You probably know someone who lost weight and looked great, and then a few months later had rebounded back to where they’d been before they lost it. Maybe that’s happened to you.



Though it’s all too common for people to regain their lost weight and even to gain more body fat than they lost, it doesn’t have to be that way.



There are many reasons for the rebound, and a new study points to a new cause most people are unfamiliar with. First, I’ll cover the common and obvious causes of weight regain. Then I’ll delve into the new one discovered through a new study published by Nature. I hope it enlightens both fitness professionals and everyday Janes and Joes alike.



1. Going Back to Bad Habits



I’m not a fan of fitness goals with deadlines, like weddings or physique contests. In some instances, they’re unavoidable, but most people who set a fat loss goal for a specific event end up dropping their program right after the event.



As I explained in a previous article, you create an endpoint for your efforts when you set a specific, time-based goal. I’ve heard people in the gym talk about being two weeks into a 12-week diet, and how they were already fantasizing about the junk they’d eat when it was over.



I don’t usually recommend strict diets for this very reason. If someone has it in their head that they can return to their old ways at some future date, they will indeed do that. The diet and lifestyle choices that made them fat and unhealthy before the diet will undoubtedly do the same if they go back to them after the diet is over.



Instead, adopting some simple, nonnegotiable nutrition and lifestyle choices is better. In doing so, the weight will come off over time, and they’ll be less risk of regaining it because those simple choices become lifelong habits.



2. Lost Muscle Mass and Dysfunctional Metabolism



Weight loss and fat loss are not the same things. Many low-calorie diets, especially those that don’t include enough protein, cause significant muscle loss along with some fat loss. This problem is further compounded when you add in excessive amounts of cardio.



Muscle loss leads to a lower metabolic rate and less carbohydrate storage capacity. On top of that, thyroid levels plummet on long-term, low-calorie diets. In some people, hypothyroidism could become a permanent condition.



Is some level of calorie restriction required for fat loss? Yes, but not as much as most people think, especially if that reduced-calorie diet is high in protein.



Imagine combining the slower, dysfunctional metabolism with a return to the old diet and lifestyle habits I discussed in the previous section. This can lead people to gain more body fat than before.



3. Reduced Exercise and Activity



I’ve met gym members over the years who’d spend two hours a day doing cardio leading up to a special event. I also know of many people who take up marathons and triathlons as a means of weight loss. What happens after the event of competition is over? 



They stop doing so much cardio or endurance exercise. Some stop altogether.



They stop expending so much energy but often end up eating the same as they had while exercising. And often, these foods are high in carbohydrates, which helps them quickly pack the body fat back on.



This is why I use cardio sparingly when I work with clients. If someone is obese, it can be a helpful tool to trigger weight loss (along with a good nutrition plan), but over time, I reduce cardio and increase resistance training. For those closer to a healthy weight, I might never have them do cardio, focusing only on their nutrition program and resistance training.



Read also: Can you lose weight without cardio?



4. Insufficient Post-Diet Protein Intake



Of all the dietary choices you could make after a diet, continuing to eat high-protein is the most important. Research shows that a high-protein diet will help you keep your appetite in check, fill you up so you won’t eat as much junk,