How does burning calories for energy work? If someone runs on an empty stomach and burns 1500 calories, where did that energy come from? When they eventually eat something, how does the body know what to do with those calories?

Question

How does burning calories for energy work? If someone runs on an empty stomach and burns 1500 calories, where did that energy come from? When they eventually eat something, how does the body know what to do with those calories?

In: Biology

Answer 1

Okay, so first thing you need to understand is that a calorie isn’t a physical thing, it’s a measurement of energy like joules or watts. If you want to get technical one calorie is the amount of energy it takes to raise the temperature of 1 gram of water by 1 degree centigrade.

So, when food packaging says that one serving contains X number of calories it means that food will provide that much energy. Saying a chocolate bar contains 300 calories is like saying your phone battery contains so 2000mAh.

Your body stores excess energy as fat. That’s why you get fat by eating too much or not exercising enough. If you eat 2000 calories in a day and only burn 1500 through activity, your body will store the extra 500 as fat.

By the same token, if you only eat 1500 calories worth of food and burn 2000, your body will use your body’s stored fat to make up the shortfall.

So, if I get up in the morning and go for a five mile run on a completely empty stomach, because running burns between 80 – 140 calories per mile, my body will burn around 550 calories worth of by body’s stored fat. When I get home and eat breakfast, my body will use that energy as I go about my day and store anything left over.