Phase 1 · Glycogen depletion
The first stretch after your last meal. Your liver is still releasing stored glucose. Blood sugar drifts gently. Ketosis hasn't started.
Fasting Insights · Glucose-Ketone Index
An honest read on your glucose ketone index.
Your CGM measures glucose. We estimate where you are in ketosis from your glucose curve and fasting duration. A directional read, in plain language — not a blood-strip replacement.
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Free for core fasting features.
Hour 22. Your watch buzzes. Your CGM is steady. Where are you, actually?
If you fast — for autophagy, metabolic health, perimenopause, autoimmune flares, or just because it makes your head clearer — there's a question that hangs over every long stretch. Am I in ketosis yet?
The honest answer used to require a finger-prick and a Keto-Mojo strip. That's still the only way to know for sure. But your CGM already records what your blood sugar is doing hour by hour, and your phone already knows how long it's been since your last meal. That's enough to make a careful estimate.
Body Insights does the estimate, and is plain about what it is: a directional read, not a clinical measurement. When the difference matters, you measure.
Estimated, not measured. We are loud about this.
The real glucose ketone index — the one developed by Dr. Thomas Seyfried for therapeutic protocols — is the ratio of blood glucose to blood ketones, both in mmol/L. It needs a ketone meter. A Keto-Mojo or Precision Xtra. A finger-prick.
Body Insights does not have access to your blood ketones. CGMs do not measure ketones. What we have is your glucose trajectory, your fasting duration, and the well-described physiology that connects the two. From that we estimate the most likely ketosis range — none, early, light, moderate, or deep.
If you're using fasting casually, the estimate is plenty. If you're using fasting therapeutically — for seizure management, as a cancer adjunct, for a metabolic condition under a clinician's care — measure with strips. We will say this on the screen, not just here.
The four phases your body actually moves through.
Fasting isn't one switch. It's a series of metabolic transitions — and your CGM is reading the surface of every one.
Phase 2 · The transition
Glycogen is mostly gone. Your body begins shifting fuel sources. Glucose drops more noticeably, and the first ketones start showing up in blood.
Phase 3 · Ketosis deepens
The shift is on. Glucose has settled lower. Ketones rise. This is the window most fasting protocols target.
Phase 4 · Deep ketosis
The long-fast territory. Glucose is low and stable, ketones are high, autophagy is well underway. This is medical territory — not a place to wander alone.
Body Insights infers which phase you're most likely in from your live glucose and your time-in-fast. Plain-language label, not a number you'll game.
See your fast, in context
Free for core features. CGM optional.
Whether today is a fasting day at all.
Long before the GKI question comes the more basic one: should you be fasting this morning? For a healthy thirty-year-old the answer is usually yes. For a body managing chronic illness, perimenopause, a hard week, or post-viral recovery, the answer is — sometimes.
Body Insights reads your overnight HRV, your sleep duration and efficiency, your morning glucose, and yesterday's strain. It returns a plain-language read: "Today has room for a longer fast" — or "Eat breakfast today. Your nervous system is asking for fuel."
Sleep and HRV carry the most weight; glucose stability rounds it out. The point is to put rest, fasting, and feeding on the same page rather than treating fasting as a discipline metric.
For people who want the science.
Everything below is what an honest fasting analytics tool should tell you. If you're comparing options — Zero, Fastic, LIFE Fasting, Senza — you're in the right place.
What we read, what we estimate.
Reads from Apple Health. Your Apple Watch tracks HRV and sleep. Your CGM writes glucose. We translate.
Glucose trajectory
The shape of your CGM curve through the fast — not the absolute number. The trajectory is what matters.
Time-in-fast
How long since your last meal, tracked from the fasting timer or inferred from your eating window.
Ketosis level estimate
None, early, light, moderate, or deep — labeled clearly as an estimate. Confirm with strips when it matters.
Metabolic flexibility
Across many fasts: how quickly your glucose settles, how stable it stays, how your body handles refeeding. The trend is the read.
Post-fast glucose response
What happens when you break the fast. A clean return is a sign of metabolic adaptation. A long, high spike is worth noticing.
Today's tolerance
HRV, sleep, glucose, and yesterday's strain combined into a plain-language read on whether today's a fasting day.
The medical caveat we will not bury.
Therapeutic ketosis — the kind used as an adjunct in epilepsy management, certain cancer protocols, metabolic disorders, and some neurological conditions — is medicine. It's run with clinicians. It uses blood ketone meters, not estimates. It's titrated against symptoms and labs.
Body Insights is not a medical device, not a diagnostic tool, and not a substitute for your care team. If you're fasting under clinical supervision, use the tools your clinician recommends and bring our trends in as context, not authority.
If you have diabetes, eating-disorder history, are pregnant or breastfeeding, take medications that interact with fasting, or are managing a serious chronic illness, talk to a clinician before extending fasts.
Fasting Insights vs Zero, Fastic, the others.
Most fasting apps are timers with streaks. The point of this one is different.
| Zero / Fastic | Keto-Mojo + paper | Body Insights | |
|---|---|---|---|
| Fasting timer | Yes | No | Yes |
| GKI from blood strips | No | Yes (gold standard) | Manual entry supported |
| GKI estimated from CGM | No | No | Yes |
| Today's tolerance read | No | No | Yes |
| Metabolic flexibility trend | No | Manual | Yes |
| Post-fast glucose response | No | No | Yes |
| Subscription | $5–10 / month | Strips ~$1 each | Free for core |
If you're managing therapeutic ketosis, Keto-Mojo with paper logs is still the right call. If you're fasting for metabolic health and want a read on what your body is actually doing — without buying strips for every fast — that's where this fits.
Questions you're probably asking.
What is the glucose ketone index?
The ratio of blood glucose to blood ketones, both in mmol/L. Developed by Dr. Thomas Seyfried as a single number describing depth of therapeutic ketosis. Lower numbers mean deeper ketosis.
Can Body Insights actually measure my GKI?
No. We estimate it from your CGM glucose and fasting duration. A true GKI needs blood ketones from a meter. We give you a directional read — not a clinical measurement.
How accurate is the estimate?
Directional, not diagnostic. Useful for trend-reading and casual fasting. For therapeutic protocols, epilepsy management, or oncology adjunct work — measure with strips.
Is therapeutic ketosis safe?
It's a medical intervention used as an adjunct in epilepsy, certain cancers, and metabolic conditions. It requires clinical supervision. We are not your care team — talk to one before pursuing it.
Do I need a CGM?
For the GKI estimation layer, yes — Stelo, Dexcom, or Libre. Without one, you still get today's tolerance read from HRV and sleep, but the ketosis estimate drops out.
How much does it cost?
Free for the core fasting timer and tolerance read. GKI estimation and metabolic flexibility tracking are premium. No card needed to start.
Your fasting data stays yours.
On-device. No account. No subscription required. No data sold. Ever.
- Reads from Apple Health
- Stays on your iPhone
- You choose what to share
Where you are in ketosis, in plain language.
Free for core fasting features. Premium adds GKI estimation and metabolic flexibility tracking.
