A look at how metabolic inflexibility persists, even outside the body
We often talk about health as if it’s something we can fully control. If you eat better, sleep more, exercise harder, take the right supplement… better health will follow. These messages not only ignore the very real barriers many people face because of time, money, and access, they also ignore our biology. Personal accountability is not a substitute for healthcare. If you’re interested in this broader conversation in the context of obesity, two papers on the topic came out this week: one showing how genetics shape responses to GLP‑1 receptor agonists (PMID: 41951734)1, and another highlighting social bias and lack of coherence in obesity prevention efforts (PMID: 41945228)2.
While what we eat and how well we sleep clearly matter, effort alone does not always lead to health. Bodies respond differently, and those differences are not always visible from the outside. This variation has been the focus of my work for decades ask I’ve worked to understand the underlying biology contributing to weight gain and obesity, independent of lifestyle. It began with this paper I published in 2012 (PMID: 22024640)3.
Reader’s Note: I use the phrase “people with obesity” to reflect that obesity as a complex, chronic disease. The findings discussed describe group‑level patterns, not individual predictions, as does the clinical designation of obesity by BMI. People with obesity are metabolically diverse, and my ongoing research focuses on understanding this heterogeneity and its origins.
The background
This post is the second in my From the Paper series, where I explain my peer‑reviewed research studies, beginning with my earliest work. This post builds upon the previous piece, where we showed that skeletal muscle responses to a short high‑fat diet vary from person to person, and that reduced metabolic flexibility is linked to obesity.
Long before that diet study, work by our team and others, had shown that people with obesity and insulin resistance tend to have lower fat oxidation, as well as lower mitochondrial metabolism and excess fats stored in their skeletal muscle. Similar patterns were also observed in skeletal muscle satellite cells cultured from people with obesity or with type 2 diabetes. In several studies, lipid oxidation measured in these cultured satellite cells, closely mirrored metabolism in the muscle from the living donors (PMID: 16007256)4, suggesting that some aspects of metabolic inflexibility may be preserved at the cellular level.
You may know mitochondria as the ‘powerhouse of the cell’, the primary location for oxidative metabolism. Oxidative metabolism refers to the chemical process that uses oxygen to break down our nutrients, like carbohydrates or fats, to generate energy as adenosine triphosphate, or ATP. Because all of this occurs in the mitochondria, if we want to measure fat oxidation and metabolic flexibility, we need to need to take a deeper look at the mitochondria.
The prior paper focused on how muscle gene expression changed after a five-day high-fat diet in people with obesity (BMI > 30) or normal weight (BMI < 25) . We collected muscle samples and quickly froze them to capture changes as they are in the body (in vivo), where circulating hormones, nutrients, and environment all influence metabolism. This paper builds on that work, taking a closer look at the underlying biology. Here, we examined the skeletal muscle satellite cells from the same participants. By growing these cells in the lab (in vitro), we can strip away the behaviors, hormones, and environmental influences, allowing us to ask whether muscle cells themselves differ in their mitochondrial response to fats.
Satellite cells are the resident progenitor cells of skeletal muscle. These cells reside alongside muscle fibers and are responsible for repair, remodeling, and adaptation of muscle tissue throughout life. During a muscle biopsy, we can isolate these cells and grow them in culture in the lab. Once isolated, these cells are allowed to proliferate in the lab and are then cryopreserved before experiments. This means that the cells we study were never inside the body. They are many generations removed from the original cells collected. Importantly, even these progeny cells appear to retain a ‘biological memory’, displaying similar phenotypes in vitro to those of their donor in vivo. Metabolic health that’s embedded at the cellular level.
I recently wrote a standalone essay exploring progenitor cells more broadly: what they are, where they come from, and why they matter for health. If you’d like to read about these cells, and satellite cells in particular, you can read it HERE.
The question
In this study, we asked: Do differences in fat metabolism and metabolic inflexibility seen in obesity persist when skeletal muscle cells are removed from the body and grown in culture?
Why does this matter? If muscle cells retain metabolic inflexibility when cultured outside the body, it suggests that some aspects of metabolic health shaped by intrinsic cellular biology, which may help explain why individuals can respond so differently to the same diets, environments, or even medications.
What we did
We cultured muscle satellite cells from biopsies of young adults with obesity and with normal weight, then differentiated these progenitor cells to mature muscle cells in the lab. To mimic a short, high-fat diet, we then exposed cells for 24-hours to either control media containing mostly sugars (glucose) or media supplemented with excess fats commonly found in our diets (oleate and palmitate).
After this high-fat challenge, we then permeabilized the cells using a detergent to ‘poke holes’ in the cell membrane. This allowed us to directly access the mitochondria, while keeping their structure intact. We measured mitochondrial metabolism by placing the permeabilized cells in a sealed, high resolution respirometry chamber. This system tracks small changes in oxygen concentration as the mitochondria consume oxygen to create energy. Our primary focus was fat oxidation during state 3 respiration, which is measured with excess adenosine diphosphate (ADP) present to stimulate physiological conversion to ATP. We also measured carbohydrate oxidation and maximal respiration capacity. Lastly, we measured mitochondrial content, allowing us to distinguish potential differences in mitochondrial content versus how well the mitochondria functioned.
