The best way to lose weight, believe me, is to eat less, said Boris Johnson as he defended the Governments food strategy after proposals for a salt and sugar tax were ignored.

With that believe me, the Prime Minister hit a nerve for serial dieters: which parts of the vast smorgasbord of advice out there should we be following to ensure our health and happiness?

Along with the recent introduction of calories onto restaurant menus, it can feel as if were being shamed into giving up food altogether.

For Professor Tim Spector, the Kings College London epidemiologist celebrated for his work with identical twins, as well as diet and the microbiome, all of the above represents a backward step in the public understanding of how humans respond to and process food.

For the past 100 years, weve been obsessed with calories, and it has really stopped us thinking about anything else, says the 63-year-old author of The Diet Myth and Spoon-fed. He has been working hard to change that thinking.

When The Diet Myth was published in 2015, few people had an inkling of the role that the estimated 100 trillion microbes in our gut play in our digestion. Spectors work has helped to put kefir in our fridges and kimchi in our jars. Via the Zoe Project, the worlds largest nutrition study, he has encouraged us all to join up and analyse our unique guts, blood fats and blood sugar responses.

Today, though, his number one myth target is that calories are a useful way to monitor our diet. Not only are calorie estimates often less accurate than we might hope, Spectors studies of twins have shown that humans vary hugely in how much energy they extract from a given food.

The daily allowances for men and women, Spector says, are not based on hard data. So I ask him: what should we be aiming for? Even asking the question, he says, gives credence to the idea that there is a perfect figure. If it was only 1,900, would that make a difference? No, it wouldnt.

And when people are told to avoid calorie-dense foods, Spector says that advice can be taken to be encouraging the consumption of low-calorie drinks and low-fat foods. Its why we support this multi-billion-pound diet industry of low-calorie shakes and Weight Watchers, and all that other stuff.

So, what other diet myths are we swallowing, according to Professor Spector?

Exercise does require energy, but our metabolism adjusts to that loss by storing more energy as fat the next time we eat.

Our body is programmed to keep our biology steady, known as homeostasis, so if our energy levels are drastically changed with lots more exercise and less food, our metabolism will respond by slowing the weight loss down and eventually put it back on very quickly when we go back to normal activity and food which is what we see in yo-yo dieters who put all the weight they initially lose back on. Saying that exercise alone is a good way to reach a healthy weight in the long-term is complete rubbish, Prof Spector says.

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11 of the most common diet myths, busted by Professor Tim Spector - The Telegraph