Muscle loss is often treated as par for the course when dieting. Want to lose fat and get lean? Well, you also have to accept that some muscle loss will occur.
Part of the confusion stems from what researchers measure. A study might report a loss of “lean mass” or “fat-free mass”, but these terms include more than what most people picture when they think of ‘muscle’. Further, in many of these studies, participants aren’t lifting weights or eating enough protein — two things that can help mitigate muscle loss during an energy deficit.
So before we go any further, we need to understand some terminology because it’ll help lay the groundwork for the rest of the article.
Fat-free mass, lean mass, and skeletal muscle
There are a number of terms used interchangeably to refer to muscle mass, but not all of them refer to muscles like your biceps.
- Fat-free mass (FFM) is everything in your body that isn’t fat. That includes skeletal muscle, water, organs, glycogen, connective tissue, protein and bone mineral.
- Lean mass is a loose term for non-fat tissue.
- Skeletal muscle is the specific muscle attached to your skeleton (your biceps, quads, glutes and so on).
So when you hear that a certain diet strategy caused participants to lose a kilogram of fat-free or lean mass, that doesn’t mean they lost a kilogram of skeletal muscle. That loss could reflect changes in all the other non-fat tissue like glycogen, water, gut contents, organs, and connective tissue.
For example, Bosy-Westphal and colleagues conducted a study involving 45 women with overweight or obesity who followed an 800-1000 kcal diet for about 13 weeks. They lost an average of 9.5 kg, including 1.5 kg of fat-free mass. The researchers estimated that 0.9 kg came from skeletal muscle and around 0.1 kg from the liver, heart, and kidneys. The remaining 0.5 kg was attributed to other components, including the fat-free portion of adipose tissue.
Because, yep, even fat tissue contains some fat-free components, which would be classified as “fat-free mass.” So when you lose fat, you’re also losing some of this adipose tissue fat-free mass.
A 2019 paper by Abe and colleagues is a good example of how this can change apparent lean mass losses.
The authors revisited two weight loss studies and adjusted the reported lean mass losses to account for the fat-free component of the lost adipose tissue. When they did that, the estimated lean mass losses fell from 2.7 to 1.6 kg in the cardio group and from 1.7 to 0.5 kg in the combined-training group. In the strength-training group, a reported 1 kg loss flipped to an estimated 0.3 kg gain.
So to quickly recap: Fat-free mass refers to all of the non-fat components of the body, including skeletal muscle – the stuff you’re trying to build when you do biceps curls. However, just because someone lost fat-free mass, that doesn’t necessarily mean they lost skeletal muscle. With that distinction in mind, let’s look at what the research says about muscle loss when people diet and get leaner.
How much muscle loss actually occurs?
There’s a claim that 25% of the weight you lose during weight loss will come from muscle. In other words, if you dieted and lost 50 lbs, you would have lost ~12 lbs of muscle. Just to be sure, that’s a lot of muscle to lose.
If, like me, you were surprised that dieting could cause someone to lose such a large amount of muscle, you might decide to chase down where this claim originated from. If you did that, you would find a 2014 review that did all the legwork for you.
The researchers traced the 25% figure back to a 1984 study by Webster and colleagues, who looked at the body composition of 104 women ranging from lean to obese. Based on their analysis, they estimated that the ‘excess’ weight carried by women with obesity was roughly 78% fat and 22% fat-free mass. From here, the researchers recommended that when someone with obesity loses weight, no more than 22% of the weight loss should come from lean tissue. That 22% was later rounded to 25%, giving rise to what the 2014 review called the ‘Quarter FFM Rule’. But you’ll note this is still fat-free mass, and not skeletal muscle.
Even the authors of the review weren’t particularly sold on the idea, writing that the Quarter FFM Rule was “at best an approximation” with a “limited mechanistic basis.”
Furthermore, dozens of factors can influence how much muscle someone might lose, such as their body fat level at the start of the diet, the size of the caloric deficit, diet composition, sex, and the type of training they do.
In a more recent paper, Heymsfield and colleagues used whole-body MRI to estimate how much skeletal muscle people lost during calorie restriction without a structured exercise programme.
After doing a bunch of complicated math that most people reading this probably don’t give a fuck about, they estimated that, without structured exercise, men with overweight or obesity might lose roughly 2-2.5 kg of skeletal muscle per 10 kg of weight loss, while women might lose roughly 1-1.5 kg.
Now, the above might hold true for someone who isn’t lifting. So the next question is: how much muscle would someone who’s doing all the ‘right stuff’ expect to lose? Notably, lifting weights, eating enough protein, and losing weight gradually.
How do lifting, protein, and rate of loss impact muscle retention?
You need to give your body a reason to maintain muscle, and without the stimulus of resistance training, the risk of muscle loss increases. That “reason” is strength training. When you lift, the tension your muscles experience triggers a bunch of chemical reactions inside your muscle cells that stimulate the production of muscle proteins. In other words, lifting signals to your body that you still need your muscles and it probably shouldn’t throw them out quite yet.
Recently, Deller and colleagues compared calorie restriction alone with calorie restriction plus strength training, endurance training, or a combination of the two.
