Science Behind Calorie Burning

Science Behind Calorie Burning
As a science teacher, I was tasked with teaching biological, chemical, and physical science concepts to high school students. They were usually unabashedly disinterested unless I could make the abstract and foreign seem more accessible and understandable.

But I’m sure YOU were always paying close attention in science class and never passing notes. Still, nevertheless, we need a refresher on the science of weight loss. We need to know how our bodies work so our weight loss can be permanent and sustainable.

Don’t worry; it’ll be fun!

Energy Is Fantastic Stuff

We need the energy to live, so that makes it pretty grand in my book.

We need it to run, jump, climb trees, think, create, dream, have sex, play with our kids, and just get up in the morning. So to do all those cool things, we get our energy from the food we eat. Since we can’t convert light into sugars (like plants do through photosynthesis), we have to consume energy in the form of food (plants and animals).

We must eat to live!

There is another way to talk about the energy in food that you have probably heard of; calories. A calorie is a tiny amount of energy in the grand scheme of things; it is the amount of energy required to heat 1 gram of water by 1°C. So when you look at a nutrition label, the “calories” you see are kilo-calories, that is, 1,000 calories.

So Calories (capital C means 1,000 calories) are the units of energy that our bodies extract in our food.

Now, in science, we have these nifty things called scientific Laws. They have been tested and retested over and over so many times, and the result is always the same.

A scientific Law can be considered a fact. If someone could disprove it, they’d win a Nobel prize, heaps of cash, and never have to work again! Energy has its exceptional Law: The Law of Conservation of Energy. It’s a severe and fancy way of saying that: all the energy in the universe is constant. Energy cannot be created or destroyed, but it can be converted from one form to another. So the chemical energy in your food can be converted into kinetic (moving) energy in your muscles.

But energy is never created or destroyed in this process.

The Law of Conservation of Energy

Calories In, Calories Out

When you are using Calories to run, jump, climb trees, etc., at the cellular and molecular levels, your body is converting your stored energy (triglycerides-fat, glycogen-sugar) and using it for movement and body functions; these are your Calories Out. When you eat and digest food, your body converts the energy from the food into stored energy in the form of triglycerides and glycogen; these are your Calories In.

  • To maintain your current weight, your Calories In must equal Calories Out.
  • To gain weight, your Calories In must be greater than Calories Out.
  • To lose weight, your Calories In must be less than Calories Out.

Your body weight is entirely a result of how much energy goes in and how much energy comes out. Fortunately, we have control over some of these two variables.

Weight Maintenance Science

For now, let’s examine maintenance, that is, staying the same weight:

If your goal is not to lose or gain any weight, let’s say for just a day, you need to start with one key piece of intel; how many Calories you burn doing all the things you do during the 24-hour day.

Fun fact: even when you lay perfectly still, you’re burning calories! Indeed not as many as when you’re sprinting to catch a train. But the cells and molecules in your body are doing something.

There are several ways of determining how many calories you burn in a day, but here are two that you can do from home:

  • Use a Fitbit or another wearable activity tracker that calculates calories out by using your heart rate to measure exertion and your steps to measure movement.It is the most accurate way to track your Calories Out that is accessible to just about everyone. It does not require any trips to a laboratory setting to be hooked up to a claustrophobia-inducing full face mask and being forced to run like a hamster on a wheel. (Who wants to do that? Not me!)
  • Use an online calculator to find your TDEE (total daily energy expenditure, a fancy way of saying: “Calories Out”). This method is less accurate than the Fitbit but perfectly reasonable if you can accurately estimate your daily activity level.


On Sunday, my Fitbit calculated that I burned just over 2,600 calories. As a fantastic coincidence would have it, I went out and celebrated my birthday at a lovely French restaurant in SoHo. I decided that enjoying myself was more important than losing weight, for that day only. I ate at “maintenance.” What that means is that I ate all the calories I burned, no more, no less. I ate 2,600 calories on Sunday, including breakfast, lunch, and of course, dinner of steak fries, strawberry shortcake, champagne, and appetizers! So yummy!

Calories In: I ate 2,600 calories.

Calories Out: I burned 2,600 calories.

It is maintenance; I ATE the same number of calories I burned, and my weight stays the same.

Suppose you want to practice “maintenance” for a day or week while losing weight; go for it! Practicing is a great way to prepare yourself to plan what to do when you get to your goal weight. I have “maintenance days” if I need to, and I never have “cheat days.” Notice the difference?

For a maintenance day, I simply maintain my weight and enjoy my life. A “cheat day” implies that I have done something inherently bad. Moral judgment is not necessary or helpful in the long run. You will find yourself feeling more confident in your ability to stay on your plan if you know how to take a “maintenance day” once in a while. ?


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