From hour 14
Ketosis
The liver converts more fat into ketones, which the brain and muscles can use when glucose is scarce.
- Typical onset
- 12–18 h
- Varies most with
- Activity · last meal
- Measurable at home
- Only by testing
What’s understood
Ketone production rises as fat becomes the dominant fuel. The switch from glucose is gradual rather than a moment, and its timing differs widely between people.
What people commonly report
Reduced appetite, a metallic or fruity taste, mild headache in the first few fasts, and for some a lift in concentration. Sleep is often lighter.
What it isn’t
Not a state this app can detect — ketosis is measurable only by testing blood or breath, never by a clock.
Where the evidence is thin
Ketone levels don't map neatly onto benefits, and nothing about reaching ketosis in a single fast has been shown to change long-term health on its own. We can't tell you whether you're in it — only what hour you're at.
Sources
- Effects of intermittent fasting on health, aging and disease de Cabo R, Mattson MP · N Engl J Med · 2019 · Review
- Fasting: molecular mechanisms and clinical applications Longo VD, Mattson MP · Cell Metab · 2014 · Review