Day four of a fast is where something shifts. The brutal hunger from days one to three starts to ease. Your energy dips slightly, but it’s manageable. And deep inside, your body is switching fuel sources — from glucose to fat, from quick burns to steady reserves.
I know this because I’m tracking it. Glucose down from 93.6 to 55.8. Ketones up from 0.5 to 4.0. GKI collapsed from 10.4 to 0.8. Those aren’t just numbers. They’re your body telling you it’s changed gears.
But here’s what no one warns you about: the cold. That bone-deep chill that sticks with you all day. The hunger pangs that still come in bursts, even when you thought you’d passed them. The sleep that won’t stick around.
Day four isn’t easy. But it’s the day you realise you’ve turned a corner.
Day 4: When the Gears Start to Change
Energy dipped today. Not drastically, but I could feel it. The kind of tiredness that sits just behind your eyes and makes you move a bit slower.
Hunger was more manageable though. Still present, but nowhere near as sharp or painful as those first two days. Instead of constant gnawing, it came in short bursts. Maybe two or three across the day. Each one passed within minutes.
Sleep was rough again. Just three and a half hours overnight, then a nap at lunchtime to offset the deficit. That deep cold sensation stuck with me from head to toe. No amount of layers helped.
But mood? Steady. Chirpy, even. I went out with some friends who were quite stressed, and it didn’t affect me. Normally, I absorb some of that emotion. Not today. My emotional state felt level in a way it doesn’t always in the first few days of a fast.
The Numbers: Metabolic Shift in Action
Here’s where it gets interesting. I’m now at 13 stone 8 pounds — that’s 9 pounds down in four days. I know most of that is water, but the shift is clear.
More important are the biomarkers:
- Glucose: 93.6 (Day 1) → 55.8 (Day 4)
- Ketones: 0.5 (Day 1) → 4.0 (Day 4)
- GKI: 10.4 (Day 1) → 0.8 (Day 4)
That GKI drop is a huge metabolic change. It means my body has fully shifted from burning glucose to burning fat for fuel.
To support this, I got in an early morning walk. Didn’t sleep, so might as well. Did 50 ruck squats carrying the amount of weight I’ve lost — so I was squatting back at my original 18 stone from when I started this journey. That felt good.
I clocked some sunlight, hydrated with water and black coffee, had a gentle afternoon walk, and took that nap to offset the lack of sleep. No major temptation today. Just a steady rhythm.
Why You Feel Cold During a Fast
That deep cold feeling? It’s actually your body redirecting blood flow and energy while insulin drops and fat metabolism ramps up.
Think of it like this: your body is switching from petrol to diesel. The engine still runs, but the fuel system is adjusting. Blood flow priorities change. Energy distribution shifts. It’s uncomfortable, but meaningful.
As ketones rise, many people find the chill eases off as the body adapts. I haven’t so far. But it’s a sign that your system is shifting gears.
This isn’t failure. It’s your body doing exactly what it’s supposed to do.
What This Means for You
If I could say something to my future self or to anyone else watching: the first three days are definitely the hardest. But after that, it does ease up for a stretch — certainly until around day seven to ten.
Day four feels like I’ve turned a bit of a corner. Hunger is less dominant. Mood is balanced. The numbers are showing real change.
The challenge now is to keep supporting my body with rest, light movement, and patience.
Remember, this journey isn’t just about weight. It’s about real health — physical, emotional, metabolic, all of it.
🎯 Key Takeaways
Mood stabilises — Emotional resilience often improves as ketones rise
Days 1-3 are the hardest — Day 4 is where hunger starts to ease and metabolic shift accelerates
GKI 0.8 signals deep ketosis — Your body has fully switched from glucose to fat burning
Cold sensation is normal — Blood flow redirects as insulin drops and fat metabolism ramps up
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🔽 Read the Video Transcript
0:02Hey everyone, welcome to day four of my
0:04fasting journey. Physically today,
0:07energy dipped a little, not drastically,
0:09but I could feel it. Hunger was
0:12definitely more manageable, though.
0:13Still present, but nowhere near as sharp
0:15or painful as those first two days.
0:18Sleep was rough again, just three and a
0:20half hours overnight. Then I had a nap
0:22at lunchtime.
0:23And that deep cold sensation stuck with
0:26me for most of the day from head to toe.
0:29Moodwise though, I felt chirpy and
0:31steady.
0:33Even when I went out with some friends
0:35who were quite stressed, it didn’t
0:37affect me, which is unusual
0:39because I’m known to absorb some of that
0:42emotion from time to time.
0:44It feels like my emotional state has
Biomarkers: glucose 55.8, ketones 4, GKI 0.8
0:47leveled out in a way that doesn’t always
0:49happen in the first few days of a fast,
0:52but because I was in such an upbeat
0:54place, it was fine. Hunger pangs came in
0:57short bursts, maybe two or three across
0:59the day and were much less much more
1:02less intense than before. To support
1:05myself today, I got in an early morning
1:07walk. Didn’t sleep, so might as well.
1:10Did 50 rook squats carrying the amount
1:12of weight that I’ve lost. So, I was
1:14squatting back at my original 18 stone
1:16from when I started this journey. That
1:18felt good. I cked some sunlight,
1:20hydrated with some water and black
1:22coffee, had a gentle afternoon walk, and
1:25took that nap to offset the lack of
1:27sleep. No major temptation today, just a
Movement: rucked squats at 18 stone + walks
1:31steady rhythm on
1:33on the numbers side. I’m now at 138,
1:36which is 9 pounds down in four days. I
1:38know that most of that is water, but the
1:40shift is clear. Glucose has dropped from
1:4293.6 in day one to 55.8 today. ketones
1:46from 0.5 to four and GKI has fallen from
1:5010.4 to 0.8 which is a huge metabolic
1:53change.
1:55What I’ve noticed is that a night walk
1:57has really supported me and the squats
1:59are helping too. And if I could say
2:01something to my future self or to anyone
2:03else watching
2:05every time those first three days are
2:08definitely the hardest but after that it
2:11does ease up for a stretch certainly
2:13until around day seven to 10.
Why the deep cold happens (metabolism explained)
2:17educational moment. That deep cold
2:19feeling is actually your body
2:21redirecting blood flow and energy while
2:24insulin is dropping and fat metabolism
2:28ramps up. It’s a sign that your system
2:30is shifting gears. Uncomfortable but
2:33meaningful. As ketones rise, many f
2:36people find the chill eases off as the
2:38body adapts. I haven’t so far. So, day
2:42four feels like I’ve turned a bit of a
2:43corner. Hunger is definitely less
2:45dominant. Mood is definitely balanced
2:47and the numbers are showing real change.
Days 1–3 are hardest — then it eases
2:50The challenge now is to keep supporting
2:52body with rest, light movement, and
2:54patience.
2:56Remember, this journey isn’t just about
2:58weight. It’s about real health,
3:00physical, emotional, metabolic, all of
3:04it. If you like this video, please hit
3:06like and subscribe. It really helps to
3:08get the message out to more people. I’ll
3:10see you tomorrow on the last day of this
3:12fast, day five.
⚠️ Important: This is educational content based on evidence and personal experience. It is not medical advice. Speak to your GP before making changes to your fasting pattern, diet, or medication.
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