The Study
Dietary protein restriction elevates FGF21 levels and energy requirements to maintain body weight in lean men
This study is like a fair test where guys ate less protein for a while and then more again, and scientists measured what happened. They found that when they ate less protein, their bodies needed more food to stay the same weight — but that doesn’t mean eating less protein makes you lose weight. It just means your body burns a bit more energy when protein is low.
Analysis score
Maximum 90 for a randomized controlled trial.
Where the score came from
When men ate less protein but kept their weight the same, their bodies burned more energy, so they had to eat more food to stay even.
Where does this study sit?
Reviews of RCTs (Meta-analyses)
Max 100Randomized Trials
Max 90Reviews of Cohort Studies
Max 85Cohort Studies
Max 72Reviews of Case-Control Studies
Max 63Case-Control Studies
Max 58Cross-Sectional & Case Series
Max 50Expert Opinion
Max 559 / 100
Quality score
Participants are randomly assigned to treatment or control groups, minimizing bias. The gold standard for testing whether an intervention causes an effect.
Key takeaways
Summary
Based on the study abstract and findings.
- 1Yes — this means low-protein diets may make you naturally crave more food, even if you're not trying to gain weight.
- 2Eating only 9% protein made men eat 19–21% more calories to stay the same weight.
- 3Their FGF21 hormone jumped 208–361%, and their fat cells changed how they used energy.
Score breakdown, methodology, conflicts of interest, evidence analysis & raw study data
Publication
Journal
Nature Metabolism
Year
2025
Authors
T. S. Nicolaisen, A. E. Lyster, K. Sjøberg, Daniel T. Haas, Christian T. Voldstedlund, Annemarie Lundsgaard, J. K. Jensen, Ea M Madsen, Casper K Nielsen, Mads Bloch-Ibenfeldt, N. J. Wewer Albrechtsen, Adam J. Rose, Natalie Krahmer, Christoffer Clemmensen, Erik A. Richter, A. Fritzen, Bente Kiens
Related Content
Claims (5)
When lean men reduce their dietary protein intake, their blood levels of a signaling molecule called FGF21 rise significantly within five weeks, and this rise is closely tied to the increased calorie intake needed to keep their body weight stable, indicating FGF21 may play a role in connecting protein consumption to energy use.
When lean, healthy men consume a diet with very low protein for five weeks while maintaining the same total calorie intake, they naturally eat more food to keep their weight stable, suggesting that low protein intake increases the body’s energy needs for weight maintenance.
In lean men, reducing protein intake alters mitochondrial proteins in fat tissue, increasing some components involved in energy production while decreasing others, which may lead to higher energy use.
When male mice are fed a low-protein diet, their fat tissue undergoes changes in mitochondrial function, but these changes do not occur if the mice lack the FGF21 protein, showing that FGF21 is necessary for the metabolic response to protein restriction.
When people reduce the amount of protein in their diet to 10% of their total calories, they tend to eat more total calories without trying.
Not medical advice. For informational purposes only. Always consult a qualified healthcare professional before making health decisions.