The Science of Weight Loss: Part I

As a science teacher, I was tasked with teaching concepts of biological, chemical, and physical science 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 (haha) but 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 energy to live, so that makes it pretty grand in my book. We need it in order to run, jump, climb trees, think, create, dream, have sex, play with our kids, and just get up in the morning. So in order 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 really 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 actually 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 the food we eat.

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 it’s own very special Law: The Law of Conservation of Energy. It’s a fancy and serious 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.

When you are using Calories to run, jump, climb trees, etc., at the cellular and molecular levels our 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 is converting 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.

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

if your goal is to not 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! Certainly 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 ( I wear the Fitbit Charge 2 HR all day, even while sleeping) or another wearable activity tracker that will calculate Calories Out by using your heart rate to measure exertion and your steps to measure movement.
    • This is the most accurate way to track your Calories Out that is accessible to just about everyone and 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 a perfectly reasonable method if you can accurately estimate your daily activity level.

Me at Balthazar in SoHo!

On Sunday, my Fitbit calculated that I burned just over 2,600 calories. As fantastic coincidence would have it, I went out and celebrated my birthday at a lovely French restaurant in SoHo (Balthazar) and decided that enjoying myself was more important that 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 frites, strawberry shortcake, champagne, and appetizers! So yummy!

Calories In: I ate 2,600 calories
Calories Out: I burned 2,600 calories

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

If you want to practice “maintenance” for a day or week while in the process of losing weight, go for it! Practicing is a great way to prepare yourself for having a plan for what to do when you get to your goal weight. I personally 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. The moral judgement is not necessary or helpful in the long run, and 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.

Have you had trouble with maintaining your weight in the past? Has it felt easy or difficult and mysterious. I’d love to hear from you! Like, Comment, Share, if you enjoy the post!

Next time in “The Science of WEight Loss: Part II,” I’ll explain the science Of losing and gaining, and what you can do to take charge!

Erin Hughes is an irreverent writer and successful weight-loser. She's on a mission to stamp out the bad advice from well-meaning colleagues and make weight loss attainable for others. Get her free QuickStart Guide To Weight Loss for strategies to get started on the right track.

  • Fran says:

    Do you think all calories are created equal? Even though a gourmet chocolate and a few cups of broccoli have the same number of calories, I think the body processes them differently and they have a different result with weight and your body. Should I be looking at that differently? I feel that when I eat clean calories the weight comes off much more easily than when I eat empty calories. Maybe its just due to PCOS issues and how my body processes carbs in general? Thanks for sharing this great explanation of calories! I’m almost to maintenance and excited to try a new process…maintaining, where I’ve failed so many times!

    • says:

      Thank you for your comment! Scientifically speaking, all calories are created equal because a calorie is a defined unit of energy that is standard across space and time. If a small square of gourmet chocolate and two cups of broccoli have the same number of calories once metabolized, the calories themselves effect weight the same way, because that is just a unit of energy. But there are other things in the chocolate and brocolli, fiber being the big one, that exerts other beneficial effects so the brocolli is still the healthier choice. Though sometimes, the magnesium and antioxidants in the chocolate during menses might be better if it helps one stay on track and rebalance magnesium deficiency at that time of the month. I have PCOS too, for the last 15+ years, and there are unfortunately some side effects which cause us to burn fewer calories at rest. It totally sucks. Medication (metformin)has helped me to negate some of that effect but not all of it. This affects the “calories out” side of the equation but not the equation itself. Congratulations on making it to maintenance! I’m planning a post in the near future on how to ease into maintenance! Keep on rockin my cyster! 🙂

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