Extended Fasting Affects Blood Chemistry Results

Extended fasting (2-4 days) has a significant impact on laboratory results, particularly those related to metabolism, including blood glucose, triglycerides, ketone bodies, and amino acids.

To differentiate between fasting effects and underlying health conditions, it's crucial to consider the individual's baseline values and medical history and perform additional tests as needed. 

Lab Results Affected by Extended Fasting:

Metabolic Markers:

• Blood Glucose: Decreases as the body shifts from using glucose to using fat for energy. 
• Triglycerides: May decrease initially but can increase during refeeding. 
• Ketone Bodies: Increase significantly as the body starts to break down fat for energy. 
• Free Fatty Acids (FFAs): Increase as the body mobilizes fat stores. 
• Uric Acid: May increase. 

Other Potential Changes:

• Liver Enzymes (ALT, AST, AP, GGT): Can be elevated or decreased depending on the duration and severity of the fast. 
• Kidney Parameters (GFR, Urea, Creatinine): May show changes. 
• Amino Acids: Changes in the content and types of amino acids can occur. 
• Red Blood Cell Count, Hemoglobin, Hematocrit: May decrease. 
• White Blood Cell Count: May increase. 
• Iron: May increase. 

Differentiating Fasting Effects from Underlying Conditions:

Baseline Values:

Compare current lab results to the individual's previous results to see if the changes are within expected limits for fasting. 

Medical History:

Consider any pre-existing conditions or medications that could affect lab results. 

Physical Exam:

A thorough physical exam can help identify any signs or symptoms of underlying conditions. 

Additional Tests:

Depending on the results and clinical suspicion, additional tests may be necessary to rule out other potential conditions. 

Timing of Blood Draw:

Ensure that blood samples are drawn at the appropriate time relative to the last meal or fasting period. 

Fasting Status:

Confirm that the individual is fasting before blood tests that require fasting. 

Re-feeding:

Observe the individual's response to re-feeding to see if the changes are reversed, which would suggest a fasting-related effect

Optimal Takeaways

• Fasting Alters Key Metabolic Markers: Extended fasting can lower blood glucose and increase ketone bodies, free fatty acids, and uric acid, reflecting a shift to fat metabolism.
• Triglycerides and Amino Acids Fluctuate: Triglycerides may decrease during fasting but can increase again with refeeding; amino acid levels also fluctuate due to reduced dietary intake.
• Other Lab Changes May Occur: Liver enzymes, kidney function markers, red and white blood cell counts, and iron levels may shift depending on the duration and intensity of fasting.
• Baseline Comparison Is Critical: Always compare lab results to previous values and consider the person’s fasting status to determine whether results are due to fasting or an underlying issue.
• Consider Clinical Context: A thorough medical history, physical exam, and potentially additional tests are essential to distinguish between fasting effects and medical conditions.
• Refeeding Response Helps Confirm Cause: If lab changes normalize after eating resumes, it likely indicates the results were fasting-related, not disease-related.

References

Dai, Zhongquan et al. “Analysis of physiological and biochemical changes and metabolic shifts during 21-Day fasting hypometabolism.” Scientific reports vol. 14,1 28550. 18 Nov. 2024, doi:10.1038/s41598-024-80049-2

Ding, Xiao-Qi et al. “Effects of a 72 hours fasting on brain metabolism in healthy women studied in vivo with magnetic resonance spectroscopic imaging.” Journal of cerebral blood flow and metabolism : official journal of the International Society of Cerebral Blood Flow and Metabolism vol. 38,3 (2018): 469-478. doi:10.1177/0271678X17697721

Gabriel, Sahmla et al. “A Six-Week Follow-Up Study on the Sustained Effects of Prolonged Water-Only Fasting and Refeeding on Markers of Cardiometabolic Risk.” Nutrients vol. 14,20 4313. 15 Oct. 2022, doi:10.3390/nu14204313

Horton, T J, and J O Hill. “Prolonged fasting significantly changes nutrient oxidation and glucose tolerance after a normal mixed meal.” Journal of applied physiology (Bethesda, Md. : 1985) vol. 90,1 (2001): 155-63. doi:10.1152/jappl.2001.90.1.155

Jørgensen, Sine W et al. “Impact of prolonged fasting on insulin secretion, insulin action, and hepatic versus whole body insulin secretion disposition indices in healthy young males.” American journal of physiology. Endocrinology and metabolism vol. 320,2 (2021): E281-E290. doi:10.1152/ajpendo.00433.2020

Longo, Valter D, and Mark P Mattson. “Fasting: molecular mechanisms and clinical applications.” Cell metabolism vol. 19,2 (2014): 181-92. doi:10.1016/j.cmet.2013.12.008

Moser, Othmar et al. “Impact of a Single 36 Hours Prolonged Fasting Period in Adults With Type 1 Diabetes - A Cross-Over Controlled Trial.” Frontiers in endocrinology vol. 12 656346. 6 Jul. 2021, doi:10.3389/fendo.2021.656346

Sanvictores, Terrence, et al. “Physiology, Fasting.” StatPearls, StatPearls Publishing, 24 July 2023.

Šupak-Smolčić, Vesna et al. “Influence of a prolonged fasting and mild activity on routine laboratory tests.” Clinical biochemistry vol. 48,1-2 (2015): 85-8. doi:10.1016/j.clinbiochem.2014.10.005

Wang, Yiren, and Ruilin Wu. “The Effect of Fasting on Human Metabolism and Psychological Health.” Disease markers vol. 2022 5653739. 5 Jan. 2022, doi:10.1155/2022/5653739

Wilhelmi de Toledo, Françoise et al. “Influence of Long-Term Fasting on Blood Redox Status in Humans.” Antioxidants (Basel, Switzerland) vol. 9,6 496. 6 Jun. 2020, doi:10.3390/antiox9060496