The Obesity Code: Unlocking the Secrets of Weight Loss is a work of nonfiction by Dr. Jason Fung, a Canadian nephrologist and expert in intermittent fasting. Founder of The Fasting Method, Fung argues that intermittent fasting, combined with a diet low in refined carbohydrates and high in natural fats, combats obesity and obesity-related diseases, notably, type 2 diabetes. Drawing on decades of clinical experience and secondary research, Fung debunks deeply engrained myths surrounding obesity, presenting insulin, a hormone that helps regulate the metabolism, as the prime factor driving the disease. Combining rigorous research and an engaging writing style, The Obesity Code is an accessible introduction to the science of weight loss and the power of intermittent fasting.

This guide refers to the 2016 edition published by Greystone Books.

Summary

The Obesity Code comprises an Introduction and six parts, followed by appendices with sample meal plans, fasting protocols, and guidelines to reduce cortisol. The Introduction outlines Fung’s evidenced-based approach to treating obesity and obesity-related diseases, such as type 2 diabetes, as well as identifying high insulin levels and insulin resistance as key factors driving weight gain.

Part 1, “The Epidemic,” examines the timeline of the obesity epidemic, which emerged in North America in the 1970s, when people began eating more refined carbohydrates. Following a critique of the “Eat Less, Move More” approach to weight loss, Fung argues that obesity is a multifactorial disease that results from a combination of genetic and environmental factors. Hormonal imbalances account for approximately 70% of the tendency to gain weight, leaving about 30% open to environmental control.

Part 2, “The Calorie Deception,” critiques the “calories in, calories out” theory of weight loss. Homeostasis, the biological principle describing the body’s tendency toward equilibrium, explains why caloric reduction doesn’t produce weight loss. Research shows that the body adapts to caloric reduction by slowing the metabolism and expending less energy, leading to a weight loss plateau, weight gain, and self-recrimination. The body also adapts by raising hunger hormones and lowering satiety hormones, both of which increase the desire to eat. Despite its proven inefficacy, health experts continue to promote caloric reduction, alongside exercise, as the path to lasting weight loss.

Part 3, “A New Model of Obesity,” introduces a hormonal theory of obesity. Fung argues that obesity has many contributing factors, including genetics and environmental factors. Of these, high insulin levels and insulin resistance are the most important. Certain foods cause spikes in insulin, while persistently high insulin levels lead to insulin resistance. Managing insulin levels through low-carbohydrate diets, such as the Atkins Diet, does not produce long term benefits. Lasting solutions to obesity must address homeostasis with a combination of meal composition and meal timing.

Part 4, “The Social Phenomenon of Obesity,” considers societal factors related to the obesity epidemic. Keen to increase their profits, food corporations fueled obesity in the closing decades of the 20th century by promoting snacks as a new category of food and by paying health organizations and practitioners to endorse their products. Obesity rates rose in response, particularly among the poor, whose diets comprise largely of affordable, refined carbohydrates. Childhood obesity also skyrocketed in this period, including in infants aged zero to six months, who developed high insulin levels in the womb.

Part 5, “What’s Wrong with Our Diet?” examines the role of the three macronutrients—carbohydrates, protein, and fat—in weight gain, identifying fructose, which is found in soda, as the main culprit. Fructose has a low glycemic index compared to glucose, but it increases the risk of fatty liver when consumed in large quantities, a condition that results in insulin resistance and weight gain. Processed foods are fueling the obesity epidemic, particularly refined carbohydrates, which are quickly absorbed and result in insulin spikes. Some proteins also raise insulin levels. By contrast, dietary fats not only have a weak insulin-stimulating effect but may even protect against obesity by tempering insulin surges from other foods.

Part 6, “The Solution,” offers guidelines to achieve lasting weight loss. To reduce insulin levels, Fung recommends consuming fewer added sugars and refined grains, keeping protein consumption moderate, and eating more fiber and healthy fat. He presents intermittent fasting as an effective treatment for insulin resistance, one that avoids the negative effects of calorie-reduced diets. Controlling cortisol, a hormone that impacts insulin levels, through stress management and sleep improvement is also important. In short, Fung recommends a multipronged treatment for obesity, focusing on breaking the cycle of high, persistent insulin.