According to NASM 50% of dieters gain weight back. Why? Because most diets do not teach you to make healthy decisions every day. You are sold a product that requires an extreme measure that cannot be sustained over time. Only daily, healthy choices are the key to keeping weight off. Think of weight loss in terms of sustainability.
One pound equals 3500 calories. So, to lose one pound in one week, you must have a caloric deficit of around 500 calories each day. In other words, you would eat 500 calories less than what your body needs, or you have to would burn an extra 500 calories each day. For two pounds a week you would need to forgo 1000 calories per day. This is relatively sustainable for most people.
One to two pounds per week can seem pretty slow, especially for those who have a lot of weight to lose. However, an overweight person who normally exceeds their body’s caloric needs may lose more weight at first just by staying within their personal daily caloric needs.
Fast weight loss requires an excessive daily caloric deficit. Because your body needs calories just to exist, depriving yourself so severely simply cannot be sustained over time. Any diet that requires some sort of starvation or exuberant deprivation should be avoided. Bottom line, if it is not sustainable, it is probably not healthy.
Be patient and remember that weight loss will happen as you adopt a healthy lifestyle and make good choices. You will be more likely to keep the weight off because you have made healthy, sustainable habits for life.