The Australian Guide to Healthy Eating. So what, if not overeating, is causing an obesity crisis of epidemic proportions? There are two things to consider, says Ludwig, who examines the epidemic and foods that act as "fat fertilisers" in his new book, Always Hungry. Firstly, what we are eating is a big problem. "It's the low-fat, very high carbohydrate diet that we've been eating for the last 40 years, which raises levels of the hormone insulin and programs fat cells to go into calorie storage overdrive," he explains.

"I like to think of insulin as the ultimate fat cell fertiliser." The calories become so well stored in the fat cells that our bodies cannot access them to burn for energy. This means we always feel hungry, as our bodies cry out for fuel they can use and simply trying to eat less exacerbates the problem without addressing the real issue. "When we cut back on calories, our body responds by increasing hunger and slowing metabolism," Ludwig says. "We think of obesity as a state of excess, but it's really more akin to a state of starvation." He continues: "If the fat cells are storing too many calories, the brain doesn't have access to enough to make sure that metabolism runs properly.

"So the brain makes us hungry in an attempt to solve that problem, and we overeat and feel better temporarily. But if the fat cells continue to take in too many calories, then we get stuck in this never-ending cycle of overeating and weight gain. The problem isn't that there are too many calories in the fat cells, it's that there's too few in the bloodstream, and cutting back on calories can't work. "And that makes weight loss progressively more and more difficult on a standard low calorie diet." The second part of the problem is predetermined by genetics. In this sense, fat people and thin people aren't all that different, but it explains why the same diet can have dramatically different outcomes. We have a body weight ''set-point'' Ludwig explains, which seems heavily based on the genes we were born with.

"If you inherited obesity genes from your parents, the biological responses that defend body weight will kick in for you at a higher range compared to someone who didn't inherit those genes," Ludwig says in a new op-ed for Medium. "We've been following the wrong advice for too long. Dieting doesn't need to be this hard. The key to long-term weight loss isn't counting calories; it's eating in a way that lowers insulin levels, calms chronic inflammation and, by so doing, readjusts the body weight set-point to a lower level." This involves nourishing through nutrition instead of eating by numbers. And nutrition guidelines are finally starting to catch up with ever-evolving nutrition science. This year, sugar took over from fat as public food enemy number one. In the States at least, where new dietary guidelines advise significantly cutting back on sugar - specifically limiting added sugars to 10 per cent of daily calories. The Australian dietary guidelines, which were revised in 2013, are a little more open to interpretation. They advise consuming added sugar "only sometimes and in small amounts".

Both the Australian and American guidelines still recommend low or reduced fat dairy and both largely feature grains ("mostly wholegrain") as a fibre and carbohydrate source. Ludwig's recommendations are not altogether in line with the guidelines. His plan involves ditching processed carbohydrates, added sugars and grain products for the first two weeks. He suggests non-starchy vegetables, fruits and beans as the carbohydrate sources during this phase, while the body resets. "After two weeks, we reintroduce whole kernel grains, potatoes – except for white potatoes – and a little bit of added sugars." He does not shy away from fat (the new American guidelines recommend limiting saturated fat intake to 10 per cent of daily calorie intake), in fact, he encourages it, provided it is coming from wholefood sources like avocado, nuts, extra virgin olive oil or flaxseed.

"The quickest way to lower insulin is to cut back on processed carbohydrates and to get the right balance of protein and fat in your diet," he says. "A high fat diet is really the fastest way to shift metabolism. It lowers insulin, calms fat cells down and gets people out of the cycle of hunger, craving and overeating." It also gets them out of the cycle of calorie counting, reiterating that the battle of the bulge is much more than just a numbers game. "Cutting back on calories won't do it. That doesn't change biology. To change biology, you have to change the kinds of foods you're eating."