Having read your most recent book, Why We Get Fat, I agree with you that the conventional wisdom on how to eat healthily and lose weight needs to be closely scrutinised. We’ve been told for years to observe the healthy eating pyramid, which seems, on the face of it, very sensible. And yet we are in the midst of an obesity epidemic, so either these recommendations are wrong or the vast majority of people are unable to follow them.

My purpose in writing Good Calories, Bad Calories was to get people to pay attention to an alternative hypothesis. In an ideal world, the book would have been called The Alternative Hypothesis, but my editors were not going to let me write a book – which they hoped would be very popular – with the word “hypothesis” in the title. I became more of an advocate in the second book, Why We Get Fat. As I say in the author’s note, this was partly to force the issue, especially with the research community. But it was also because I was getting such positive feedback that it seemed a risk worth taking.

Before we get to your alternative hypothesis, I want to hear more about the view you are critiquing. Are you saying that the scientific evidence for the food pyramid the government recommends is lacking?

Yes. The [US] government, to its credit, has dropped the food pyramid for My Plate. But the general message hasn’t changed much, which is that most of our calories should come from carbohydrate-rich foods and fat should make up as little of our diet as possible. But we have never done a clinical trial or experiment that could demonstrate that this approach actually makes people healthier. The existence of the obesity epidemic in association with this message certainly suggests that the message could be wrong.

So you advocate a more open-minded approach?

I advocate an open-minded approach that takes into account all the evidence and not just part of it. I would argue that this a scientific approach, although the people who disagree with me would argue that the last thing I am is a scientist.

In your book you argue that conventional wisdom started going down the wrong track in the 1940s and 50s. Is that why a lot of the books you’re recommending are quite old?

Yes. Before World War II in Europe, there were two general themes that were running through nutrition and obesity science. One was that the refinement of food – particularly the refinement of carbohydrates and sugars – was causing major health problems. Obesity and diabetes were the most obvious results, but there was a general decay of health in populations that ate diets rich in refined grains and sugars. The other was that clinicians studying obesity had come to believe it was foolish to think obesity was caused merely by people consuming more calories than they expended. They thought it was a hormonal, enzymatic disorder, just like any other growth disorder. If you have gigantism, when people grow far too tall, or dwarfism, when they don’t grow to a normal adult height, it’s to do with grown hormones and growth hormone receptors.

These researchers argued that what was true of vertical growth disorders was almost as surely true of horizontal growth disorders – obesity, in other words. All we have to do is fully understand which hormones and enzymes regulate fat accumulation and fat tissue, and we’ll know what causes obesity. Then in the 1960s, long after this European school had evaporated, it became clear that the hormone regulating fat accumulation was insulin. We secrete insulin primarily in response to the carbohydrate content of our diet. This fit perfectly with the pre-World War II observations that populations that ate refined carbohydrates had obesity, diabetes and other chronic diseases, and populations that didn’t eat these foods didn’t. That’s the story I’m telling in my books.

This could help explain something that I’ve never fully understood – that some people who are quite seriously overweight don’t actually eat very much.

Yes, they don’t necessarily eat any more than lean people do. One of the things I try to do in Why We Get Fat is to argue against the concept of overeating as the cause of obesity, and one of the arguments I use is that no one can even define what overeating is. We know, for instance, that [Olympic swimmer] Michael Phelps eats 12,000 calories a day. But he’s obviously not overeating, because he’s not obese. The whole concept of overeating can’t be defined, unless you know the person is obese. There’s a circular logic to it. I can eat 3,000 calories of food and be perfectly lean, and my twin brother can eat 3,000 calories and be obese. The question is why? What’s happening in his body that’s making him fat, while the same 3,000 calories don’t make me fat?