Where Does Fat Go When You Lose Weight? (surprising)

Written by Tim Skwiat

Where Does Fat Go When You Lose Weight

How does fat loss actually happen? Where does the fat go? When we lose weight, where does it go? Do any of these questions sound familiar…

  • Obviously, it’s burned off (heat/energy), right?
  • Surely, we sweat it out, don’t we?
  • Everyone knows that fat turns to muscle when we exercise, doesn’t it?
  • Have you ever heard that you pee or poop it out?

If you’re thinking one—or more—of the above, I don’t blame you. In fact, according to a recent study published in the British Medical Journal, researchers found that health professionals (like doctors, dietitians, and personal trainers) think along the very same lines.1 Here are the results of a survey the researchers took:

Where Fat Goes When You Lose Weight

So, where does fat go when you lose weight? Before we answer that question, let’s take a step back.

Calories In Versus Calories Out

Ever heard of the first law of thermodynamics (also known as the law of conservation of energy), which basically states that energy/heat can neither be created nor destroyed? Well, our bodies abide by that law. In fact, that’s where the ol’ “calories in versus calories out” (often referred to as “CICO”) mantra comes from, and it basically says that bodyweight is a function of energy balance.

In other words, you gain weight when energy in exceeds energy out, and you lose weight when you expend more energy than you consume. For what it’s worth, the energy balance equation is a constantly fluctuating depiction of the principle of energy conservation.2 Also, you can gain or lose weight regardless of the composition of your diet (e.g., low carb, low fat, and anything in between).3

Of course, the body’s main storage form of fuel—and “extra” weight—is fat. And the body is quite adept at storing “extra” dietary fat as body fat, and it is completely capable of converting “extra” carbohydrate (and to a lesser degree, “extra” protein) into triglycerides and storing them as body fat.

One other thing that’s important to point out is that we are in a dynamic state of energy imbalance. In other words, under normal conditions, there may be times when you’re in an energy surplus and other times when you’re in an energy deficit. It’s only when the scale is tipped in favor of energy in or energy out consistently over time that weight gain or loss, respectively, ensues.

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Eat Less, Move More

With a better understanding of CICO, you can see why the simplest recommendation for weight loss is often “eat less, move more,” which is just another way of saying reduce energy in and increase energy out. (Of course, we know that weight loss is WAY more complex than that.) For all intents and purposes, when we’re talking about losing weight, we’re talking about losing stored body fat (and holding onto precious fat-free mass, like muscle, bones, organs, etc.), which look a little something like this:

Because weight loss usually gets loosely translated as “burning” more fat and “burning” more calories, most people believe that fat is converted to energy or heat when it’s “lost.” However, scientists argue that this violates the law of conservation of mass.1 Yes, it’s true that the body oxidizes (“burns”) stored fat and releases energy (stored in the chemical bonds, which are represented by the dashes in the image above) to meet the increased demand.

But, as you can see in the picture above, stored fat also contains a BUNCH of carbon (C) and hydrogen (H). In fact, the average stored triglyceride contains about 55 C and 104 H! What happens to them?

With a little help from the oxygen we breathe in, the stored body fat is converted to a BUNCH of carbon dioxide (CO2) and some water (H20). In fact, according to recent research, when someone loses 10 kg (22 lbs) of body fat, 8.4 kg (18.5 lbs) is exhaled as CO2 while the remaining 1.6 kg (3.5 lbs) is lost as water (in urine, feces, sweat, tears, and other bodily fluids).1

Where Does Fat Go When You Lose Weight

So, let’s ask the “burning” question again: Where does the fat go when you lose weight? For the most part, it literally vanishes into thin air! Yes, you read that right. You lose fat through your breath. Essentially, this is metabolism in a nutshell. In fact, one tool scientists commonly use to measure metabolic rate—and find out how many calories are burned during an activity, over the course of the day, etc.—is called indirect calorimetry, which measures the consumption of oxygen and the production of carbon dioxide.

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Metabolic Water

Do you know what else that means? It means:

  • Your lungs are the primary organ responsible for excreting fat.
  • Fat is not “lost” as heat.
  • Fat is not turned into energy per se.
  • Fat does not get turned into muscle (or vice versa).
  • A relatively small amount of fat is actually lost in your pee, poop, and sweat.

This last point shouldn’t be lost. Even though it pains me to say it, I suppose it is true that some fat is “lost” as sweat. More specifically, about 10% or so of the fat that’s lost is excreted as water. This is important to mention because it means fat is a source of metabolic water.

