Fasting While on GLP-1: Does It Burn More Fat?

C

casprow

Explorer
Joined
Apr 12, 2026
Messages
34
Reaction score
0
Location
Germany
Once you quit eating, insulin levels fall, and that drop flips your body from running on sugar to running on fat.

So if someone fasts daily for roughly 4+ hours, does that translate into more fat loss? Or does it all come down to calories in the end?

Also, if I'm not wrong, stacking Retatrutide on top should amplify those effects too
 
I recall coming across information about the upsides of fasting, along with plenty of debated points—such as how well it works for men compared to women.

For myself, I picked it as a method to get where I wanted to be, and I followed 16/8 for the entire time I was losing weight. What drove me most was getting rid of visceral fat, since I had that classic older guy belly shape.

All the material I found in favor of it mentioned that 12 hours was a decent place to begin—12/12—and that the actual advantages grew beyond that point.

I paired it with eating fewer calories and setting a bare minimum for activity. (10k steps)

A handful of subreddits are worth browsing, and there is a ton of information on the internet.

What you find online could give you a fuller picture of both the good and bad sides.

Diabetes or pre-diabetes was not something I had, so it never worried me.
 
casprow said:

Once you quit eating, insulin levels fall, and that drop flips your body from running on sugar to running on fat.

So if someone fasts daily for roughly 4+ hours, does that translate into more fat loss? Or does it all come down to calories in the end?

Also, if I'm not wrong, stacking Retatrutide on top should amplify those effects too
Click to expand...
For over 10 years, I have used intermittent fasting as a way to keep reflux in check. The advantages, in my view, are completely worth it. The biggest downside? Having dinner with friends is off the table, which cuts down on social opportunities.
 
If you're able to stick with it, fasting works great. That said, the only thing that drives weight loss is a calorie deficit. Still, fasting's effects could make shedding pounds feel less difficult.
 
On Tirzepatide, OMAD just happens for me without effort, as long as the level in my system remains high enough. To smooth out that level, I divide my doses. I'm aware my protein intake has been too low, yet dropping pounds was my main priority. After I hit my target weight, I intend to keep daytime eating modest, with an emphasis on protein and overall nutrition.
 
Smiter said:

If you're able to stick with it, fasting works great. That said, the only thing that drives weight loss is a calorie deficit. Still, fasting's effects could make shedding pounds feel less difficult.
Click to expand...

Oh for crying out loud. How many physicians have shown this doesn't hold up? 1,000? It's baffling that people keep pushing that CICO rubbish.
 
ColdSmoke said:

Smiter said:

If you're able to stick with it, fasting works great. That said, the only thing that drives weight loss is a calorie deficit. Still, fasting's effects could make shedding pounds feel less difficult.
Click to expand...

Oh for crying out loud. How many physicians have shown this doesn't hold up? 1,000? It's baffling that people keep pushing that CICO rubbish.
Click to expand...
To put it the way a fellow member did, the laws of thermodynamics still win every time.
 
People taking GLP-1s already have messed-up hormone and metabolic regulation. Sure, a calorie deficit is probably necessary to drop weight, but that's far from the whole picture.

When you do IF while on GLP-1s, the real danger is failing to hit your protein needs inside that narrow window for eating. To get your macros you'd need larger meals — tough when appetite is gone and your stomach is touchy. On top of that, IF's whole point is lowering glucose so fat gets burned, and GLP-1s already handle that by themselves. So the upside looks pretty thin to me. I recently heard a podcast covering this — I'll try to find it.
 
Tirzepatide has me doing intermittent fasting most days, though it's not something I plan. Breakfast and lunch happen, then come evening I rarely feel like eating. So I skip dinner.
 
ColdSmoke said:

People taking GLP-1s already have messed-up hormone and metabolic regulation. Sure, a calorie deficit is probably necessary to drop weight, but that's far from the whole picture.

When you do IF while on GLP-1s, the real danger is failing to hit your protein needs inside that narrow window for eating. To get your macros you'd need larger meals — tough when appetite is gone and your stomach is touchy. On top of that, IF's whole point is lowering glucose so fat gets burned, and GLP-1s already handle that by themselves. So the upside looks pretty thin to me. I recently heard a podcast covering this — I'll try to find it.
Click to expand...
The reason I take GLP's is that I overeat. Thanks to GLP's, I've cut down on how much I eat. So fewer calories in = fat loss?

🤔
 
ColdSmoke said:

People taking GLP-1s already have messed-up hormone and metabolic regulation. Sure, a calorie deficit is probably necessary to drop weight, but that's far from the whole picture.

