Scree

Part 2: Getting Into Fasting

Published 2 Hours Ago · All posts on Mar 4

I’ve been doing 16:8 intermittent fasting for years and recently started 48-hour fasts — dropping about three pounds each fast, gaining one or two back, and trending steadily downward. I wanted to understand what the research actually says about what I’m doing to myself, so I worked with Claude (Anthropic’s AI) to produce this series. I set the structure, chose the topics, pushed back on claims that felt hand-wavy, and guided the editorial tone. Claude did the writing and research synthesis. My curiosity driving Claude’s research and prose.

Routines, Scenarios, and What to Expect

Research brief — the practical onramp. 16:8 IF as a starting point, Craig’s specific eating window, extended fasting scenarios anchored to a weekly rhythm, and what keto flu actually is.

Start With 16:8

You may want to start by getting into 16:8 intermittent fasting — an 8-hour eating window and 16-hour fast — before attempting anything longer.

Is this actually supported? Honestly, no one has studied whether practicing 16:8 first makes longer fasts easier. It’s conventional wisdom without a clinical trial behind it. But the physiological rationale is sound: regular time-restricted eating develops metabolic flexibility — the ability to switch between glucose and fat/ketone oxidation. Someone who does this daily would be expected to enter ketosis faster and with less discomfort during an extended fast. And the practical experience of managing hunger, learning your body’s signals, and knowing what electrolyte depletion feels like are real benefits of prior fasting experience, even if unstudied. (1)

What the research says about 16:8 on its own:

Satchin Panda’s group at the Salk Institute pioneered time-restricted eating (TRE) research, starting with a 2012 mouse study that showed mice on a high-fat diet restricted to an 8-hour eating window didn’t gain weight compared to ad libitum fed mice eating the same food. (2) Human trials with 6–10 hour eating windows have generally shown weight loss, improved glucose regulation, reduced hunger, and improved energy.

A 2020 study by Wilkinson et al. put 19 participants with metabolic syndrome on a 10-hour eating window for 12 weeks and found reductions in weight, blood pressure, and atherogenic lipids. Single-arm trial (no control group), but notable results. (3)

The Eating Window: Skip Breakfast

The simplest way to do 16:8 is to skip breakfast. Eat your first meal around 11:30 AM, finish eating by 7:30 PM. That’s it.

Example daily rhythm:

  • 11:30 AM — First meal (“break fast”)
  • 2:00–3:00 PM — Protein-heavy snack if hungry (0% fat yogurt, protein shake with fruit)
  • 5:30–6:00 PM — Dinner
  • Nothing after 7:30 PM

This gives you a clean 16-hour overnight fast every day. No special foods, no supplements, no elaborate protocols. Just… don’t eat in the morning.

But isn’t an earlier window better? Courtney Peterson’s group published the first controlled feeding trial of early time-restricted feeding (eTRF) in 2018. Men with prediabetes ate within a 6-hour window ending at 3 PM vs. a 12-hour window. eTRF improved insulin sensitivity, blood pressure, oxidative stress, and appetite — even without weight loss. (4)

Peterson’s data suggests an earlier window (say, 8 AM–4 PM) may have slightly better metabolic outcomes due to circadian alignment — insulin sensitivity and glucose tolerance are naturally higher in the morning. But the practical reality is that most people can’t eat their last meal at 3 PM. Social eating, family dinners, real life — the later window is far more sustainable. Peterson’s data shows early TRE is better, not that late TRE is bad. An 11:30–7:30 window still captures the core benefits of a compressed eating window and extended overnight fast.

Extended Fasting Scenarios

Once 16:8 feels routine, you can extend into longer fasts. All scenarios below are anchored to a Sunday ~5:30 PM family meal as the break-fast, working backward:

  • ~22 hrs — Saturday dinner (7:30 PM) Extended OMAD. Easy entry point beyond 16:8.
  • ~30 hrs — Saturday lunch (11:30 AM) Skip one full day of eating. You sleep through the hardest part.
  • ~46 hrs — Friday dinner (7:30 PM) Nearly two full days. Well into ketosis and autophagy is active. Two sleeps make this more manageable than it sounds.
  • ~54 hrs — Friday lunch (11:30 AM) Deep into the autophagy/stem cell priming window. Friday afternoon might be the grittiest stretch — you’re still burning through glucose stores and haven’t hit the ketone clarity yet.
  • ~70 hrs — Thursday dinner (7:30 PM) Three full days. Firmly in Longo’s regenerative territory. Most people report hunger actually diminishing after day 2.
  • ~78 hrs — Thursday lunch (11:30 AM) The “long weekend fast.” Maximizes time in the 48–72+ hour zone where stem cell and deep autophagy research is strongest.

I’ve recently been fasting in the 46-hour to 54-hour options to hit a sweet spot — you get the major benefits (see Parts 3–6), Sunday dinner is a natural social re-entry, and you’re only navigating one or two workdays while fasting. The jump from 54 to 70+ is where most people feel they’re making a real lifestyle commitment for the week rather than a weekend experiment.

Keto Flu: What It Is and Why It Happens

Sometime in the 24–72 hour window, you’ll likely hit the “keto flu” — headache, fatigue, brain fog, irritability, maybe muscle cramps. It actually feels a bit like you’re coming down with the flu. It’s not illness. It’s electrolytes. When I stop eating around Noon on Friday, this happens (if I don’t address the underlying cause beforehand) in the middle of the night Saturday.

Here’s the mechanism:

  1. Insulin drops → your kidneys stop retaining sodium and excrete more water. (Insulin normally signals the kidneys to reabsorb sodium.)
  2. SGLT2 effect: The sodium-glucose transport protein 2 normally reabsorbs glucose and sodium together in a 1:1 ratio. With blood glucose low, SGLT2 activity drops, and more sodium goes out in your urine.
  3. Sodium loss drags potassium and magnesium along with it through coupled renal transport.
  4. The resulting electrolyte imbalance — particularly sodium depletion — drives the entire symptom cluster.

This is well-established physiology, not speculative. (5)

The mental fog typically lifts as the brain adapts to using ketones for fuel. Many people describe a shift to unusual mental clarity once adaptation kicks in — probably related to the brain’s efficient use of beta-hydroxybutyrate (BHB) as fuel.

For me, Friday afternoon until bed is the grittiest stretch on a 46–54 hour fast; By Saturday midday I feel exceptionally sharp.

Sources

  1. de Cabo & Mattson 2019, NEJM — metabolic switching and fasting overview: https://pubmed.ncbi.nlm.nih.gov/31881139/
  2. Panda lab TRE review 2021 — time-restricted eating research overview: https://pubmed.ncbi.nlm.nih.gov/34550357/
  3. Wilkinson et al. 2020, Cell Metabolism — 10-hour TRE in metabolic syndrome: https://pubmed.ncbi.nlm.nih.gov/31813824/ (free full text: https://pmc.ncbi.nlm.nih.gov/articles/PMC6953486/)
  4. Sutton et al. 2018, Cell Metabolism — early TRE improves insulin sensitivity without weight loss: https://pubmed.ncbi.nlm.nih.gov/29754952/ (full text: https://www.cell.com/cell-metabolism/fulltext/S1550-4131(18)30253-5)
  5. Electrolyte mechanism — insulin-sodium-kidney pathway is standard renal physiology; see also general overview at https://science.drinklmnt.com/low-carb/keto-electrolytes/ for accessible explanation of the SGLT2 and insulin mechanisms.