Many people try to lose weight by just dieting alone, they cut the amount of calories they consume or try some fad diet. In many cases this tactic might work in the beginning but its unsustainable, why can’t you lose weight by just dieting?
As you lose weight your metabolism slows, the metabolic rate is the rate at which you burn calories the more you weigh the higher your metabolic rate will be, as you begin to weigh less your body will burn less calories. You may think a simple answer to this problem is to just cut calories further but that can slow your metabolism even more you need to take into account your BMR (Basal Metabolic Rate), this is the energy (calories) your body burns just to keep you alive.
Your body will only release enough energy in this basal state for the functioning of your vital organs, such as your heart, lungs, brain, etc.
Heavier people will have a higher BMR than lighter people. Cutting calories too far will not only decrease your metabolic rate in the short term but cause loss of lean mass (muscle) in the long term, the less muscle you have the more fat you’ll put on when you start eating normally.
This is why losing way by dieting alone is unsustainable, to lose weight long term and keep it off you need to increase your metabolic rate whilst being more active and burning more calories.
Somebody who eats 2000 calories a day and burns 2000 calories a day is going to stay at the same weight, their not going to lose any fat, get any heavier or lighter.
If they took up exercise and created a calorie deficit burning more calories than they ate, they would lose fat whilst maintaining a fast metabolic rate. Maintaining this faster metabolism means they would continue to burn more calories than they ate and continue their weight loss.