Merely and intuitively speaking, to lose weight you have to eat fewer calories than your body uses. This may sound like a cliche but that’s the way it goes the healthy way. Calories are the amount of energy in the food you eat. More precisely and technically, a calorie is the amount of heat needed to rise the temperature of 1 gram of water to 1 degree Celsius.Foods that are high in fat and sugar are also high in calories so if you eat more calories than your body uses, the extra calories will be stored as body excess fat. A pound of fat is about 3.500 calories.
This means that in order to lose 1 pound of fat in a week, you have to eat 3.500 fewer calories (that is 500 fewer calories a day), or you have to burn 3.500 extra calories.Let’s take a short practical example: 6 Ritz crackers, seemingly small, weighs 20 grams but contains 6 grams of sugar and fat. Those 6 grams of fat and sugar pack 100 calories of stored energy, which if unused will be stored as body fat. To burn off the energy in those crackers you would need to: go swimming for ~15 minutes; go jogging for ~12 minutes; sprint (run as fast as you can) for ~8 minutes.
Note that these are just approximate. The number of calories you burn depends upon the activity your are doing and the intensity level you are exercising at. Any activity that you perform can be done at a variety of intensity levels.Your body weight makes a difference also. The larger you are the more energy you need to use go faster. So a 240 lb. man will burn through calories approx. twice as fast than a 120 lb. man doing the same exercise at the same intensity level. Another essential factor is the body metabolism, which is a variable of genetics.