And for many mammals, fat is a major source of metabolic water, which is used by many animals to survive periods of water shortage. Whales, for example, obtain much of their water from burning fat. Meanwhile, desert animals (like the ol’ sand rat, camel, and oryx) have high body fat, which they use to generate water during times of need.

What the heck does this have to do with us? Basically, storing more fat is a means for providing metabolic water and protecting against water shortage. Some researchers suggest that this built-in survival mechanism may in part explain why some people have so much trouble losing fat.4 And it’s a darn good reason to make sure you’re drinking enough water and consuming plenty of low-energy-dense foods (like vegetables and fruits).

You May Be Losing More Than Fat

Since we’re talking fat loss, let’s take a minute to mention persistent organic pollutants, or POPs for short, which are potentially harmful compounds (pesticides and environmental contaminants, for example). POPs are “lipophilic,” which means they have a tendency to accumulate in fat tissue, and generally speaking, obese folks have higher levels of POPs than lean people. When fat is lost—like described above—these POPs increase significantly in the bloodstream.

Studies have shown that POPs have been linked to various endocrine, immune, nervous, and reproductive system issues. On one hand, body fat plays a protective role against the effects of POPs, which it sequesters. However, when fat is lost and these POPs are released into the bloodstream, it’s possible that may explain some of the unexpected adverse effects sometimes experienced after weight loss.5–7 There’s a lot that we don’t know on this topic, but it’s something worth mentioning. If you’ve lost a lot of weight and experienced unintended consequences, I’d love to hear your feedback.

Simply put, losing weight involves “unlocking the carbon stored in fat cells.” This reinforces the common advice to “eat less, move more,” which may be more appropriately stated as “eat less, exhale more,” although hyperventilating won’t do you any good.

Being more active and exercising more—and doing so consistently—can help swing the balance in favor of weight (carbon) loss. (There’s a reason why you breathe more when you’re moving and grooving.) However, exercise alone is rarely enough, and that’s why it’s so important to complement it with a healthy eating plan that helps you reduce your carbon load.

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More From Tim Skwiat


  • The Top Foods That Burn Belly Fat
  • 1. Meerman R, Brown AJ. When somebody loses weight, where does the fat go? BMJ. 2014;349:g7257. doi:10.1136/bmj.g7257
  • 2. Hall KD, Heymsfield SB, Kemnitz JW, Klein S, Schoeller DA, Speakman JR. Energy balance and its components: implications for body weight regulation. Am J Clin Nutr. 2012;95(4):989-994. doi:10.3945/ajcn.112.036350
  • 3. Hall KD, Bemis T, Brychta R, et al. Calorie for calorie, dietary fat restriction results in more body fat loss than carbohydrate restriction in people with obesity. Cell Metab. 2015;22(3):427-436. doi:10.1016/j.cmet.2015.07.021
  • 4. Johnson RJ, Stenvinkel P, Jensen T, et al. Metabolic and kidney diseases in the setting of climate change, water shortage, and survival factors. J Am Soc Nephrol JASN. 2016;27(8):2247-2256. doi:10.1681/ASN.2015121314
  • 5. Cheikh Rouhou M, Karelis AD, St-Pierre DH, Lamontagne L. Adverse effects of weight loss: Are persistent organic pollutants a potential culprit? Diabetes Metab. 2016;42(4):215-223. doi:10.1016/j.diabet.2016.05.009
  • 6. Jansen A, Lyche JL, Polder A, Aaseth J, Skaug MA. Increased blood levels of persistent organic pollutants (POP) in obese individuals after weight loss—a review. J Toxicol Environ Health Part B. 2017;20(1):22-37. doi:10.1080/10937404.2016.1246391
  • 7. Lee YM, Kim KS, Jacobs DR, Lee DH. Persistent organic pollutants in adipose tissue should be considered in obesity research. Obes Rev. 2016;18(2):129-139. doi:10.1111/obr.12481
  • Brown Fat vs White Fat
  • SargintRock

    Interesting! I shall add some of this information with my intermittent fasting plan!

  • Hi Sandra,

    First off, congrats on the improvements that you are making; that’s awesome!

    That is a great question! Hormones aren’t stored in fat; however, adipose (i.e., fat) tissue is an endocrine gland. More simply put, body fat is responsible for producing and secreting certain hormones, such as leptin, adiponectin, and resistin.

