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The fitness community often says that you can't slim down and build muscle at the same time. The reason for this is said to be that the body is in a “catabolic state” with calorie deficit. In order to be in an “anabolic state”, there should be no calorie deficit. This is repeated over and over again, but I have never found any scientifically reliable evidence for it. For example:

Since anabolism and catabolism are parts of your metabolism, these processes affect your body weight. Remember: When you’re in an anabolic state, you’re building and maintaining your muscle mass. When you’re in a catabolic state, you’re breaking down or losing overall mass, both fat and muscle.

And then there are athletes who claim to lose weight and build muscle at the same time − is that true or even possible? Has this “catabolic state” thing ever been scientifically proven? And, if so, is it so drastic that, for a recreational athlete, it is inefficient to do strength training when he is slimming down?

I realize that fat is not converted into muscle through exercise (as is a common motif in marketing), but is there scientifically substantiated evidence that during a fat-reducing process, at the same time, no muscle-building process can take place, even if stimuli are set through training?

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    Hello Maron. Welcome to the site. I've trimmed down the question a bit and added a "notable claim" for answers to address. When you have a chance, please take the tour and see the help center. – fredsbend Jul 9 at 21:16
  • The quote is completely nonsensical, and further, is not fully in context. Losing fat and building muscle are completely unrelated, and as a commonplace matter both happen at once. – Fattie Jul 14 at 16:02
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    Losing weight and losing fat are different things. If you perform significant exercise, you will almost certainly lose fat. But you will only lose weight by cutting calories. Cutting calories makes it much harder to gain muscle. – Gort the Robot Jul 16 at 3:42
  • Gort, there's a danger of adding confusion.. (1) "exercise" means one of two totally different things, either (a) aerobic exercise or (b) lifting weights. (a) and (b) are utterly different and totally unconnected. (2) "If you perform significant exercise, you will almost certainly lose fat." the mechanism to "lose fat" is to reduce carbs (and, as a limiter, moderate calorie intake). exercise-(a) has no effect on this equation at all, other than slightly changing your calorie equation. (exercise-(b) is completely unrelated to fat.) as every builder says, "you lose fat in the kitchen" – Fattie Jul 18 at 11:22
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It's completely commonplace to increase muscle mass while reducing body fat.

It's trivial to find 1000s of articles and papers on this, random examples https://health.usnews.com/wellness/fitness/articles/can-you-gain-muscle-while-losing-weight , https://www.healthline.com/nutrition/body-recomposition#how-it-works , https://www.verywellfit.com/mistakes-to-avoid-when-building-muscle-and-losing-fat-3498333

The given quote about "anabolism and catabolism", sic, is nonsensical, and indeed way out of context.

  1. If you work muscles (and, obviously, get at least enough protein), muscle size will increase.

  2. If you cut carbohydrates, and arguably calories, body fat will reduce.

The two are totally unrelated.

(Obviously, there's the ridiculous example where you don't eat ANYTHING, hence zero protein, and of course you cannot increase muscle size, since, muscles are made of protein.)

You only need a relatively small amount of protein to be able to build muscle size; it has no limitation or bearing at all on reducing carbohydrate/calories intake in terms of reducing body fat.

As an extreme example, if you eat a kilogram !!! of chicken breast each day,

https://www.calorieking.com/us/en/foods/f/calories-in-chicken-chicken-breast-without-skin-raw/e4qJu59DSBqeWXUN6cS5iw

enter image description here

You'd be getting an astonishing ~ 1.5 kilograms of protein a week (it is inconceivable your muscle building regime could need more protein than that), and zero carbohydrates and only about 1100 calories a day.

Muscle-building is achieved by muscle-building training (and, sure, you of course have to eat a relatively small amount of protein every day). Body fat reduction is caused by carbohydrate intake (and arguably calorie intake), which has no connection to protein intake.

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  • I'm seeing a lot of begging the question here. "anabolism and catabolism", sic, is nonsensical, The two [muscle building and fat burning] are totally unrelated, and You only need a relatively small amount of protein to be able to build muscle size; it has no limitation or bearing at all on reducing carbohydrate/calories intake in terms of reducing body fat. All of these just repeat your "yes, both can occur simultaneously" answer without any supporting quotes or evidence. You need to quote the relevant items in those top links. – fredsbend Jul 14 at 18:01
  • @fredsbend , thanks for taking the time. I truly agree with you that more point-to-the-text refs would be better. Hopefully someone will have more time for a longer answer. For me the question is a bit should-be-closed since the claim is not a usual, normal, common claim but rather an oddball one, easily and widely refuted - so I've just thrown in a quick answer. I'll just delete it if someone has time for a comprehensive answer – Fattie Jul 14 at 19:21
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    I don't think you need to delete. The only issue is that the links may or may not support your answer. Without clicking and reading them, readers have to take you at your word. There's no rush to make this better in that regard, so you can take up to a week to do that. – fredsbend Jul 14 at 19:26
  • IMHO the claim totally unrelated is too strong since protein can also be metabolized to produce energy. In the hypothetical chicken breast diet the question is IMHO less whether muscle building needs more than 1.5 kg of protein per week but how much protein would be left for muscle building and how much will be used for energy metabolism (including gluconeogenesis to provide the brain with glucose, which cannot be done from lipids). And actually the links in the answer do not say it's commonplace, they say that it is possible. – cbeleites unhappy with SX Jul 16 at 23:48
  • @cbeleitesunhappywithSX hmm, details of protein pathways - again I urge anyone else to put in an even more detailed answer with more and better refs. It's pretty common on this site that broadly incorrect, easily answered questions, where you "can't really prove a negative" just get straightforward quick answers. – Fattie Jul 17 at 12:10
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The solution of the apparent contradiction may be in the scales of time:

  • At any point in time, you can either slim down (catabolic, during excercise) or build muscle (anabolic, during rest after excercise), but
  • on a more coarse time scale where excercise and rest phases are "at the same time" both can happen "together".
    See e.g. the paper cited in @SyrenBaran's answer reporting that this does actually happen.

