The Study
Effects of seven days’ fasting on physical performance and metabolic adaptation during exercise in humans
This study watched 13 healthy people before and after 7 days of fasting. It shows what changed in their bodies, but because everyone did the same thing with no comparison group, we can't be sure the fasting alone caused the changes. It's like noticing your plants grow taller after you start singing to them — they changed, but we don’t know if it was the singing or something else.
Analysis score
Maximum 44 for a cross-sectional study.
Where the score came from
Your body keeps your muscle strength even if you lose muscle size, but you get tired faster during intense exercise because your body can't use sugar well.
Where does this study sit?
Systematic Reviews & Meta-analyses
Max 100Randomized Trials
Max 90Cohort Studies
Max 72Case-Control
Max 58Cross-Sectional
Max 44Case Reports & Series
Max 30Expert Opinion
Max 538 / 100
Quality score
Detailed descriptions of individual patients or small groups. Valuable for identifying new conditions or side effects, but cannot establish generalizable conclusions.
Key takeaways
Summary
Based on the study abstract and findings.
- 1Yes, it matters: you can still lift or push hard, but running or cycling hard becomes harder even if your muscles aren't weaker.
- 2People lost 4.6 kg of lean mass, muscle strength stayed the same, oxygen use dropped by 13%, muscle sugar stores were cut in half, and a key enzyme blocking sugar burning increased 13 times.
Score breakdown, methodology, conflicts of interest, evidence analysis & raw study data
Publication
Journal
Nature Communications
Year
2025
Authors
Kristoffer J. Kolnes, Emelie T. F. Nilsen, Steffen Brufladt, Allison M. Meadows, P. Jeppesen, Ø. Skattebo, E. Johansen, J. Birk, K. Højlund, J. Hingst, B. S. Skålhegg, R. Kjøbsted, J.L. Griffin, A. Kolnes, S. O’Rahilly, J. Wojtaszewski, J. Jensen
Not medical advice. For informational purposes only. Always consult a qualified healthcare professional before making health decisions.