen to 30 per cent.
But Australia is up there on the fat league tables. With a hefty 19 per cent of adults classified as obese, the OECD ranks us fourth among developed countries (behind the US, Britain, and Germany). And although we lack good data for the 1970s, we do know that obesity in Australia has risen steadily since the start of the 1980s. So a theory that explains the increase in US obesity may well have some insights Down Under.
Until Cutler, Glaeser and Shapiro came along, one of the leading theories on American obesity had been that of the University of Chicago's Thomas Philipson and Richard Posner, who argued that changes in the amount of energy spent on the job and commuting to work accounted for the rise in obesity. Yet as the Harvard trio point out, most of the changes into physically passive occupations had taken place by 1970.
Using detailed evidence from "time diaries", they then go on to show that changes in exercise patterns over the past 30 years have been fairly minimal. Over the past 25 years, the average American has gained about five kilograms. For this to be explained by exercise, Americans would have to have reduced their energy usage by the equivalent of two kilometres of walking per day. No such shift has occurred.
Other popular theories, such as television-watching, rising incomes, increasing rates of eating out, and rising female labour-force participation are also rejected.
The big news comes when the Harvard researchers look at changes in energy intake. While the amount of kilojoules consumed at dinner has fallen, breakfasts and lunches have more kilojoules and the amount of energy consumed through snacks has nearly doubled. Overall, the amount of available food per person in the US has risen markedly over the past few decades (adjusting for exports and wastage). On a kilojoules-per-day basis, the average American is guzzling the equivalent of a can of Coke more than in the 1970s.
Why are Americans consuming more? Because kilojoules are tastier, cheaper, and available more quickly than ever before. Since 1970, a dazzling array of food technologies have helped to reduce the time between when we want a snack, and when we can eat it. Stretch-wrap films, controlled atmosphere processing, advances in artificial flavouring and the widespread availability of microwave ovens have cut preparation times dramatically over the past 30 years. Cream-filled cakes - once the product only of ambitious cooks - are now available for only a few dollars. Tasty biscuits can be kept fresh enough to sell through a vending machine.
For most of us, these advances in food technology have been a boon, allowing us to spend less time preparing food, and more time on work or leisure activities. But for those with lower levels of self-control, or people with addiction problems, the changes have come at a cost. As Cutler, Glaeser and Shapiro argue, if the vending machine is 10 metres away, a person on a diet might have a mid-afternoon snack. But they are much less likely to have the snack if it requires a 10-minute walk to the corner store.
Across the developed world, countries that have restricted the availability of new food technologies tend to have lower levels of obesity. But it is unlikely that Australia would want to turn the clock back on microwaves, vending machines and tastier foods. Shapiro argues instead for a carefully targeted awareness campaign, focused on those who are most vulnerable.
With medical researchers now linking obesity to higher rates of heart disease and cancer, understanding its causes has more than cosmetic implications. And it may also affect our society in other fundamental ways. Amid rising weights across the developed world, both men and women say that their ideal partner is thinner than 30 years ago. Food technology and fashion, it seems, are pulling us in different directions. Eventually, something may have to give.