"These diets are so restrictive that of course you're going to lose weight fast because you're not eating enough calories to sustain basic activities of your body, let alone any exercise. That's nothing that any person can sustain for the long term," Hogan said. "The weight's going to come back if you do lose any weight, and then it's going to be harder to lose weight in the future."
Even tiny changes in thyroid function can meddle with the way your body processes food for fuel because thyroid hormones affect metabolism and how your body deals with weight. When thyroid hormone levels drop too low, you can start gaining weight. You may also have other symptoms that don’t seem connected — low energy, feeling cold a lot, skin issues or hair loss.
Sure, you can lose weight quickly. There are plenty of fad diets that work to shed pounds rapidly -- while leaving you feeling hungry and deprived. But what good is losing weight only to regain it? To keep pounds off permanently, it's best to lose weight slowly. And many experts say you can do that without going on a "diet." Instead, the key is making simple tweaks to your lifestyle.