When people lose weight quickly, they often lose muscle along with fat, which isn’t ideal. Scientists discovered that a special protein called FGF15 (FGF19 in humans) helps protect muscle during weight loss. In this study, researchers compared two weight-loss methods in mice: changing their diet and using a medication called semaglutide. They found that mice with normal FGF15 levels kept more muscle when dieting, but the medication caused muscle loss regardless of FGF15 levels. The findings suggest that how you lose weight matters—diet changes and medications work differently in your body, and understanding these differences could help people preserve muscle while getting healthier.
The Quick Take
- What they studied: How a gut protein called FGF15 affects whether people keep their muscle mass when they lose weight, and how different weight-loss methods (diet versus medication) compare.
- Who participated: Laboratory mice were used in this study—some with normal FGF15 and some genetically modified to lack it. The mice were put on either a regular diet or given semaglutide (a weight-loss medication).
- Key finding: Mice that kept their natural FGF15 protein preserved more muscle when they lost weight through diet changes. However, when using semaglutide medication, mice lost muscle regardless of whether they had FGF15. This suggests different weight-loss methods affect your body in different ways.
- What it means for you: If you’re losing weight, the method you choose (diet, exercise, or medication) may affect how much muscle you keep. This research suggests that boosting FGF19 (the human version of FGF15) might help preserve muscle during diet-based weight loss, though more human studies are needed before making any changes to your routine.
The Research Details
Scientists conducted an experiment using two groups of mice: one group with normal gut function and another group genetically modified to lack the FGF15 protein. Each group was then split into two weight-loss methods: one group switched to healthier food (like going on a diet), and the other received daily injections of semaglutide, a medication used for weight loss. The researchers tracked changes in body weight, muscle mass, bone health, blood sugar control, and various chemicals in the mice’s bodies over 25 days.
This type of study is called a ‘controlled experiment’ because scientists can carefully control which mice have FGF15 and which don’t, allowing them to see exactly what role this protein plays. By comparing diet-based weight loss to medication-based weight loss, they could determine whether the weight-loss method itself matters.
The researchers measured multiple health markers including lean muscle mass, fat mass, liver fat, blood sugar response, and bile acids (digestive chemicals). This comprehensive approach helps paint a complete picture of how weight loss affects the whole body, not just the number on the scale.
Understanding why some people lose muscle during weight loss is important because muscle is metabolically active—it burns calories and keeps your body strong. Losing too much muscle can make it harder to maintain weight loss long-term and can weaken your body. By identifying that FGF15 plays a protective role, scientists may be able to develop better strategies to help people lose fat while keeping muscle. Additionally, comparing different weight-loss methods (diet versus medication) helps us understand that there’s no one-size-fits-all approach—your body responds differently depending on how you lose weight.
This study was published in Diabetes, a respected scientific journal. The research used a controlled laboratory setting with genetically modified mice, which allows for precise cause-and-effect conclusions. However, because this is animal research, results may not directly translate to humans. The study is well-designed with clear comparisons between groups, but human studies would be needed to confirm these findings apply to people. The researchers measured multiple health markers, which strengthens the reliability of their conclusions.
What the Results Show
The most important finding was that mice with normal FGF15 preserved significantly more lean muscle mass when they lost weight through diet changes compared to mice lacking FGF15. Specifically, when mice switched to regular food from a high-fat diet, those with FGF15 maintained their muscle better, while those without FGF15 experienced greater muscle loss alongside fat loss.
Interestingly, when mice received semaglutide injections, all mice lost muscle mass regardless of whether they had FGF15 or not. This suggests that the medication works through a different pathway in the body that bypasses the protective effects of FGF15. The semaglutide-treated mice did show better blood sugar control compared to the diet group, indicating that different weight-loss methods have different effects on various aspects of health.
The researchers also found that diet-based weight loss was more effective at reducing liver fat (triglycerides), while semaglutide was better at improving how the body handles blood sugar. Both methods caused changes in bile acids (chemicals that help digest fat), but these changes were more pronounced in mice lacking FGF15, suggesting this protein influences how your gut and liver communicate during weight loss.