Our goal was to see how the mitochondria responded to the short, high fat challenge, in terms of fat oxidation and overall respiration.
What we found
In the control condition (no fat challenge), mitochondrial respiration did not differ between cells from donors with normal weight and those with obesity. After the 24‑hour fat challenge, however, fat‑supported mitochondrial respiration nearly doubled in permeabilized cells from normal‑weight individuals when adjusted for mitochondrial content, indicative of metabolic flexibility. In contrast, cells from individuals with obesity showed virtually no change, reflecting metabolic inflexibility. This difference was most apparent when respiration was expressed as the ratio of state 3 (ADP‑stimulated) to basal respiration. Measures of mitochondrial content followed a similar pattern: after lipid exposure, mitochondrial DNA content increased in cells from normal‑weight donors (+16%) but decreased in cells from donors with obesity (–13%).
Study Overview: Skeletal muscle satellite cells were isolated from participants with normal weight or obesity. The cells were cultured, then differentiated into muscle cells in vitro, then exposed for 24 hours to either control media containing mostly glucose or excess fats that mimic our diet. Mitochondrial oxidative metabolism (respiration) was measured under multiple conditions, including fat- and carbohydrate‑supported metabolism. Cells from normal‑weight donors showed increased fat‑supported mitochondrial respiration by 2-fold after lipid exposure, consistent with metabolic flexibility, while cells from donors with obesity showed little to no response, indicating metabolic inflexibility.
What it means
Muscle progenitor cells (satellite cells) from people with obesity were not able to increase fat oxidation in their mitochondria after the 24-hour fat challenge, while cells from people with normal weight almost doubled fat oxidation. This altered metabolism, even in cells cultured outside the body suggest that metabolic inflexibility is not solely caused by the environment inside our bodies. Rather, these data suggest intrinsic biology inside our cells may play a role in how our bodies use the foods we eat. These metabolic differences, perpetuated meal after meal, could favor excess lipid storage, reduced fat oxidation, and ultimately contribute to obesity and insulin resistance over time.
However, these results represent group-level patterns. They do not indicate that all muscle cells from people with obesity behave the same. Instead, it highlights that we’re all individuals, with subtle but meaningful differences in our biology, right down to our cells.
What we still don’t know
This study did not address whether the mitochondrial differences observed in these muscle cells are modifiable (though this study5 suggests yes!), or when metabolic differences first arise. There are at least two possibilities: these differences may have always been present, potentially contributing to the development of obesity, or they may have emerged after obesity developed, as a response to prolonged metabolic stress. This is an important distinction. If these metabolic traits precede weight gain, they may help explain why some individuals are more vulnerable to obesity despite similar environments.
This possibility, that the underlying biology precedes weight gain, has shaped my work since. It led me to a postdoctoral fellowship investigating pregnancy and fetal development, where we experience our earliest metabolic environments and when life-long health trajectories may begin.
Kristen Boyle, PhD
Dr. Boyle is a scientist, writer, and professor at the University of Colorado Anschutz School of Medicine, where she leads the Molecular Health Lab. Her group studies how early life shapes lifelong health. She writes about science, health, and truth.
Linked References
- Su, Q.J., Ashenhurst, J.R., Xu, W. et al. Genetic predictors of GLP1 receptor agonist weight loss and side effects. Nature (2026). PMID: 41951734
- Ramos Salas, X., Hussey, B., Alberga, A.S. et al. Rethinking Obesity Prevention: A Call for Strategic and Operational Clarity. Curr Obes Rep 15, 32 (2026). https://doi.org/10.1007/s13679-026-00708-5. PMID: 41945228
- Boyle KE, Zheng D, Anderson EJ, Neufer PD, Houmard JA. Mitochondrial lipid oxidation is impaired in cultured myotubes from obese humans. Int J Obes (Lond). 2012 Aug;36(8):1025-31. doi: 10.1038/ijo.2011.201. Epub 2011 Oct 25. PMID: 22024640
- Ukropcova B, McNeil M, Sereda O, de Jonge L, Xie H, Bray GA, Smith SR. Dynamic changes in fat oxidation in human primary myocytes mirror metabolic characteristics of the donor. J Clin Invest. 2005 Jul;115(7):1934-41. doi: 10.1172/JCI24332. PMID: 16007256
- Lund J, Rustan AC, Løvsletten NG, Mudry JM, Langleite TM, Feng YZ, Stensrud C, Brubak MG, Drevon CA, Birkeland KI, Kolnes KJ, Johansen EI, Tangen DS, Stadheim HK, Gulseth HL, Krook A, Kase ET, Jensen J, Thoresen GH. Exercise in vivo marks human myotubes in vitro: Training-induced increase in lipid metabolism. PLoS One. 2017 Apr 12;12(4):e0175441. doi: 10.1371/journal.pone.0175441. PMID: 28403174
