The scientist and broadcaster discusses the drawbacks of calorie-counting and BMI in measuring obesity, and how our growing understanding of genetics is leading to new treatments

Since the dawn of the 20th century, almost all weight loss guidelines have used calories as a simple measure of how much energy we’re consuming from our food. But according to Giles Yeo, a Cambridge University research scientist who studies the genetics of obesity, there’s one problem: not all calories are created equal. In his new book, Why Calories Don’t Count, Yeo explains that what really matters is not how many calories a particular food contains, but how that food is digested and absorbed by your body.

Can you explain why you feel calorie-counting is a flawed approach to weight loss?
There was an American chemist in the 19th century called Wilbur Olin Atwater who calculated the calorie numbers for different foods, by working out the total energy intake you get from them. But his calculations never took into account the energy it takes our cells to metabolise food in order to use it. This is important. It’s why for example a calorie of protein makes you feel fuller than a calorie of fat, because protein is more complex to metabolise. For every 100 calories of protein you eat, you only ever absorb 70.