Our bodies control the things that matter for survival. They have set points for temperature, blood pressure, oxygen levels, blood sugar, all sorts of chemicals in our blood, and many other variables. When changes occur, our bodies automatically react to return us to our set point, usually without us even noticing. We call this Homeostasis. It’s a basic and fundamental part of life.
Our bodies have a set point for weight, which reflects the amount of energy available (stored as fat and other tissues), should we need to survive without much food. For most of human history, storing extra calories for times of food shortage, preventing losses, and regaining as quickly as possible, have been vital survival mechanisms. There are still many places in the world where this is all that stands between survival and starvation. Unfortunately, in our obesogenic environment – where abundance of calories is the norm, and shortage is never an issue – these natural defence mechanisms may cause harm.
When weight decreases, the body reacts to avoid more weight loss and restore weight to its set point. The reactions are automatic, and most are entirely beyond our control. They involve our brains, nerves, hormones and every organ system in our bodies. They affect our mood, emotions, motivations and desires, along with our hunger, how much we need to eat to feel full, and how long it will be before we get hungry again. They change the way our body processes food, and how the body uses the energy it gets from food. How effectively a person’s body builds and defends fat stores varies a lot from person to person, based mainly on their genes.
Some responses to weight loss kick in straight away, others take longer. This is why we sometimes get ahead, losing weight with a new diet or lifestyle intervention, only to see it come back as the slower acting homeostasis responses take effect. While it’s easy to see this as a sign the diet has stopped working, or we’re not doing it well enough – mostly it’s simple biology. As time goes on, the body fights harder against our efforts, in order to get back to that set point.
This is why losing weight is hard – it’s supposed to be! Our bodies are making it as hard as possible to lose weight, so we won’t starve. Diets and lifestyle interventions aimed at weight loss consistently produce around 3-6% weight loss in the long term. More effective treatments, such as modern obesity medicines and surgery, achieve much greater weight loss by interrupting the body’s natural defences to it. But weight loss will always stop at some point. Inevitably the impact of the intervention and the body’s response to it will even out – making stable weight at any size a sign of success, not failure. And any time an intervention or treatment is stopped, the body will restore bodyweight to where it started, or even a little higher. Because the body fights weight loss – it’s trying to protect you.