Our body weight is determined by the amount of energy that we take in as food and the amount of energy we expend in the activities of our day. Energy is measured in calories. Metabolism is the sum of all chemical processes within the body that sustain life. Your basal metabolic rate is the number of calories (amount of energy) you need for your body to carry out necessary functions. If your weight remains constant, this is likely a sign that you are taking in the same amount of calories that you burn daily. If you're slowly gaining weight over time, it is likely that your caloric intake is greater than the number of calories you burn through your daily activities.

Weight loss isn’t a linear event over time. When you cut calories, you may drop weight for the first few weeks, for example, and then something changes. You eat the same number of calories but you lose less weight or no weight at all. That’s because when you lose weight you’re losing water and lean tissue as well as fat, your metabolism slows, and your body changes in other ways. So, in order to continue dropping weight each week, you need to continue cutting calories.