Roughly speaking,

  • catabolism is the part of metabolism (i.e. biochemical pathways) that break down larger molecules into smaller ones, including the part of metabolism that produces free energy.

  • anabolism are biochemical pathways that produce larger molecules from smaller ones, using free energy.

By these definitions, β-Oxidation, the process that breaks down lipids (fat) into Acetyl-CoA that can then enter e.g. the citric acid cycle for energy production is catabolic, while producing e.g. muscle protein from amino acids is anabolic.

 catabolic and anabolic pathways are almost always separate. This is required for energetic reasons [...]
Many metabolic reactions are regulated/controlled by the energetic status of the cell.
[...]
ATP-producing [catabolic] pathways are inhibited by high energy charge [that's a way to quantify the energetic status of the cell] whereas ATP-consuming [anabolic] pathways are stimulated.

(Stryer: Biochemie, Spektrum 1991, p. 338, my translation - if someone has access to the English version, feel free to replace)

So, if you look into a muscle cell during excercise, you'll find that mostly catabolic (energy "producing") pathways are "on" in order to rebuild the ATP that is consumed by the physical work, but anabolism (e.g. protein synthesis) is downregulated - that would be a further consumer of energy, which is scarce already. And the need for energy signaled throughout the body, upregulating e.g. β-Oxidation. So during excercise, you slim down but don't build muscle.

On a full body perspective, things get a bit more complicated: for example, you may be breaking down muscle protein into amino acids (catabolic) to have their liver use these amino acids for gluconeogenesis (anabolic) to produce glucose to feed the brain where the glucose enters glycolysis (catabolic). But, if we're looking at cycles of excercise vs. feeding & resting times, we can say that during excercise the metabolism is mostly catabolic, while anabolic pathways are active during feed & rest phases (there are still also catabolic pathways active: they produce the free energy and also substrates for the anabolic processes). See e.g. Rasmussen & Blake: Contractile and Nutritional Regulation of Human Muscle Growth, Exercise and Sport Sciences Reviews: July 2003 - Volume 31 - Issue 3 - p 127-131. They report increased muscle protein synthesis 2 h to up to 2 days after excercise - the review also contains time courses that show muscle protein synthesis ramping up after excercise.

At the first glance, protein metabolism seems to be unrelated to lipid metabolism. But protein is synthetized (anabolic) from amino acids, and these amino acids can also enter catabolic pathways for free energy production - and are then of course no longer available for protein synthesis. These pathways are coupled, and protein synthesis is downregulated when the more pressing need of the body is energy production. You may look at this as avoiding a futile cycle, at least if the energy need is sufficient that the body starts breaking down protein for energy production.


So the claim "you cannot slim down and build muscle at the same time" is correct in the sense that during excercise*, you can slim down but you don't build muscle. The actual building/growth of muscle happens during feed & rest phases - but only if your energy status is sufficient to not need the amino acids for energy production. I.e. when you don't slim down. The "window" to have the muscle grow and have the fat reserves grow less than what was used up during the excercise does exist, but it is not too wide. Hence the posts that @Fattie links in their answer that emphasize that this will happen only under carefully adjusted conditions.

So as net effect over several excercise and feed & rest cycles (the claim in the question looks at weeks), it is possible to slim down and build muscle.


* the so-called anabolic excercises have a net muscle growth only over longer excercise plus rest phase cycles. During these excercises, biochemical state is nevertheless catabolic. "Anabolic excercise" IMHO belongs into the same category of terms that calls a lemon an "alkaline food".

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  • Can't find the 2nd part of the quote from Stryer in my en edition 2015. A ~match for 1st part would be on p425. (My conslusion, your quote's too short ;) – LangLаngС Jul 17 at 22:54
  • An excellent post which brings to notice the language "....at the same 'time' ". The phrase "lose fat and gain muscle at the same time!", identical to say "get a degree in accounting and a degree in law at the same time!", refers to a process (say, "weeks or months") not a "moment in time". – Fattie Jul 18 at 11:10
  • @LangLаngС: it's in the discussion of the energy charge, still same paragraph, the 2nd [...] omits 3 sentences (energy charge definition in text, as formula, it is bounded in [0, 1].) and the beginning of the sentence I cite again "David Atkinson zeigte, dass ". And yes, I tried to shave it down to only what we need here. Of course, if p 338 moved to 425 within 25 years, they may have had to throw out stuff to keep it from moving to p 1000+... – cbeleites unhappy with SX Jul 18 at 19:58
  • @Fattie: thank you – cbeleites unhappy with SX Jul 18 at 20:07
  • Curiously, the name Atkinson now only appears on page 1005! ;) – LangLаngС Jul 18 at 20:29
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Yes. That's the normal state for beginners with more than enough body fat and no physical excercise.

Yes, different people get different tips on how to improve, it totally depends on your current physical state.

This even stands true even for older people, e.g. this study comes to the conclusion:

These results support the hypothesis that older persons who consume adequate or moderately high amounts of dietary protein can use RT to improve body composition, oral glucose tolerance, and skeletal muscle aPKC ζ/λ content without a change in body weight.

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  • Terrific answer to a simple question. – Fattie Jul 17 at 12:08

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