We all know that adhering to a caloric deficit will result in weight loss. “Eat less, move more,” and your weight will eventually drop.
But have you ever wondered what actually happens to fat when you lose weight? Many people think fat leaves the body as energy or heat, while others think it turns to muscle or gets pooped out1.
As you might know, energy is stored as triglycerides (i.e. fat) in our fat cells. These cells that specialize in fat storage are called adipocytes. Research suggests that the number of fat cells that we have is largely set during childhood2.
If we gain weight, we can increase the number of fat cells throughout adulthood. However, we can’t get rid of the fat cells we already have. (The only way to get rid of fat cells is to have them removed via a procedure such as liposuction.)
So, if the fat cells don’t disappear when we “lose fat,” then what happens to them?
What Happens to the Fat Cells?
When we don’t use energy from the food that we eat, the body converts the energy to a form of fat called triglycerides. A triglyceride is an ester made up of glycerol and three fatty acid groups. These triglycerides are stored in fat cells until hormones release them for energy later on.
Our fat cells make up loose, connective tissue - called adipose tissue - that helps to insulate our body. During exercise (or fasting), energy stored as fat in our adipose tissue can be broken down for muscles and other tissues to use. This process, called lipolysis, is induced by a variety of hormones, including cortisol3.
Through lipolysis, fat in triglyceride form is broken down (hydrolyzed) into fatty acids and glycerol, which leave the adipocytes to be used as energy. As a result of this process, fat cells shrink. They don’t dissolve or disappear. Imagine a deflated balloon—that’s a fat cell after the fat is broken down to be used as energy.
What Happens to the Fat?
Where does the fat go after it is broken down to be used as energy? Triglycerides and their fatty acid and glycerol components are made up of carbon, hydrogen, and oxygen. The oxidation of triglyceride results in carbon dioxide, water, and energy. The reaction for the oxidation of a single triglyceride molecule can be summarized as:
C55H104O6 + 78O2 → 55CO2 + 52H2O + energy1
Thus, after fat is broken down, it leaves the body as carbon dioxide and water. Most of it is exhaled as carbon dioxide. The rest leaves the body as water through urine, sweat, or other bodily fluids. Approximately 84% turns into carbon dioxide, and 16% turns into water1. This means that most fat is “lost” through the lungs!
This might sound weird, but if we stop and think about it, carbon is involved in nearly every process that our bodies are involved in—carbon is the basis of all life on Earth!
The food we eat also gets converted to carbon dioxide and water, and carbon dioxide is the reason why we weigh less when we wake up than we did when we fell asleep the night before. We breathe out carbon dioxide for hours while we’re sleeping and lose that mass in the process. (Then we go to the bathroom and lose even more weight via water.)
How to Lose Fat
If you take in 9 lbs of food, water, and oxygen, those 9 lbs need to come back out. But, in order to lose weight, you need to somehow get rid of more than the 9 lbs that you took in. As we discussed, this mostly translates to an increased amount of carbon dioxide leaving the body. In order to exhale more, we need to exercise.
When we inhale, oxygen makes its way into the bloodstream. The muscles produce carbon dioxide, which leaves the blood and crosses the lung membranes to be exhaled. By moving our muscles more, we can increase the amount of carbon dioxide produced by our body. Subsequently, we get rid of more energy than we took in, effectively losing weight.
How to Lose Fat
In order to lose weight, energy expenditure needs to exceed energy input. But weight is made up of muscle, water, bone, organs, and other tissue in addition to fat.
We can lose more fat and preserve more muscle when we follow a slower weight loss protocol rather than trying to find a quick-fix and lose too much weight at once4. Losing too much weight at once usually means that it’s coming from water and other components rather than fat.
Since we are talking about what happens to fat when we lose weight, we are only concerned with the physical process of losing fat during weight loss. These physical changes can have many side effects.
For example, extended periods of weight loss can result in adaptive thermogenesis, a disproportionate reduction in resting metabolic rate (RMR) when weight is lost5. Adaptive thermogenesis largely explains why it seems like metabolism slows down after long periods of dieting.
Weight loss is a complex process influenced by multiple factors. Let’s take a closer look at types of fat to better understand its connection to the weight loss process.
Types of Fat
Fat is located under the skin (subcutaneous fat) and around internal organs (visceral fat). When we lose weight, we generally lose more subcutaneous fat than visceral fat6. Unfortunately, we can’t choose where we lose fat first—or at all. Genetics determines where we lose fat.
White adipose tissue (WAT), or white fat, is what we normally think of when we think of fat. White fat acts as an energy reserve and a cushion for our internal organs. But there’s another type of fat called brown adipose tissue (BAT), or brown fat.