Calories in versus calories out. We add up all the calories we eat throughout the day, then subtract out the calories we burn off with exercise and we arrive at our net calorie surplus or deficit. If we want to lose weight, we make sure we are burning more calories than we are eating, right? As many people have already discovered, its not quite that simple. While we can get a pretty good handle on how many calories we take in, measuring how many calories we burn is by no means an exact science. Even arriving at an approximate number of calories burned can prove to be difficult as the variables are many and more so, they are ever changing.
So why is measuring calories burned so difficult? Well, to see the main reason, you have to understand how the metabolism works. Counting calories essentially treats the metabolism as a finite, fixed mechanism that is easy to measure and spits out predictable results. It looks at the metabolism a little bit like fuel mileage on the car. That is, this car gets 30mpg, so for every 90 miles you drive (i.e. you do this set amount of exercise) you will burn about three gallons of gas. So if we want to know how long until we run out of gas, we can pretty simply measure that by how far we drive. Simple right? Unfortunately, the metabolism doesn’t work this way.
The metabolism is much more variable than that. Not only from one person to the next, but also day to day depending on what inputs the body gets. To understand the metabolism, it needs to be looked at more like an open flame than a car burning fuel in a controlled space. You have an open flame, as you throw more wood on the fire (eat more food) the flame gets bigger, burns brighter, and burns quicker. Likewise, if you starve the fire for wood, the flame gets smaller, it burns dimmer and burns longer. The rate at which this happens is forever variable and this makes it impossible to determine how long it will take to burn say, 10 logs, as it completely depends on the rate at which the logs are placed onto the flame.
This is just one main factor to take into consideration, other factors include changes in hormone levels, stress levels, sleep duration and quality, water intake, timing of the meals eaten and more.
To effectively manage calorie intake, its much more useful to keep a steady diet and activity level, then check progress every few days. If you are eating 1800 calories per day and doing a set amount of activity per day, if after one week you have lost five pounds, you can conclude pretty reasonably (with the exception of water weight fluctuation) that you have created a pretty good deficit. Then you can adjust your food intake and exercise output.