Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting.
Level 5 - mechanism / opinion, no new human data
Narrative review of physiological mechanisms and theoretical frameworks without primary human data or systematic synthesis
PubMed 29086496 · doi:10.1002/oby.22065
What was done
This narrative review summarizes physiological responses across major organ systems, including the musculoskeletal system, during intermittent fasting regimens lasting from 12 hours to several days. The authors specifically examined the concept of the metabolic switch—the threshold (typically over 12 hours of fasting) where liver glycogen stores deplete and metabolism transitions toward fatty acid mobilization and ketone production.
What was found
The abstract reports no numerical findings, effect sizes, or statistical results. It conceptually describes that switching from glucose to fatty acid-derived ketones shifts metabolism away from lipid/cholesterol synthesis and storage toward fat oxidation, which may preserve muscle mass and function, activate pathways that optimize physiological performance, and influence aging and disease pathways.
Why it matters
The paper frames the glucose-to-ketone transition as a key physiological trigger point in intermittent fasting and recommends measuring plasma ketone levels as an objective compliance biomarker in future trials.
Limits
The abstract provides conceptual and mechanistic reasoning rather than systematic review methodology, empirical human trial data, or quantified outcomes. It does not report sample sizes, specific populations, or comparative efficacy across different fasting protocols.
Cited by
- supports It takes approximately 10 to 12 hours of fasting for the liver to deplete glycogen stores and initiate the metabolic switch to ketosis.
- supports On average, it takes 11 to 12 hours of fasting to deplete liver glycogen stores and initiate ketogenesis.