The transition point where your body shifts from burning glucose to burning fat.
What the evidence says
- The "metabolic switch" occurs when liver glycogen depletes and ketones are produced from fatty acids — typically 12-36 hours after food cessation, depending on glycogen content and energy expenditure[1],[2]
- Fasting-dependent switch from glucose-driven oxidative phosphorylation to ketone body and fatty acid-dependent metabolism[3]
- People with a typical Western eating pattern of three or more meals per day never flip the metabolic switch; their ketone levels remain continuously low — and insulin resistance further prolongs the time to flip the switch[2]
- The review argues the switch is accompanied by cellular and molecular adaptations of neural networks that enhance their functionality and resistance to stress, injury and disease — the mechanistic evidence behind that is preclinical[1]
- In yeast, fasting inhibits TOR-S6K signaling and activates autophagy, which plays an important role in aging and lifespan — the sentence sits in `cfs01`'s yeast section, whose surrounding argument turns on the yeast transcription factors Msn2, Msn4 and Gis1[3]
- The authors propose that intermittent metabolic switching (cycles of fasting/exercise followed by recovery) may optimize brain function across the lifespan (attribution restored 2026-09-10)[1]
- β-Hydroxybutyrate increased from 183 µmol/L (day 1) to 4782 µmol/L (day 4) of fasting — 11 subjects, and the spread is wide: 182.7 ± 262.9 rising to 4781.6 ± 3001.6, so the direction is unambiguous and the individual figure is not[4]
- `cfa01`'s authors hypothesise, in their word, "that once the metabolic switch occurs, both the tolerability and sustainability of IF regimens are greatly increased" — and say in the next sentence that "experimental studies, however, are needed to distinguish the effects of caloric restriction versus the metabolic switch induced by IF regimens on fat metabolism, hunger, and overall tolerability"[2]
- "Accumulating evidence suggests some organ systems exhibit similar cellular and molecular responses to aerobic exercise and IF (e.g., suppression of mTOR, stimulation of autophagy, and mitochondrial biogenesis)"; in muscle cells specifically, fasting "results in the activation of AMPK in muscle cells which, in turn, can activate SIRT1" (corrected 2026-09-19, following `cfa01`, from "Fasting and exercise trigger similar molecular pathways: both activate AMPK → SIRT1 … while suppressing mTOR", which dropped the review's accumulating evidence suggests and its some organ systems, and generalised a muscle-cell pathway)[2]