Most people will lose some weight if they follow a calorie deficit diet. Persistent calorie deficits produce 5-15 % weight loss at the end of 3-6 months, but most of the weight comes back soon. Studies have shown that at the end of 1 year, less than 50 % of people can keep their weight loss.
Weight loss can trigger many mechanisms by which people can put back weight. As people lose weight, the basal metabolic rate also reduces to a rate which is less than that predicted by their new weight. This means that people find it difficult to lose weight further with the same calorie deficits. This is called metabolic adaptation, and this limits further weight loss and promotes weight regain. Furthermore, as a person loses weight, there is a change in appetite-regulating hormones like Ghrelin and Leptin which leads to increased appetite that limits further dietary caloric restriction. Further, there is a “weight set point” (postulated) for an individual. Once there is weight loss with diet, there is a tendency for the body to come back to its original weight.
Studies have shown that by using weight loss medicines like Semaglutide and Tirzepatide, more patients can keep their weight down and prevent weight gain than just following a diet restriction alone. It reduces the appetite and signals the brain to limit the food intake. So, once you lose weight with a diet, it is not easy to keep it down.