For all the years that people have been dieting, especially those who count calories, they have followed the golden rule: cut 500 calories a day out of your diet and you will lose a pound a week. That is because 3500 calories is equivalent to one pound of adipose tissue (body fat).
While this is true, it does not follow that if you decrease your intake by that amount of calories over time that you will lose a pound. New research now proves this to be true! The way your body responds to a calorie deficit is dependent on baseline body composition, age, height, gender, and degree of caloric restriction. The result is a curvilinear pattern of weight loss over time rather than the linear pattern predicted by the 3500 kcal rule.
In other words, no two people are alike. Some have more muscle mass than others, and even at rest, muscle uses more energy (so muscular people, even at the same weight will need more calories). Also, quick weight loss typically results in loss of muscle mass, which then results in your body needing less calories. Finally, your body adjusts to a lower body weight once you do lose weight, so your energy needs change.
If your weight has creeped up higher than your normal weight, instead reflect on any lifestyle changes or eating habit changes that may have occurred over the years. Did you change jobs from a very active one to a sedentary one? Did you start a job at a restaurant where you now get food for free (and so you take it, even when you are not hungry)? Are you going back to school and no longer have time to cook, so eating fast food every day instead? If you focus on ways to shift into a healthier lifestyle, your body will know what weight it wants to be! So stop counting, and start living!