If the scale's needle starts creeping upward again after you've stopped dieting, we suggest you read the following. First, it will bring you comfort, because you'll learn that no one has any reason to criticize you for a lack of willpower. However, the hope that your weight would continue to decline, and the efforts you made for that, are considered to be more harmful than beneficial to your health, which might be discouraging—though we only hope it will bring you a sense of relief.
From an evolutionary perspective, weight loss is not advisable. Because Mother Nature and the Creator who aligns with her are always teaching us: "Grow and flourish abundantly!" Weighing less than what is naturally required can hardly be called flourishing. To illustrate this, we have many examples. Consider female athletes who train excessively or female patients with anorexia; they often stop menstruating at a young age. And it's not just them; women who eat only fresh fruits and raw vegetables are the same. In contrast, those with hearty appetites who can eat a lot are always the ones who last through difficult times and then quickly recover their strength. Therefore, holding on to every new gram of weight is the strategy for survival in nature, one that cannot be abandoned. Human society has been in an era of surplus production for less than a hundred years; not to mention, this prosperous era is only enjoyed by a small proportion of the population—the majority of people on Earth are still starving. So, from an evolutionary point of view, there is absolutely no reason to change this effective and reliable survival strategy.
Our bodies don't care about current mental fads; they simply interpret a reduction in calorie intake as an emergency, triggering a red alert and taking immediate precautions to prevent worse things from happening. Step one: lower the basal metabolic rate, meaning the body reduces its basic energy expenditure while at rest to conserve energy for heat production. Step two: make the most of every last bit of nutrition, and not waste these hard-to-come-by calories. Step three: increase appetite. Not only does our mind think about how to rebalance what has been lost, but our body is also very calculating. This is precisely why dieters spend all day thinking about food and drooling over imaginary delicacies.
We can indeed persist on a very low-calorie intake for a long time and become noticeably thin this way. However, our bodies have a perfect system for managing energy expenditure. After the famine period—i.e., the dieting phase—is over, it wisely reserves extra calories for future needs. Who knows when the next poor harvest or famine will come? So it must be prepared! At a minimum, the amount reserved is set a bit higher than the initial value before the last diet, because the last order wasn't enough, otherwise, why would there still be starvation? In this back-and-forth cycle, there is a slight calorie surplus each time, to ensure more calories are available for the next cycle. Consequently, this effect has a cute name: the "yo-yo effect," because it's like a yo-yo going up and down. And whether you like it or not, it also means that "dieting" ultimately makes you "fat."