Insulin is not the only hormone responsible for storing fat,
Acylation Stimulating Protein (ASP) is produced by adipocytes and also stimulates fat storage.
There's nothing really special about IF for fat loss, muscle gain, or for reducing insulin resistance. Improved insulin sensitivity is a natural result of fat loss.
There was a 10% weight loss, and their HbA1 levels (average blood sugar) returned to pre-diabetic levels (they could essentially stop insulin injections).
The increase in HGH levels that happens during a fast is not a net anabolic event, but a compensatory anti-starvation response that shifts fuel substrate use from glucose and amino acids toward lipolysis and ketone production. Part of the cause of GH release is reduced hepatic IGF-1 production, resulting in less negative feedback to the pituitary gland to produce GH. So during a fast you end up with more GH, but less IGF-1. IGF-1 is more directly active[0] in skeletal muscle in producing growth than HGH. Basically, the result is that while fasting-induced GH secretion does help counteract muscle protein breakdown, it does little to increase muscle protein synthesis when you're fasted and you remain in negative whole-body protein-balance. There is nothing anabolic about not eating, as your body has no fuel for new protein synthesis.
It's also only a matter of time before you start losing muscle. Muscle protein breakdown rates starts in between 15-30 hrs of fasting[1]. Fasting for 20 hrs decreases resting energy expenditure[2] and mTOR signaling, an important component of anabolic gene expression. This suggests that 20+ hour fasts may negatively affect you muscle mass and fat loss.
GH also drives insulin resistance[3], so more growth hormone isn't always desirable.
All in all, given some limited studies we have on IF, IF per se does not provide body recomposition advantages and does not alter energy expenditure, and fasts over 20 hours are even harmful. Any health benefits of IF are highly inconsistent and mostly limited to glucose and insulin dynamics. Being lean is much more important for our health than whether you do IF or not. Being muscular probably is too, because muscle mass improves insulin sensitivity. Combined with being lean, any benefits for insulin sensitivity of IF are arguably trivial for muscular, exercising individuals. I could give you a bunch more studies here, but I think this is enough for now :).
Having said all that, you can still use calorie cycling strategies to reduce caloric intake on "rest days" (or more precisely, outside of your anabolic windows) by using a PSMF (protein-sparing modified fast), and upping your caloric intake during anabolic windows (which could last up to 72 hours after training!).
re Insulin: I did say insulin was a major component, I agree of course it's not the only component.
> There's nothing really special about IF for fat loss, muscle gain, or for reducing insulin resistance. Improved insulin sensitivity is a natural result of fat loss.
I think it's too early to say that, which is why we need more studies. To your own point, there aren't enough. In no small part because there are so many different protocols and studies both mix-and-match protocols and also often use questionable ones. Sometimes "fasting" means eating a few hundred calories or a fasting-mimicking diet. Sometimes it means 16:8. Sometimes it means alternate-day. Sometimes it means 5 days on and 2 days off.
> It's also only a matter of time before you start losing muscle. Muscle protein breakdown rates starts in between 15-30 hrs of fasting[1].
For what it's worth, proteolysis does not imply the loss of muscle mass after re-feeding in short-term fasting. The working thesis is that while fasting your body attempts to spare as much muscle as possible (in part through HGH) and that protein breakdown slows dramatically [4].
After all, HGH increases fat oxidation and increases protein retention, so it would all make sense.
I didn't, and don't think that, fasting will increase muscle mass, but I have seen no evidence that it will, as you suggest, break down muscle in any meaningful way until far, far later in the starvation cycle. Muscle in particular is a fuel of last resort. Wouldn't make much sense otherwise. There will always be some turnover, of course.
> Resting Metabolic Rate
Studies show that during short-term fasting your metabolic rate stays pretty consistent, and increases over the first couple of days. A couple of studies pins it at up to +10% after 36-48 hours. [1,2] It doesn't actually drop until 60-96 hours into a fast. [3]
tl;dr: I guess my point is, it's too early to be dismissive. More data is needed.
> This suggests that 20+ hour fasts may negatively affect you muscle mass and fat loss.
I will say this particular line is kind of nuts, if you think about it. How can not eating anything at all slow down your fat loss when you have a caloric deficit of 2500kcal/day? That's why you have fat. That's why your body enters ketosis over the first 3 days. Why would your body burn your muscles (i.e. means of obtaining more food) when it's been storing stockpiles of energy in the form of fat? This right here, is a common myth.
There's only two tissues in your body that require glucose. Red blood cells and the brain. After 3 days 70% of the energy required by your brain is provided by ketones. The remaining glucose requirements are satisfied via gluconeogenesis from glycerol backbones of triglycerides, and yes, to a small extent, via protein breakdown. The entire rest of your energy requirements are satisfied via lipolysis, to the tune of 2/3 of a pound of fat per day. Or the equivalent of running 17 miles.
There's nothing really special about IF for fat loss, muscle gain, or for reducing insulin resistance. Improved insulin sensitivity is a natural result of fat loss.
Here's a study on diabetics that are put on a 2200 kcal diet with 400grams of carbs: https://pubmed.ncbi.nlm.nih.gov/21483296/
There was a 10% weight loss, and their HbA1 levels (average blood sugar) returned to pre-diabetic levels (they could essentially stop insulin injections).
The increase in HGH levels that happens during a fast is not a net anabolic event, but a compensatory anti-starvation response that shifts fuel substrate use from glucose and amino acids toward lipolysis and ketone production. Part of the cause of GH release is reduced hepatic IGF-1 production, resulting in less negative feedback to the pituitary gland to produce GH. So during a fast you end up with more GH, but less IGF-1. IGF-1 is more directly active[0] in skeletal muscle in producing growth than HGH. Basically, the result is that while fasting-induced GH secretion does help counteract muscle protein breakdown, it does little to increase muscle protein synthesis when you're fasted and you remain in negative whole-body protein-balance. There is nothing anabolic about not eating, as your body has no fuel for new protein synthesis.
It's also only a matter of time before you start losing muscle. Muscle protein breakdown rates starts in between 15-30 hrs of fasting[1]. Fasting for 20 hrs decreases resting energy expenditure[2] and mTOR signaling, an important component of anabolic gene expression. This suggests that 20+ hour fasts may negatively affect you muscle mass and fat loss.
GH also drives insulin resistance[3], so more growth hormone isn't always desirable.
All in all, given some limited studies we have on IF, IF per se does not provide body recomposition advantages and does not alter energy expenditure, and fasts over 20 hours are even harmful. Any health benefits of IF are highly inconsistent and mostly limited to glucose and insulin dynamics. Being lean is much more important for our health than whether you do IF or not. Being muscular probably is too, because muscle mass improves insulin sensitivity. Combined with being lean, any benefits for insulin sensitivity of IF are arguably trivial for muscular, exercising individuals. I could give you a bunch more studies here, but I think this is enough for now :).
Having said all that, you can still use calorie cycling strategies to reduce caloric intake on "rest days" (or more precisely, outside of your anabolic windows) by using a PSMF (protein-sparing modified fast), and upping your caloric intake during anabolic windows (which could last up to 72 hours after training!).
[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2439518/
[1] https://pubmed.ncbi.nlm.nih.gov/6476111/
[2] https://academic.oup.com/ajcn/article/90/5/1244/4598111
[3] https://www.sciencedirect.com/science/article/abs/pii/S10966...