When you do IF while on GLP-1s, the real danger is failing to hit your protein needs inside that narrow window for eating. To get your macros you'd need larger meals — tough when appetite is gone and your stomach is touchy. On top of that, IF's whole point is lowering glucose so fat gets burned, and GLP-1s already handle that by themselves. So the upside looks pretty thin to me. I recently heard a podcast covering this — I'll try to find it.
Click to expand...
I'm with you 100%. When you're on GL-1s and actually shedding pounds, going without food is nearly unmanageable—particularly if you're aiming for extended periods without eating.

ColdSmoke said:

People taking GLP-1s already have messed-up hormone and metabolic regulation. Sure, a calorie deficit is probably necessary to drop weight, but that's far from the whole picture.

When you do IF while on GLP-1s, the real danger is failing to hit your protein needs inside that narrow window for eating. To get your macros you'd need larger meals — tough when appetite is gone and your stomach is touchy. On top of that, IF's whole point is lowering glucose so fat gets burned, and GLP-1s already handle that by themselves. So the upside looks pretty thin to me. I recently heard a podcast covering this — I'll try to find it.
Click to expand...
Wait, what? That angle never crossed my mind. Sure, intermittent fasting could lower insulin release, but in my head the autophagy that comes with it was always the whole point of IF.
 
My understanding has always been that fat gets burned after blood sugar is depleted and insulin stops being chaotic. As for autophagy, it only starts around the 20-24 hour mark, possibly later.
 
I've been doing IF for close to 3 years, and after I began Reta it became way easier to stick with. I have 2 meals a day. The first is at about noon, the second at about 7pm. Every week I also do a 24h fast. I picked Monday because it's my busiest day and I skip workouts then. The one real drawback with Reta is getting enough protein from just 2 meals. Whey protein powder makes that problem simple to handle, though. Either way, this way of eating has become normal for me. I only make exceptions during Holidays or events such as Xmas...and then everything falls apart 😁
 
Taking a different angle from what’s already been shared, my own view is that IF may carry real risks while using these meds—particularly for someone who doesn’t have any insulin resistance to begin with.

For me, baseline BGLs sit in the 6-8 mmol/L range, but after going 6+ hours without eating on Reta 4mg, they’ve been falling below 3.5mmol fairly regularly.

You can read what happened in my case here

Post in thread 'Blood sugar drop on Reta?'

/community-link/thread/10750/
 
I followed this pattern for basically my whole time losing weight on Reta. My final calories came in before 0000, and then I ate nothing again until roughly 1500 after waking up (I work nights). On most days, my 8-9 hour "feed" window looked like this: protein shake > small meal > small meal > protein shake.

Whether it made any difference, I can't say. Nothing stood out to me as either a benefit or a drawback.
 
Smiter said:

If you're able to stick with it, fasting works great. That said, the only thing that drives weight loss is a calorie deficit. Still, fasting's effects could make shedding pounds feel less difficult.
Click to expand...
Isn’t a calorie deficit the outcome of fasting?
 
onwuz1234 said:

Smiter said:

If you're able to stick with it, fasting works great. That said, the only thing that drives weight loss is a calorie deficit. Still, fasting's effects could make shedding pounds feel less difficult.
Click to expand...
Isn’t a calorie deficit the outcome of fasting?
Click to expand...
On its own, no...Should people consume too much within their eating window...they'd end up in a surplus
 
Intermittent fasting does appear to carry some health advantages — it can improve metabolic issues like insulin resistance and liver fat — but when someone is already eating a low calorie diet while using GLP's, I'm skeptical that it shifts how much weight comes off or how quickly, provided total calories stay equal.

CICO holds up completely, as long as intake and output are genuinely tracked with precision; thermodynamics won't bend. In practice, though, accurate intake measurement basically never happens, calorie estimates are famously unreliable, and output can't really be measured directly — only inferred by working backward from weight lost, using 7700kcal/kg of fat.

People often think CICO fails because the simple version ignores how wildly energy expenditure varies between individuals, and it typically doesn't properly factor in the decline in energy expenditure that accompanies weight loss, particularly metabolic adaptation to prolonged low calorie intake. The usual age / weight / calories per day figures simply aren't precise enough to be very useful. In my own case, energy expenditure fell 50% across 10 months of weight loss, going from 145kg down to 75 kg — 50% is a bigger drop than standard formulas predict. That gap was the difference between losing 6kg per month and 0 kg per month on identical calorie intake.
 
Back
Top