We all know someone who seems to eat very little yet cannot avoid weight gain, or someone who eats everything they like while remaining slim. Why? Is it the kinds of food we eat, and when? Are our hormones to blame? Could it be chemicals in our environment? Even specialists are now questioning our understanding of the forces that shape body weight, and we are all more confused than ever.

In this book, Keith Frayn, one of the world's leading experts on metabolism, argues that all these challenges are distracting us from tackling the obesity problem in the only way it can be addressed: by rebalancing the disregarded message of 'calories in - calories out'.

Taking readers on a deep dive into the real science of energy balance, he reveals how nutrition research has been plagued by the difficulty of really knowing what people are eating and doing; why it is unlikely that some nutrients are intrinsically more fattening than others; how supposed differences between people in the speed of their metabolism vanish in the laboratory; how energy balance is altered in obese people and people who have managed to lose weight; and why these responses - honed over millennia of evolution - make dieting so hard.

With clarity and insights from expert research, Frayn provides a clear-eyed perspective on current trends mired in controversy and confusion: time-restricted eating, intermittent fasting, low-carb versus low-fat meal plans, high-protein breakfasts and other dietary trickery.

In a world where desirable, energy-rich food is increasingly plentiful, and labour-saving gadgets widespread, it becomes more and more difficult to stick to the simple message of energy balance. But, as Frayn shows, we can reshape our lives and improve our health by going back to what we know about calories, rediscovering the benefits of a more active life, and getting smart about what we eat.