There are well-documented phenomena that after some people lose weight, it often slowly comes back. Often this is put down to laziness or weakness, which of course if we are honest we ourselves we all can be that sometimes, but actually there are very clear biological reasons for this.
The first reason is called adaptive thermogenesis which i already discussed in principle 3 when I talked about our metabolic rates otherwise called basal metabolic rates or BMR. Our metabolic rates are the calories we burn just being alive, breathing, pumping your heart, creating heat, and it is the largest way we burn calories in our body.
The bad news is that as we lose weight our BMR actually slows down, so it gets harder to keep losing weight. This is a survival mechanism and in our past evolution, it was really important. When humans were evolving we lived in an environment that had little food around. So when we were starving the body reduces the BMR to stop you wasting away. So this is great for an environment that has little food and you need to survive in the short term but now we live in the opposite environment, where there is lots of food and we need to survive in the long term.
The second reason is our hormones. One year after we start reducing our intake we can see certain key hormones change. Leptin is a hormone that is released from your fat cells and the more fat cells you have, the more leptin you make. There is debate over its exact role, whether it is supposed to tell us we have had enough, or to motivate us to eat if it is low. However, we think that when it goes up it signals to the brain when you have had enough after eating. However, the longer you are overweight this hormone goes down and you may become resistant to it, which means you your body can’t tell the brain it is full and you feel more hungry. Also, your hunger hormone, called Ghrelin, goes up, which makes you even more hungry. So that sucks!
And finally being overweight for long periods has some associated changes in the brain like neuron injury in parts of the brain crucial for body weight control like the mediobasal hypothalamus.
All these things can change and over time the longer you lose weight and keep it off then these biological signals may reset closer to normal, and you can continue to lose weight in the long term. But we need to be aware of where these signals are coming from so you don’t beat yourself up if you go up and down for a while.