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I want to maintain or even grow muscle while burning fat (doesn't everyone – I know).

Assume a moderate daily deficit of 300 calories to achieve fat loss sustainably. Also assume a continuation of hypertrophy training. The variables are cardio and diet.

I want to know whether it'd make a difference if the deficit comes from cardio or eating less (or even a mixture of both).

My guess is that eating less would cause muscle reduction (BAD), whereas eating the same while doing cardio would not because the muscles would still get all the nutrients they need to grow or at least maintain (GOOD).

True or false?

2 Answers 2

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Your body doesn’t really know or care how you got into a deficit when it comes to burning lean mass for energy. Keep resistance training, maintain a modest deficit, eat a decent amount of protein (at least 1.3g/kg) and you’ve done everything you can to attenuate lean tissue loss. Therefore, given a choice between a deficit created by eating less and a deficit created by cardio, the plethora of other health benefits of aerobic training make the choice clear.

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  • "Your body doesn’t really know the difference." It couldn't tell between more vs fewer nutrients/food? Sep 4, 2023 at 12:24
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    @DennisHackethal Not really. At the end of the day, it doesn’t really matter how you got into a deficit. The things that matter for minimizing muscle loss are the size of the deficit, and whether or not you are resistance training.
    – Thomas Markov
    Sep 4, 2023 at 12:28
  • If I achieved daily maintenance eating nothing but spoonfuls of olive oil my muscles wouldn't shrink? Sep 4, 2023 at 12:31
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    @DennisHackethal Sure, but do you really think I needed to qualify my answer with “as long as you are eating the sort of diet that wont lead to death by anemia”? Just eat a well balanced diet with a decent amount of protein.
    – Thomas Markov
    Sep 4, 2023 at 12:43
  • My olive-oil example is extreme but it shows that it's not all about calories in vs calories out. Macros matter; the body can tell the difference, at least in terms of macros. You've since edited your answer to include a recommended daily protein intake; if cardio allows me to consume more protein while keeping the same daily calorie deficit, then by your own logic, my body does know the difference. Or am I missing something? Sep 4, 2023 at 12:53
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Does cardio burn muscle?

Not on its face. A calorie deficit will either slow down muscle growth or even reduce muscle depending on intensity and duration regardless of how it is achieved. Eating back all the calories you burn from cardio would then not have a negative impact on muscle building at all with one exception that I'm going to bring up later in this answer.

Protein and calories needs to be allocated for either muscle protein synthesis or moving. Moving is way more important for survival so your body is going to default to that. If you're in a calorie deficit, then more of that will be allocated to keeping you moving. If you're at maintenance or higher, then more can be allocated to muscle protein synthesis.

There are two ways cardio could negatively impact muscle building in the short-term:

  1. Increasing the calorie deficit even further. Starting at 300 calorie deficit, if you added 300 calories burned from cardio, it would have the equivalent effect of reducing your calorie intake by 300. This is speaking strictly in the context of fat reduction / muscle building. Protein and calories goes towards moving your body rather than muscle protein synthesis so it becomes increasingly difficult to build more muscle.
  2. Impacting recovery. If your cardio of choice is really hard, heart pounding then this can have a negative impact on your ability to recover. This then would negatively impact your ability to push the resistance training which is important for maintaining muscle mass. So it's usually better to choose long steady-state cardio over fast HIIT cardio. This is usually why it's advised to do cardio after the weight training session on the same day.

There are positive benefits of cardio for muscle building as well. It increases heart health. It increases endurance. These are long-term benefits that over time help translate to lifting weights better. So I don't want to discourage anybody from doing cardio because in the long-term it's very beneficial.

Personally, I also think there are benefits to increasing cardio over lower calories because more food also means more micronutrients which is always a good thing. The downsides are time, effort, and it's nearly impossible to track calories burned*. People typically find eating less to be the path of least resistance. Particularly if it's something you have to do for many weeks or months.

To summarize, as far at fat/muscle loss is concerned, it doesn't make much of a difference where the deficit comes from. The main difference comes down to what you can find is easier to adhere to.

* Watches and machines are not accurate assessments of calories burned.

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  • Maybe a conversion of red Type IIb to rose Type IIa fibres may influence hypertrophy as well, isn't it? That's not exactly "loss" but possibly "less gain". Sep 4, 2023 at 14:24
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    @PhilipKlöcking I'm not well-versed in how muscle-fiber types would impact muscle gain/loss. Would someone doing only 10 or so hours a week of cardio really have enough of an impact on their muscle fiber types to make a difference? Also according to Dr. Andy Gallpin, bodybuilders typically have mostly Type IIa fibers because their style of training is not really explosive as compared to powerlifters or sprinters. Based on that, I feel that would actually be a benefit maybe? Or at least no impact.
    – DeeV
    Sep 4, 2023 at 15:26
  • I have no clue, just an idea. Thanks for the Gallpin reference, sounds interesting. Also, 10 hours a week should indeed have an effect, yes. Sep 4, 2023 at 15:28
  • Thanks, @DeeV. Regarding point 2, I was planning on doing light cardio only, for that very reason. Also because Mike Mentzer said that high-intensity cardio burns sugar – he claimed only low-intensity cardio burns fat, ie the kind of cardio you perform at a 'conversational pace'. That does sound like there's more to fat loss than just caloric deficit, doesn't it? Sep 4, 2023 at 18:43
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    @DennisHackethal that's old school thinking that hasn't held up to science. Steady State Burns more fat in the moment, but that means there's more sugar left over and you burn less fat throughout the rest of the day. The rate of fat loss will be the same at the end of the day.
    – DeeV
    Sep 4, 2023 at 19:15

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