Your body needs energy to operate
We will start with energy because weight loss is all about how your body uses and stores energy.

Basal Metabolism: Your body needs a certain amount of energy to function – to power your heart, lungs, brain, kidneys, and other organs and keep them in good repair. The amount of energy you use when you are completely at rest is called your basal metabolism. The rate at which you burn energy when you are completely rest is called your basal metabolism. The rate at which you burn energy when you are completely at rest is called your basal metabolism rate (BMR).

Each person has his own BMR determined partly by heredity and partly by lifestyle. Your heredity sets the upper and lower limits of your BMR. You can maximize your BMR within this range by doing daily aerobic exercise and by eating a lot of nutrient-dense, fiber-rich carbohydrates. The higher your BMR, the faster and more easily you lose weight.

Physical Activity: Unlike your basal metabolic rate, physical activity is not fixed within a range. The more exercise you do, the more energy you need to fuel it.

Total energy needs: Your total energy needs are the sum of your basal metabolism plus the amount of physical activity you do. The higher your BMR and the more active you are, the more energy you need.

Food Supplies Energy
Where does your body get energy to power your basal metabolism + physical activity? Just as energy stored in batteries powers machines to do work, energy stored in the food you eat powers your body.

The energy in food comes packed in three types of nutrients: fats, carbohydrates, and proteins. The amount of energy supplied by each of these nutrients is measured in calories.

Calorie Facts: A calorie is a unit of measurement of the amount of energy stored in food. The nutrients that contribute calories (energy) are carbohydrates (sugars and starches), fats, and proteins. Alcohol also contributes calories. Total calories are the sum of fat, carbohydrate, and protein calories: Total Calories = Fat calories + Carbohydrate calories + Protein calories. If alcohol is consumed, the alcohol calories are included in the total.

Not all calories are euqal
Many people believe that whether you eat fats, carbohydrates, or proteins, if you eat too much, the energy is stored and becomes fat. Not true. What is true is that the body handles carbohydrates, fats, and proteins differently and the calories from each have a totally different effect on weight gain.