Hello Wellness Warriors! Since it is back to school season, I thought it might be time for us to brush up on our science. Before you click off, hear me out. I promise that I won’t ask you to write the equations for Newton’s Laws of Motion or to recite the Periodic Table. However, one of the most commonly asked questions is where does body fat go when you lose weight? And science can answer it for us.

I came across an article awhile back with the headline that something like 97% of trainers couldn’t correctly answer the question where does fat go when you lose weight? And that made me mad. As most people seek out personal trainers for weight management goals we, as a group, should be able to answer that one. And then it made me sad. How many clients (maybe you, reader) have been misled or misinformed over the years. So let me see if I can clear it up.

We trainers love to hide behind the First Law of Thermodynamics. I’ve even mentioned it here on MMW several times. Plus, it just sounds fancy and smart. The Laws of Thermodynamics are principles of physics that govern how the universe functions and have many applications in other sciences. And trust me, living with an engineer, I hear about it a lot. The First Law of Thermodynamics also works in symbiosis (there’s a seventh grade biology term for you!) with the Law of Conservation of Energy.

Conservation of Energy states that energy cannot be created or destroyed, it can only change forms. The First Law of Thermodynamics states that energy can pass into or out of a system via different forms, primarily as matter, heat, or work. But how do those fancy-schmancy definitions translate into weight loss?

First, we need to think of the human body as a system. Truthfully, that’s not too hard. Our bodies are set up to do all the things we need to stay alive, all in one nice neat skin-encased package. However, systems are typically set up to interact with other systems. In the case of our bodies, they are interacting with the food we consume and the activity we perform.

In terms of weight management, if we consume more energy than we use, we gain weight. And the opposite is true, too, if we use more energy than we consume, we lose weight. But again, we can’t create energy (or weight) from nothing, so where is it coming from?

Food. If you have ever looked at the nutrition panel on a food item and searched out the calorie count, what you have really located is the energy content of that particular food. Calories, or more correctly, kilocalories, is the energy held in the food we eat. As we digest said food, the chemical bonds are broken which releases the calorie energy into our bodies.

The calorie energy is used by our cells to run all of their processes and keep us alive. In many instances, the calorie energy from the food we eat is used to create a compound called ATP which is what our muscles need to contract. Or, on a broader level, ATP is what allows our bodies to move and exercise.

If we don’t move enough to use the all the calorie energy, our bodies reassemble the chemical components into the body’s energy storage format, fat. The fat remains until we need it. Basically, when we consume too few calories to support the amount of activity, the body will mobilize stored body fat and break it back down to generate the ATP it needs.

The breakdown process of fat requires activity or exercise, time, and oxygen. As the body transforms the fat (stored energy) into ATP (energy of movement), the body also releases heat (another form of energy), water, and carbon dioxide as byproducts.

Back to our fancy Thermodynamics definition. Energy enters our system (body) as calories. The chemical bonds are broken and the energy is transformed to either ATP (for motion or work) or to body fat (energy storage). When body fat is needed to supplement energy intake (i.e. caloric deficit), the storage form is transformed into ATP (energy for work). As your body works, heat energy is also released and internal temperature rises, which the body must combat and does so via sweat (excreting both heat and water). Additionally, during work, the body must rid itself of excess carbon dioxide, a natural chemical byproduct of work.

In other words, when you lose weight, the energy (i.e. stored body fat) merely leaves your system in some other form of energy or matter. The motion created by the body during exercise is a form of work (energy). The heat created by the increase in metabolic rate is another form of energy. The water and carbon dioxide are matter (a liquid and a gas to be exact).

The body fat that you lose when you are in calorie deficit doesn’t just disappear. Thermodynamics won’t allow that to happen. It becomes energy for work, heat, or material byproducts of the work your body has done. And now the next time one of us tries to hide behind a big fancy scientific concept, you can call us out!

Until next time, be well friends!