Beyond muscle preservation, the study revealed that semaglutide treatment caused more dramatic changes in bile acid composition, particularly in mice without FGF15. Bile acids are important for digestion and also send signals throughout the body that affect metabolism. The changes in bile acids with semaglutide suggest this medication works partly by altering how your digestive system functions. Additionally, when mice stopped taking semaglutide, their weight, fat mass, and lean mass all returned to baseline levels, indicating the medication’s effects are temporary and dependent on continued use. The diet-based weight loss showed more stable changes in liver health markers.
This research builds on earlier findings showing that FGF19 (the human version of FGF15) predicts how well people maintain muscle during very-low-calorie diets. Previous studies in mice showed that FGF15 protected against muscle loss after gastric sleeve surgery. This new study extends that knowledge by directly comparing diet-based weight loss to medication-based weight loss and showing that FGF15’s protective effects work specifically with dietary interventions, not with semaglutide. The findings align with growing evidence that weight-loss method matters—it’s not just about calories in versus calories out, but how your body’s internal signaling systems respond to different approaches.
This study used laboratory mice, not humans, so the results may not directly apply to people. Mice have different metabolisms and lifespans than humans, and their responses to weight loss may differ. The study didn’t include exercise as a weight-loss method, so it’s unclear how FGF15 affects muscle preservation during physical activity combined with diet. The researchers also didn’t test whether artificially boosting FGF15 or FGF19 levels could improve muscle preservation in humans—they only showed what happens when it’s absent. Additionally, the study duration was relatively short (25 days), so long-term effects remain unknown. Finally, while the research identifies that FGF15 matters, it doesn’t fully explain the mechanisms of how it works, limiting our ability to develop targeted treatments.
The Bottom Line
Based on this research, if you’re planning to lose weight through diet changes, maintaining adequate nutrition and protein intake may be especially important to preserve muscle—this aligns with FGF15’s protective role. If you’re considering semaglutide or similar medications, be aware that muscle loss may occur regardless of FGF15 levels, so combining medication with strength training could help offset this effect. However, these are preliminary findings from animal studies, and you should consult your doctor before making any weight-loss decisions. The confidence level for these recommendations is moderate—the research is solid but needs human confirmation.
This research is most relevant to people planning significant weight loss who want to preserve muscle mass, including those considering weight-loss medications like semaglutide. It’s also important for people with type 2 diabetes or metabolic conditions, since the study examined glucose tolerance. Healthcare providers and researchers developing weight-loss strategies should pay attention to these findings. People who are not planning weight loss or who have medical conditions preventing them from dieting or taking these medications don’t need to apply this information. Pregnant women, children, and people with certain medical conditions should not use semaglutide and should consult doctors about any weight-loss approach.
If you’re using diet-based weight loss, muscle preservation effects would likely become apparent within weeks, though the full benefits may take several months. If using semaglutide, muscle loss could begin within days to weeks of starting treatment, which is why combining it with strength training is important. The study showed changes over 25 days, but real-world results in humans typically take longer. Don’t expect overnight results—sustainable weight loss and muscle preservation usually take 3-6 months to become clearly noticeable.
Want to Apply This Research?
- Track weekly body composition changes (muscle mass vs. fat loss) using a smart scale or periodic measurements, rather than just total weight. Record your weight-loss method (diet, medication, exercise) alongside these measurements to see patterns in how your body responds. If possible, note protein intake and strength training sessions to correlate with muscle preservation.
- If using the app for weight loss, add a daily protein intake goal (aim for 0.7-1 gram per pound of body weight) and log strength training sessions at least 2-3 times per week. Set reminders to track body measurements monthly rather than just daily weigh-ins, since muscle weighs more than fat and the scale doesn’t tell the whole story. If taking semaglutide or similar medications, use the app to monitor for muscle loss and adjust exercise accordingly.
- Create a long-term tracking dashboard that shows weight, estimated muscle mass, fat mass, and your chosen weight-loss method over time. Set monthly check-ins to assess whether you’re losing primarily fat or a concerning amount of muscle. If using medication, compare your muscle preservation to baseline measurements taken before starting treatment. Share these trends with your healthcare provider during regular check-ups to adjust your approach if needed.
This research is from animal studies and has not yet been confirmed in humans. The findings suggest potential mechanisms but should not be used to make medical decisions without consulting a healthcare provider. Semaglutide and other weight-loss medications require medical supervision and are not appropriate for everyone. Before starting any weight-loss program, diet change, or medication, speak with your doctor about what’s safe and appropriate for your individual health situation. This article is for educational purposes only and does not constitute medical advice.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.