BMR is the most important factor, the amount of calories you need while at rest. Accounting for body size, activity level and some other factors, you can calculate what you need to function, but to lose weight you need to reduce this amount by 500-700 calories, It is almost impossible not to lose weight in this manner. Its no fad, just sound weight loss. To increase body weight, you need to do the opposite.
There are plenty of online calculators to accomplish this.