In 2016, researchers at the National Institute of Health shocked the world, when they publish results of what would be dubbed the biggest loser study. After falling 14 out of 16 contestants from season eight of the popular show for six years, they made a surprising observation. Even though six years had passed since the end of the competition, their bodies were still fighting back to regain the weight they had worked so hard to lose. By the end of the 30 week contest, the average resting metabolic rate or metabolism of the contestants was 600 calories per day slower than at the start of the competition. That however was expected. When a person loses weight, he or she loses muscle, as well as fat. As a result, metabolism slows. Six years later, most of the contestants had regained some of the weight they had lost. So, their metabolism was expected to recover in proportion to their body composition, yet instead, their metabolism slowed even further. The average resting metabolic rate of the contestants was 500 calories per day slower, than someone who was at their same weight and with the same amount of muscle and fat mass, who had never lost such a massive amount of weight. This difference between their expected metabolism based on their body composition and their measured metabolism is called metabolic adaptation. It represents the degree to which the body fights back against hard-earned weight-loss, even six years later. If these individuals who had the support of the world's best trainers and nutritionists couldn't succeed at sustained weight loss, what does that mean for other people struggling with weight, who don't have these resources? A flurry of headlines debate at these questions. Five years prior, another groundbreaking study published in the New England Journal of Medicine had made another peculiar observation that changed the way we view obesity. Rather than looking at calories we burn after weight loss, these researchers looked at calories we take in, or a hungry level after weight loss. They placed 50 obese and overweight people on a very low calorie diet of 500 to 550 calories per day for 10 weeks. They measured each participant's appetite hormones at the start, after the 10-week diet, and then a year later. Following weight loss from the very low calorie diet, these participants had a higher amount of the appetite hormone that signals hunger called ghrelin, and a lower amount of the appetite hormones that signals to tidy. Their brains were getting the signal that they were hungrier and less satisfied after eating, to drive their weight backup, and this lasted a year later. Like the biggest loser study, what they were seeing is that our bodies somehow fight back against weight loss. The authors concluded, the high-grade of relapse among obese people who have lost weight has a strong physiologic basis, and is not simply the result of the voluntary resumption of old habits. Understanding how and why, and finding ways to overcome these strong compensatory responses is the reason most people struggle with their weight, it's the reason weight regain or yo-yoing is so common after weight loss. But rather than viewing this as a failure, we're learning that our bodies are fighting hard to drive us to regain weight. Our body continues to send signals that are hard, sometimes impossible to fight. Losing weight successfully requires a reprogramming of the signals. This has become the holy grail of Obesity Research. Over the coming weeks, I want to share with you the things we have learned so far, the take-home is that managing your weight as a balancing act between calories in versus calories out is important, but simply outdated. Sustained weight loss is far more complicated. What I hope you will learn from this course is a far more comprehensive strategy.