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
It’s dead simple. Just look at this colorful graph:
https://upload.wikimedia.org/wikipedia/commons/0/0b/Citric_acid_cycle_with_aconitate_2.svg
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.
Body has temporary stores of energy from previously consumed food. You have some very small stores of instant use energy in your muscles. The next step is glycogen stored in the body and bloodstream to supply longer needs. After that is your fat reserves.
Think of glycogen as your refrigerator, and fat as your freezer. You can eat your fridge food right away, but freezer food has to be defrosted.
Where food you consume goes is determined by hormones. Insulin is the master hormone that causes most of the direction. Consumed foods first refill depleted glycogen reserves. Then if that’s filled, body will convert and store as fat for later usage.
These systems are necessary because prior to civilization, the body never knew when it’s next meal was coming. So you would eat everything you could when you got a meal, and your body stores what’s not needed right away for later since it could be days or weeks before another meal.
Calories are stored in the body as sugar. Excess sugar gets stored in muscles and body fat, and when starving, the body starts burning fat, then muscles
The liver stores about 2000 calories worth of energy in the form of glycogen. Once you burn through that, your body starts to enter a state called ketosis, where is breaks down fats for energy. When you eat again, it depends on what you eat. Carbohydrates are used more or less directly to replenish the glycogen stores in the liver and to supple the muscles. Fats and proteins are used less directly, and often stored or used to repair tissues.
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