Skinny Fat Body Composition – Before & After Weight Loss

Recent headlines (1, 2, 3, 4, 5, 6) link Ozempic/Wegovy/semaglutide (GLP-1) weight loss drugs to muscle tissue loss, supposedly resulting in a skinny fat body composition. The typical studies referenced (7, 8, 9, 10) use DEXA/DXA scans due to cost-effectiveness, regardless of accuracy, as currently:

“The FDA suggests that only a fraction of participants in Phase 3 trials ought to undergo body composition assessment and the FDA does not require any tests of muscle function, mobility, or strength. Investigations into body composition or muscle-related changes associated with GLP-1RAs are thus limited and likely underpowered and, consequently, the effect on muscle health and function is largely unknown. “

The fact is, DEXA/DXA scans are inaccurate (11, 12, 13, 14, 15, 16) measures of body composition. They cannot discern skinny fat (17, 18, 19) which is different (20) than regular white/yellow fat (body fat %, common adipose tissue). The only technology capable of accurately measuring skinny fat is a specific MRI scan (21).

Skinny Fat Body Composition – Before & After Weight Loss

At this time, there are no scientific studies that use an MRI to accurately determine body composition before and after weight loss. Just as there are no MRI studies that know how much genetic muscle tissue each person had (or did not have) at birth, or ever. The truth is, 99%+ of human beings and their doctors have no idea how much muscle tissue they were born with or without (skinny fat).

Thus, the typical studies referenced (7, 8, 9, 10) inaccurately ASSUME each person’s level of muscle mass before and after weight loss:

“Indeed, research has shown that lean mass composition correlates with body weight and varies greatly among individuals.³⁹ In addition, up to 15% of adipose tissue can actually consist of fat-free mass (FFM; which largely consists of lean mass), meaning a large loss of adipose tissue could significantly (and variably) contribute to the overall lean mass loss and inaccurately reflect changes in muscle mass in weight loss trials.^{39, 40}

There is a widely cited ‘rule’ stating the expected loss of FFM for a given amount of body weight loss that is commonly used as a reference for lean mass loss. This rule, called the quarter FFM rule, states that approximately one-quarter of weight loss will be FFM (i.e., ΔFFM/ΔWeight = ∼0.25), with the remaining three-quarters being fat mass. In other words, when an individual loses weight purposefully, it is assumed that approximately 75% of weight is lost as fat mass, and 25% of weight is lost as FFM.

However, an in-depth review of the quarter FFM rule concluded that the rule is at best an approximation, with limited mechanistic basis and questionable accuracy, as the proportion of weight lost as lean tissue varies over time and is determined by multiple factors including level of energy intake, diet composition, sex, baseline adiposity, presence of inactivity or type and level of added activity, and potentially the subject’s metabolic state or hormonal response.⁴¹ This observation, together with the variable results of the effects on lean mass from different obesity interventions, leaves us without a proper reference for what should be considered a clinically important amount of lean mass loss during weight loss. Therefore, given the current body of evidence, the clinical significance of the GLP-1-based effects on muscle mass (distinct from lean mass) remains unclear.

Unfortunately, [the] data may not be readily available in the near future. According to the US Food and Drug Administration (FDA) guidelines⁴² for assessing weight management therapies, the only acceptable primary efficacy endpoints for weight loss drug trials are those related directly to changes in body weight. Body composition metrics such as muscle mass or lean mass, by contrast, are considered safety endpoints, which require far smaller cohorts for testing.”

GLP-1 Drugs & Body Composition – Muscle Loss or Preservation?

Moreover, one study (23) indicates that supposed muscle tissue loss revealed no significant change in physical function — IE muscle function was maintained despite the supposed significant loss of muscle. This suggests that muscle was not actually lost, it was never present to begin with, likely from birth. There are even contradictory studies (24, 25, 26) that indicate that GLP-1 drugs protect against muscle tissue atrophy.

So, what is the reality of body composition, skinny fat, GLP-1 drugs, and weight loss, in general? Basic anatomy deems that lost muscle tissue — from weight loss and/or poor diet, exercise, or/and lifestyle — does not magically become some kind of fat/skinny fat. This is simply not (27, 28, 29) how human tissue works.

If that were true, that lost muscle becomes skinny fat, then it would apply equally to 100% all human beings. Not some, many, or a select few. All. But it does not apply to genetically blessed Christian Bale. Around 2003, he lost a lot of weight for a movie role including obvious muscle tissue:

Yet, even while emaciated at a rough underweight BMI of 16.5 the muscle definition including 6-pack abs and muscle on his love handles, at least, is clear. With no signs of any skinny fat! Genetics.

People who are experiencing skinny fat after weight loss, be it via GLP-1 drugs, a calorie deficit, or the like, were already experiencing skinny fat before they lost the weight.

Before & After Weight Loss – Skinny Fat Body Composition

Research Participant 1053 (30)(RP 1053) started their weight loss journey at an obese BMI of 32. 1 with obvious cellulite and thin fat tissue (skinny fat)(17, 19) where default genetic muscle tissue (18) should be but is not:

They safely lost weight down to an overweight BMI of 26.3, yet the same thin fat and cellulite tissue (skinny fat)(17, 19) exists in all the same places it did when they were at BMI 32.1. Genetics.

