This is the million dollar question! The answer to this question is not simple but let’s give it a go shall we?
First of all, by definition, a diet is finite.
It get you from A to B. So what happens usually is that A is your current status and B is your goal. You reach your goal and then there’s no plan, no maintenance and this is where people naturally go back to habits they had when they were at point A - taking them right back to it.
Then there’s the fact that, especially with fad-diets, when your energy balance changes your body adapts. What does that mean? Well it means that your body learns to be more efficient driving down energy intake and driving up energy expenditure to compensate for the lack of energy.
So you have two things happening: one is the diet you are on is meant to get you to point B and then leaves you hanging. Two this journey causes a physiological response so that when you do reach B you just go back to A.
Let me give you an example to make this clearer.
Let’s say you go from A to B by creating an energy deficit, aka eating less calories, lose 0.5kg per week for 4 weeks. Now, you are 2kgs lighter than you were at A, but if you went back to the same diet you had when you were at A you’d be increasing your caloric intake to what it was before and hence going back to A. So to stay at B and maintain you’d have to consume the same amount of calories that got you to B for as long as you wish to stay at B. This is also a reason why diets need to be sustainable in the long run.
People miss this point more often than not. They think they’ve followed the diet to get them to B and that’s it job done!
Did you know that not only do a lot of people on diets tend to regain the weight they’ve lost, BUT they often gain more. There are physiological reasons for that and it’s because the body is designed to help you survive with deficit next time it happens. Next time you decide to go on a diet and lose weight you’ll have more energy stores to help your body cope with the “deprivation” and you end up with a bigger number on the scale than you were before and you’ll need to go on a even more extreme diet to get back to B.
I find it interesting how fascinated people are with new fad-diets and how amazed they are by the weight-loss experienced when they cut out whole food groups. It is pretty normal and expected to lose weight when you don’t eat a long list of foods and consume 800 calories per day, Anyone can come up with a new fad-diet and it would work.
The challenge is maintenance.