Obesity science has reached a crossroads: The carbohydrate hypothesis is poised to overtake “calories in, calories out” as the predominant understanding of weight gain. Physicians, dietitians and trainers have come to treat “carbs are bad” as a new gospel, preaching a lifestyle that strays very far from the true scientific consensus. In "What Is Fat For?" Ignatius Brady, a weight loss physician and science writer, presents a fresh perspective on obesity based on critical new research that has gone largely overlooked.

The protein leverage hypothesis holds that neither dietary fat nor dietary carbohydrate “cause” us to gain weight. The obesity debate is re-framed as an imbalance between “protein” and “non-protein” energy. This imbalance has caused a widespread human adaptation: the obesity epidemic.

In a thorough yet readable style, the book takes the reader through normal human weight regulation, the time frame needed for weight loss, and what’s missing in the “carbs are bad” thinking. This is not a diet book, but a scientific exploration of the inner workings of human biology and our interactions with the modern nutritional environment. The question, “What is fat for?” drives this narrative, which takes nothing for granted, analyzes all possibilities, and presents detailed evidence for the reader’s best judgment. Biases are overturned, accepted wisdom is re-considered and new answers are discovered.