Optimal - The Blog
April 8, 2025
Extended fasting (lasting 2–4 days) can cause significant changes in lab test results, especially those related to metabolism.
Blood glucose typically drops, while ketone bodies, free fatty acids, and uric acid levels rise as the body shifts from using carbohydrates to burning fat for energy.
Triglycerides may drop during fasting but can increase during refeeding. Other laboratory markers, such as liver enzymes, kidney function markers, amino acids, and blood cell counts, may also shift during extended fasts.
To determine whether these changes are due to fasting or an underlying condition, it’s essential to compare the results to a person’s baseline, consider their medical history, and possibly conduct additional tests.
A physical exam and knowing the timing of the blood draw in relation to the fast are also helpful.
If lab abnormalities normalize after eating resumes, they’re more likely related to fasting rather than a health issue.
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