When You Lose Weight, Where Does it Go? The Answer May Surprise You

Recently, I was sitting and thinking about all of the diet and exercise suggestions that constantly bombard us from all sides. While trying to determine which techniques would likely yield the largest benefits, I decided to start from the beginning and attempted to answer a seemingly simple question: When we lose weight, where does the weight go? When the fat from our waistline disappears, what happens to it? Answering this question was actually way more difficult than I imagined at the start, and forced me to think back to my time as a molecular biology major in order to answer the question effectively.

After uncovering the answer for myself, I asked others to think about the question to see if the solution was more obvious to them than it was to me. Shockingly, even many physicians I asked were unable to answer this question accurately and completely. Below are the most popular answers people gave to the question and they are both completely wrong.

1. You Poop it Out (Digestion is a one way process for the most part, when we lose weight we aren’t expelling material into our digestive track)
2. It Gets Converted to Energy (So wrong. Law of Conservation of Mass)

A few people were able to give a response that was technically correct, that we urinate it out, but could not account for what happens to the non-water components of our bodies.

The surprising answer to this question is that we breathe it out. Yes, that’s right, the fat in your waist eventually ends up in the air as you breathe it out over time. I’ll explain how this works conceptually, then will run some calculations to estimate the weight loss associated with an average breath, then will discuss how this relates to exercise and some additional nuances of the problem.

The simplest way to think about this requires you to dust off your high school science skills and I will assume at least this level of comprehension. We breathe in oxygen, with molecular formula O₂, and breathe out carbon dioxide, with molecular formula, CO₂. Notice, that carbon dioxide has the exact same formula as oxygen, but with a carbon attached. This extra carbon has mass associated with it, 12 grams per mole to be precise. That means that for every mole of oxygen that you breathe in and use in cellular respiration, a mole of carbon dioxide is produced, and thus 12 grams of mass are transferred from your body to the atmosphere.

If you don’t remember how cellular respiration works, here is a refresher:

The basic formula for cellular respiration is that one mole of glucose combines with 6 moles of oxygen to produce 6 moles of water,  6 moles of carbon dioxide and releases chemical energy in the form of ATP. Thus, if we just think simplistically about one mole of glucose representing the weight you are losing, 108 grams of water and 72 grams of carbon are released for every mole of glucose consumed in cellular respiration. The water ends up getting expelled mostly through urination and the carbon dioxide expelled through breathing.

Now let’s look at what all this means for the average person and the average breath while resting. Again, get ready to brush off that high school science and basic math. First, we need to determine the difference in mass between an average resting inhale and average resting exhale. (See the Table Below – Again this is driven by inhaling lots of oxygen and very little carbon dioxide, and then exhaling relatively more carbon dioxide and relatively less oxygen).

Based on some online research, I found that the average inhale has 21% oxygen and 0.06% carbon dioxide based on their average concentrations in the atmosphere. Similarly, the average exhale has 17% oxygen and 4% carbon dioxide. Given that the average resting tidal volume of each breath is 0.75L, and there are 24.5L in each mole of gas, this converts to a difference of 0.014 grams per each breath cycle. This is obviously tiny, but adds up over time. Given that there are 453 grams in a pound, it would take 32,623 breaths to breathe out a net pound of mass. Finally, since the average person takes 15 breaths per minute at rest, it would take 36 hours to breathe out a net pound for a resting human being.

Given that the carbon dioxide only represents one route for weight to be lost, lets look at a given pound of body mass and estimate where relative percentages of it go. First, I will assume that the human body is 60% water and that when we lose weight, we lose it in the same proportion from the water excretion due to shrinking cells and by the conversion of glucose into water and carbon dioxide. Of the glucose converted, 40% (72/(72+108)) is expelled as carbon in carbon dioxide and the other 60% is expelled as water. Thus, for a given pound of mass, 84% leaves the body as water (60% + (60%*40%)) and 16% leaves the body as the carbon in carbon dioxide ((40%*40%)). Technically, there are some other substances such as the nitrogen found in proteins, which gets expelled as urea, but this should provide a rough estimate of where weight goes when you lose it.

This analysis makes it clear why exercise is so powerful in the weight loss equation. In the example above, there are levers that can be pulled that will speed up the process of weight loss – the proportion of CO₂ that is present during exhale, the tidal volume of the average breath, and the amount of breaths per minute. Exercise can increase all of these factors. The process works like this: exercise increases the rate of cellular respiration, which converts more oxygen, and thus expels more CO₂ and water into the bloodstream, which then get carried to the lungs and bladder and ultimately released . We all know that during exercise we breathe harder and faster and this is because the rise in CO₂ in our blood stream triggers this response. Thus, during exercise, energy stores (like glucose) are consumed, and CO₂ and water is expelled at a faster rate, leading to weight loss in the long term all things else being equal.

No matter which complicated suggestion for weight loss is proposed, the basic equation is the same. You need to expel more mass than you take in. The composition of the food you eat, your genetic disposition, and your lifestyle all affect how easy it is to achieve this, but all weight loss techniques are essentially acting on this basic fact.

Thank you for reading and please let me know if you have any comments, questions, or insights to provide on this topic!