No indications exist that they had the default genetic muscle tissue before they lost weight. And there are no indications that the default genetic muscle tissue was hiding under the excess regular white/yellow fat. Of course, the only way to definitively prove this is with a proper MRI scan, which our Scientific Skinny Fat MRI Study (21) is working on.

Whereas, even when genetically blessed Joe Rogan is overweight he has undeniably defined abs under the excess regular white/yellow body fat. And no skinny fat. Genetics.

Skinny Fat Body Composition is Genetic

Like so many people around the world, RP 1053 never had the default genetic muscle tissue to begin with. They have been experiencing skinny fat (a lack of genetic muscle tissue) from birth, thanks to their unique genetics. We are working on proving this once and for all with our MRI Study (21).

No different than the aforementioned inaccurate scientific studies that claim GLP-1 drugs cause muscle tissue loss and subsequently a skinny fat body composition. People experiencing skinny fat after weight loss were experiencing it always, from birth. There is no evidence that RP 1053 had the default genetic muscle tissue before they lost weight. As well, there is no evidence they lost muscle tissue during weight loss that somehow magically caused them to become skinny fat.

Skinny fat body composition is genetic. The more genetic skinny fat a person is experiencing, the slower (31) their metabolism and the more prone they are to insulin resistance (32, 18, 31), type-2 diabetes (33, 18, 31), being overweight (18, 31, 32, 33), and obesity (18, 31, 32, 33). Are you experiencing skinny fat? Take the Scientific Body Type Quiz to find out.

References

1. NBC News, Health News: Weight loss drugs can lead to muscle loss, too. Is that a bad thing?, May 20, 2023, Kaitlin Sullivan. https://www.nbcnews.com/health/health-news/weight-loss-drugs-muscle-loss-rcna84936
2. BestLife: Ozempic May Cause Dangerous Muscle Loss, Doctors Warn—Here’s How to Prevent That, February 9, 2024, Abby Reinhard. https://bestlifeonline.com/ozempic-muscle-loss-news/
3. The Harvard Gazette: Losing fat is good, but losing muscle isn’t, March 18, 2024, Alvin Powell. https://news.harvard.edu/gazette/story/2024/03/losing-fat-is-good-but-losing-muscle-isnt/
4. MedicalNewsToday: How to preserve muscle on weight-loss drugs like Wegovy, January 7, 2025, Deep Shukla (Fact checked by Jill Seladi-Schulman, Ph.D). https://www.medicalnewstoday.com/articles/how-to-preserve-muscle-mass-on-weight-loss-drugs-like-wegovy
5. The Lancet, Diabetes & Endocrinology: Muscle matters: the effects of medically induced weight loss on skeletal muscle, Volume 12, November 2024, Issue 11p785-787, Carla M Prado, Stuart M Phillips, M Cristina Gonzalez, Steven B Heymsfield. https://www.thelancet.com/journals/landia/article/PIIS2213-8587(24)00272-9/abstract
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7. Diabetes, Obesity, and Metabolism: Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies, June 27, 2024, Ian J. Neeland MD, Jennifer Linge PhD, and Andreas L. Birkenfeld MD. https://dom-pubs.onlinelibrary.wiley.com/doi/10.1111/dom.15728
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17. Skinny Fat Science: What Is Skinny Fat?, July 26, 2024. https://skinnyfat.fellowone.com/skinny-fat-science/what-is-skinny-fat/
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20. Skinny Fat Science: Are Regular Fat and Skinny Fat the Same Thing?, August 28, 2024. https://skinnyfat.fellowone.com/skinny-fat-science/are-regular-fat-and-skinny-fat-the-same-thing/
21. Skinny Fat Science: Scientific Skinny Fat MRI Study – Proving What Skinny Fat Is, March 26, 2025. https://skinnyfat.fellowone.com/skinny-fat-science/scientific-skinny-fat-mri-study-proving-what-skinny-fat-is/
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24. NIH, National Library of Medicine: GLP-1RA Liraglutide and Semaglutide Improves Obesity-Induced Muscle Atrophy via SIRT1 Pathway, August 15, 2023, Jie Xiang, Liyan Qin, Jinling Zhong, Ning Xia, and Yuzhen Liang. https://pubmed.ncbi.nlm.nih.gov/37602204/
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26. Drug Discovery & Development: Evidence on GLP-1 drugs and muscle loss remains murky, November 27, 2023, Brian Buntz. https://www.drugdiscoverytrends.com/glp-1-impact-lean-mass/
27. Healthline: Does Fat Turn into Muscle? What to Know, March 2, 2021, Katey Davidson, MScFN, RD, CPT (Medically reviewed by Jake Tipane, CPT). https://www.healthline.com/nutrition/does-fat-turn-into-muscle
28. NIH, National Library of Medicine: Muscle-to-fat interaction: a two-way street?, January 1, 2010, Bente K Pedersen. https://pmc.ncbi.nlm.nih.gov/articles/PMC2821541/
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