    For example, leptin levels tend to be proportionate to the amount of body fat that you carry. And as you lose fat, levels of leptin (which is a satiety and energy-sensing hormone) drop, which is one reason why it can become increasingly difficult to lose fat over time and maintain weight loss. (For more on leptin, check out this excellent article by Coach Cristina.)

    Because the endocrine system is just that — a system — it’s important to realize that hormones don’t work in a vacuum; they influence one another, and it’s certainly plausible that fluctuations in these adipose-derived hormones can affect production of other hormones (e.g., estrogens). So, that’s an indirect means by which fat loss may be contributing to what you’ve noticed.

    Another thing to consider is that fat tissue also produces an enzyme called aromatase, which is responsible for the conversion of androgens into estrogens. In other words, adipose tissue can significantly contribute to the body’s estrogen pool directly.

    So, it certainly seems plausible that fat loss could be contributing to what you’re experiencing. After all, as you are probably already aware, it’s thought that a reduction in estrogen that accompanies (post)menopause that contributes to hot flashes (although there’s probably more to the story than this, such as imbalances between estrogen and progesterone). For more on the topic, you might want to take a look at the following article where I provide some practical tips:

    Ask the Coaches: Is Menopause Causing My Weight Gain?

    I hope that you find this helpful, Sandra, and if you have any additional feedback or questions, please feel free to share.

    Keep up the great work!

  • Hi Katey,

    Thank you so for taking the time to read this article and comment; it means more to me than you know. First off, I’m very proud of you for making the lifestyle changes that you did, which resulted in significant weight loss. I have a feeling that improved your life and quality of life in ways that were not described here. (Right on with 80/20!)

    Having said that, I am very sorry to hear about your cancer diagnosis. However, it sounds like they caught it very early, and so far, the treatment protocol has been effective. That’s good news! As far as tamoxifen/nolvadex, unfortunately, I’m not qualified to speak to that or offer any suggestions. I do appreciate you taking the route of caution and seeking opinions, and I would advise that you ask some experts their thoughts (on the costs/benefits).

    In one of the other comments, I discussed how fat tissue is an endocrine organ (secreting hormones) and may both directly and indirectly contribute to the body’s levels of estrogens. In fact, fat tissue secretes more than 260 different compounds (e.g., proteins, peptides, inflammatory chemicals). In other words, it’s not just a storage depot…although it is most certainly that as well.

    As discussed in the article above, fat tissue stores more than just body fat, and there may be other compounds (e.g., toxins) that are released when fat is “lost.” I only mentioned POPs in the article, but that’s not to say there aren’t other “suspects” in play. For instance, fat tissue can also “sequester” vitamin D, and while vitamin D is not necessarily harmful, it does give insight into the fact that other fat-soluble compounds can be stored in adipose tissue…and when we “lose” fat, what happens to those compounds?

    These are important questions that we hope to have more answers (and solutions) for in the future.

    Thanks again for sharing, Katey, and please keep us posted on your health progress. Wishing you the best!

  • Hi Leonie,

    I hope this finds you doing well! Thank you so much for taking the time to read this article and others over the years. I can’t tell you how much it pleases me to hear that you have been enjoying them — and hopefully finding some practical take-aways.

    I am truly humbled to have had even a minor impact in your journey. Speaking of which, congratulations on the tremendous progress that you’ve been making on your weight loss program. It sounds like you’re kicking some serious tail; that’s awesome!

    I also greatly appreciate you taking the time to share your experiences. That’s very interesting, and your testimony and some of the others just go to show that weight loss is not always sunshine and rainbows.

    Having said that, I think we also have to take a look at all the factors that could be involved. For example, could the joint discomfort be a function of increased physical activity and exercise? Could the headaches be a result of changing eating habits (e.g., lower nutrient intake, introduction of food sensitivities), stress, or sleep loss?

    As far as your question, at this time, we do not ship our products directly to South Africa (from BioTrust.com). However, are products are also available on Amazon.com, and you may be able to have them shipped to South Africa. I am not sure about that, but it’s worth checking. Just head over to the Official BioTrust Store on Amazon to find out more.

    I hope this helps, Leonie; keep up the good work!

  • Adam

    That is very interesting article, but one additional thing that I learned in my classes towards my Bachelor’s in Exercise Science is that one never loses fat cells. they just get larger until they divide into two cells. You may lose the fat from inside the cell, but the cell still remains. That is why many people who lose a lot of weight, especially if they lose it quickly, have excess skin that needs to be addressed.