The actual number depends on body size, age and activity level. While there are formulas to narrow down this range, none of them are exact for any one individual. Figure out how many calories you're eating, and whether you're gaining weight eating that much. When you hit the level that your weight is stable, that's how many calories you need. For example, some fad diets can cause deficiencies in calcium.
Adequate calcium is needed to ward off osteoporosis.
Other fad diets restrict carbohydrates and offer unlimited fat. But a high-fat diet can lead to cardiovascular problems. There is no biological reason that it's harder to lose weight the second time or any time thereafter, said Dr. Ryan. In fact, she said, people often learn from failed attempts to lose or keep off weight. If they figure out what went wrong and why, they might be more successful the next time.