Across 34 randomised controlled trials involving ~1400 adults with overweight or obesity, adding exercise preserved an additional 0.9 kg of fat-free mass compared with dieting alone. The evidence was stronger for strength and mixed training, with less certainty around endurance training. So while all forms of exercise are great, if you want to retain muscle in a deficit, your best bet is strength training or a combination of strength and cardio.
It’s worth noting these results mainly concern fat-free mass, and not skeletal muscle. And the losses with exercise were generally fairly small. The two studies that looked at skeletal muscle specifically also found smaller average losses with resistance training than with dieting alone (although the difference between groups wasn’t statistically significant in one).
Of course, lifting weights and getting ‘swol’ is only one part of the equation. We also know that protein intake plays a role in muscle retention.
First, it supplies amino acids, the building blocks your body uses to make new muscle proteins. As an interesting aside, researchers have actually traced amino acids from ingested protein into newly made muscle proteins. In this case, you literally are what you eat.
Second, a caloric deficit reduces muscle protein synthesis – the fancy name for the process your body uses to build new muscle – and protein intake, along with resistance training, can help mitigate this effect.
A 2025 review of 29 studies looked at the relationship between protein intake and changes in fat-free mass in people with resistance-training experience during a caloric deficit. Higher protein intakes were generally associated with better fat-free mass retention.
Lastly, to round this all off, we have to look at the rate of loss. Ans I gotta be honest, the research isn’t that clear in this particular area.
Looking at the broader evidence, a 2020 meta-analysis of seven trials in adults with overweight or obesity found that a slower weight loss rate generally favoured lean mass retention, but the difference wasn’t statistically significant. However, the review excluded exercise interventions, and the faster-loss groups generally ate less protein. So it’s hard to say whether the lean mass loss was due to losing weight more quickly or to a combination of factors.
Garthe and colleagues conducted a study that’s a bit more relevant to Physiqonomics readers. 24 elite athletes lifted weights four times per week, while eating around 1.6 grams of protein per kilogram. Both groups lost around 5% of their body weight. But those losing 0.7% of body weight per week increased lean mass by 2.1%, while those losing 1.0% of body weight per week more or less maintained their lean mass.
Meanwhile, in a four-week study, 15 recreationally trained women lost either ~1% or ~1.7% of their body weight per week while eating around 1.5 grams of protein per kilogram of body weight (~0.7g/lb). Neither group experienced a statistically significant reduction in lean mass.
Finally, research in bodybuilders gives us a look at what happens at the extreme end of leanness. I’m aware this doesn’t neatly tie into this section’s discussion of rate of loss, but how lean you get is another factor that can influence lean mass retention, and I think it’s worth exploring.
Some studies in male bodybuilders and female physique competitors have found relatively little loss of lean mass despite the participants getting super lean.
And even large drops in measured lean mass need to be contextualised.
In a 2026 study of three male physique competitors, DEXA measurements suggested they lost 4-12 lb (~2-5 kg) of lean mass during contest prep. However, a month into the recovery phase, their lean body mass “rapidly increased” by 5-8 lbs (~2-4 kg), “resulting in negligible net changes from baseline.” The researchers attributed the rapid regain in lean body mass to changes in glycogen, water, and sodium intake.
Obviously, this doesn’t prove that no muscle loss occurred, but it does show why we should be a bit cautious about interpreting every pound of lost lean mass as a pound of lost skeletal muscle.
In sum
If you’d asked me this question years ago, I might have told you that losing some muscle just comes with the fat-loss territory. These days, I’m not so sure. As you’ve seen, it’s possible to retain almost all your lean mass even when getting far leaner than most people reading this ever intend to.
There’s individual variation, of course. But losing muscle isn’t an unavoidable part of fat loss, especially when you’re doing the things we know can give you the best shot at retaining it.
Notably, lifting weights with a focus on progression and a good amount of effort. Combining the caloric deficit with adequate protein intake. And while the research isn’t that clear-cut on how faster rates of loss impact muscle retention, I’d generally favour a moderate rate of loss over trying to lose weight too quickly.
If those things are in place, retaining most, if not all, of your muscle is a very realistic outcome.
TL;DR
- Fat-free or lean mass isn’t the same as skeletal muscle. The former refers to all the non-fat components of the body, while the latter refers to what most people would consider ‘muscle’.
- Changes in water, glycogen, gut contents, and even the fat-free components of adipose tissue can make it seem like you lost more muscle than you really did.
- The claim that 25% of your weight loss will be muscle is a rough approximation, and doesn’t apply to individuals lifting weights and eating adequate protein.
- In resistance-trained individuals eating enough protein, muscle loss is fairly negligible when it does occur.
- The research isn’t that clear on how faster rates of loss impact muscle retention. But I’d still favour a moderate rate of weight loss (and by extension, a moderate caloric deficit) over rates of loss that are too aggressive.
- Muscle loss isn’t an inevitable part of fat loss. In fact, retaining most, if not all, of your muscle is a very realistic outcome for a well-put-together fat loss plan.
And if you want to learn how to put together a smart fat-loss plan, check this out.
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