Why do we gain weight after dieting: A full explanation.

Why do we gain weight after dieting: A full explanation.

A balanced diet and correct nutritional intake are two of the best ways to lose weight and tone up. However, what most people don’t know is that dieting triggers the body’s defense system which causes weight re-gain once a diet is complete. One of the scariest things about the defense system is that it sets you up for weight-regain before you have even completed a diet which means the moment you fuel your body with a higher calorie intake than your diet, it will seek to capture those calories and store them in the body to prevent future weight loss and replenish the body from its depleted fat stores.

In this post you will learn how the body’s defense system works and how to prevent and limit weight regain after a successful diet.

What is the body’s defense system?

During a diet to lose weight, also known as a caloric deficit, the body will naturally make hormonal and metabolic changes. You’ll notice these biological changes because at this point you will hit a plateau. Weight loss will slow down and the caloric deficit will no longer feel like you are dieting as you may feel more satiated than when you initially started. If you are uncertain about calories and caloric deficits, you can learn about this here.

What does the body’s defense system do?

There are 3 stages to the body’s defense. The 1st stage is known as metabolic adaptation. This is initially triggered by a decrease in fat cells which will naturally occur during a weight loss journey. In this way, The metabolic adaptation defends the body against the caloric restriction you’ve placed it under, or in other words, your metabolism slows down to defend the body against starvation.

To better explain this phase of the defense system, let’s take a look at an example.

Let’s presume we are currently consuming 2000 calories per day, and let’s presume this is the amount of calories we shall need to maintain your current weight exactly. If we begin your weight loss journey by eating less, so as now we are only consuming 1500 calories per day, this will prompt the body’s defense system to activate. This is due to the combination of eating less, being hungry as we are not eating what your body requires to remain in homeostasis (staying the exact same weight), and beginning to lose weight meaning that our fat cells will be shrinking. Our fat cells not only shrink however, but are also broken down to release energy for our daily gym session or daily activities and errands to make up for the 500 calories we are missing from your diet. As a result of these changes, the second stage of the body’s defense occurs - a hormone called leptin is released and begins sending signals to our brains.

How does Leptin work?

Leptin is a hormone that responds to changes in the body’s fat cells. Think of this hormone as an air conditioning system. An air conditioning system can activate itself if the temperature of a room falls below a certain temperature, in order to bring the temperature back to that which it is attempting to maintain. Vice versa, if the temperature goes above the set point, it will turn off to bring the temperature back down again.

Leptin works in a very similar way. The human body naturally wants to stay in a state of homeostasis, and therefore in addition there is a natural percentage of  body fat it will wish to retain, which is sensed via the size of each individual fat cell. When these fat cells shrink or begin to breakdown to produce energy for the body, you will enter the ‘starvation mode’. As a result, fat cells will produce less leptin reducing the metabolic rate and will try to drive the body to consume more calories by increasing another hormone called ghrelin which skyrockets hunger and appetite.

At this point you may experience weakness, extreme hunger and sugary cravings that are very difficult to resist. Sooner or later the majority of us give into this hunger and in most cases we over-eat which causes those fat cells to expand again.

The last phase of the defense system

Due to the combination of all these biological changes, the body seeks to become more efficient at storing fat in the future, to try and prevent the shortage of calories in the future.

To clarify this point, lets continue with our previously mentioned example. Remember, if we initially eat 2000 calories per day but lower our calories to 1500 calories per day we create an energy deficit to promote weight loss. At this phase, we would be happy with our weight, and we would have lost all the weight we targeted losing over the course of our diet, and now we would be ready to return to eating a normal pre-diet calorie amount (2000 calories). However, due to the hormonal and metabolic changes we experience, and the fact that we have taken our body weight down to a level where it only requires 1500 calories per day to remain the same weight, we will now be overeating by 500 (2000-1500) calories. This is despite the fact that these were calories that maintained our previous weight.

As we discussed your fat cells shrink when you are in the dieting process, however post diet, our fat cells will expand and sometimes depending on how drastic the weight loss was in terms of timescale and number of calories we choose to cut, your body’s defense system can also create  brand new fat cells to protect us from this event in the future. This is the primary reason why the vast majority of people who perform a successful diet can end up weighing more than when they initially started a diet, as our body’s defense system attempts to protect us from starvation and drastic weight loss in the future.
Over time, with repeated “quick weight loss diets”, the body’s natural response can be amplified, and the generation of new fat stores will become more frequent as the body learns to store as much energy as possible for the inevitable next period of dieting, without sufficient caloric intake.


During a diet our body will naturally make hormonal changes and metabolic changes to defend itself against future weight loss. You’ll know when this happens initially because you will reach a plateau with your weight loss. If you monitor your body changes closely, and stick to your diet well, you’ll realize when this occurs. All the biological changes causes your body fat to overshoot. This means your body fat is on stand-by to catch any calories and nutrients it can. So, once you give your body enough food for energy, it will grab all it can to restore what you have depleted it from, and prevents the weight loss from happening again. To minimize weight regain, you will need to look at option to increase your metabolism, some great options are thermogenic foods, reverse diet and HIIT workouts.

Written by Paula @ Nutrition2change


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