Why do you gain weight so fast after losing it

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Have you been dieting for a while and lost all the weight you wanted to lose just to gain it back? Don’t worry, you’re not alone; many people face a similar problem.

About 80% of people will regain their weight within a year of quitting their diet. Within two years, about 85% of people will regain their weight, and within three years, that figure rises to 95%. (Norton and Baker, 2019). 

Unexplained weight gain happens due to a number of reasons, and specifically due to your body’s defence system. We’ve created this post to help you understand why you keep gaining weight.

Why am I gaining weight?

During a diet, the body goes through a number of changes, including changes to the hormones that control your cravings and appetite, as well as changes to your metabolism.

All of these modifications occur to protect the body from fat loss, which the body interprets as a sign that it is hungry and needs more food. As a result, your hormones will start to drive your appetite, and your metabolism will slow down, causing you to store the majority of the calories you eat. This is why it is so easy to gain the weight back on.

So how does this work?

Your body is very clever; it has its own defence system, and this is called the ‘starvation mode’.

When on a calorie deficit, the body will make hormonal and metabolic adaptations to defend itself from further weight loss (diet). When this happens, you’ll know because you’ll hit a plateau.  If you keep a close eye on your physical changes, you’ll be able to tell when this happens.

As these changes to hormones and metabolism occur, the body fat starts to overshoot. This means it is on standby to grab any calories and nutrients it can get its hands on. As a result, when you eat any food, your body will do whatever it can to replenish what you’ve depleted it from, preventing weight loss from occurring again.

Furthermore, this makes the body more efficient at storing calories as body fat, which is why gaining weight after dieting happens so quickly.

In the next part, we will take a look at each stage of the defence system, and how it affects your body.

The first stage of the defence system

The first stage of the defense system is known as metabolic adaptation. This is to defend your body against the caloric restriction you have placed it under. Essentially, your metabolism slows down to defend against starvation.

The best way to describe this process is with an example. So, let’s presume that before you began your diet you consumed 2000 calories per day. Then, you decide to go on a diet on which you’d eat 1500 calories per day. The defence system will be triggered at that point.

How? Since you will be on a diet, you will feel hungry, and your body will begin to break down your fat stores to release calories for your everyday activities and exercise. When your body fat begins to break down, a hormone known as leptin reacts by sending a signal to your brain, informing it that your fat stores are being broken down

How does the leptin hormone work?

Consider it as an air conditioning system: the temperature is set at a certain level, and if it drops below that level, it will switch on to increase the temperature back to that level, and vice versa.

The hormone Leptin works in the same way. The body fat setpoint is sensed via the size of each individual fat cell, as these fat cells begin to shrink during ‘starvation mode’ they release less leptin which reduces your metabolic rate and it tries to drive it back up by increasing another hormone called ghrelin which skyrockets your hunger. A lot of people will give in to that hunger and overeat so those fat cells expand again. This will increase the metabolic rate and take it back to its original set point.

It’s not just short term dieting, even people who have been dieting for months or years, their body will fight back to its original set point.

The last step of the defence system

By now you should have a fair understanding on how a diet makes biological changes to your body. Therefore, you’ll know that due to those biological changes your body prepares itself for weight regain before you even finished your diet by increasing hunger and reducing your metabolism. What does this mean for you? It means your body will be much more efficient at storing calories as fat.

You’re about to learn the last step of the defence system, but the best way to do this is to continue with the initial example. Remember, before you began your diet you consumed 2000 calories per day, then you reduced your calories to 1500 to lose weight. Let’s now say you have completed your diet and you are happy with your weight loss and begin to eat 2000 calories again. By this point, all those biological changes have occurred and your metabolism has slowed too which means that you will now be overeating by 500 calories.

Normally, fat cells shrink when you lose weight, but they grow afterward and depending on how severe the weight loss was, the body’s defence system can also produce new fat cells.

So, why do I keep gaining weight?

When you subtract 500 calories from your regular calorie intake, your metabolic rate slows to 1500 calories per day, which means those 1500 calories are now your maintenance calories.

As a result, after you finish your diet and return to your original daily calorie intake of 2000 then you are now consuming 500 calories more than you need. At this stage, the body would have found enough fuel (calories) to replenish its fat reserves and protect itself from further depletion by forming new fat cells. To put it another way, it will collect and store all of the food and ‘extra’ calories it can.

How to maintain weight after losing it?

In order to maintain your weight loss and minimise weight regain you need to speed up your metabolism. The best way to speed up your metabolism is through High Intensity Interval Training (HIIT) and Reverse Dieting. If you have been struggling to lose weight and you’re worried about your health, you can now get a weight loss test to identify the underlying reasons why you are struggling to lose weight.


In conclusion, the reason why you are gaining weight is due to hormonal and metabolic changes. This is why it’s so important to perform a diet that you can adhere to. Diet becomes a lifestyle, it’s not just a diet. If you are planning to do intermittent fasting or a ketogenic diet, just ask yourself if you can do this as a lifestyle? If you can, perfect go ahead. But the only thing that will drive your weight loss is a caloric deficit. And, the only thing that will reduce your weight regain is reverse dieting.

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Norton, L. and Baker, P. (2019). Fat loss forever. 1st ed

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