So as a taster, you can do multiple 24-hour fasts and get the same experience from a health point of view (everything else is different, though).
That said, at some point, I’m interested in trying a multiple-day fast myself but haven’t taken the plunge yet.
NO, that's incorrect. The length matters a lot. In fact, a major change occurs after the 2-3 day mark, when body glycogen stores are depleted and your body shifts to ketosis etc. Another major marker has to do with when a much coveted state called autophagy is activated.
Ketosis is literally the start of starvation, so I'm dubious of widespread health benefits. I know it is extremely beneficial to people with a history of seizures, but that's it.
I've been considering trying ketosis but all I've heard are good things, what are the downsides?
https://en.wikipedia.org/wiki/Ketosis#Safety
Oh and this gem from the article on starvation.
> After two or three days of fasting, the liver begins to synthesize ketone bodies from precursors obtained from fatty acid breakdown. The brain uses these ketone bodies as fuel, thus cutting its requirement for glucose. After fasting for three days, the brain gets 30% of its energy from ketone bodies. After four days, this may increase to 70% or more.[11] Thus, the production of ketone bodies cuts the brain's glucose requirement from 80 g per day to 30 g per day, about 35% of normal, with 65% derived from ketone bodies. But of the brain's remaining 30 g requirement, 20 g per day can be produced by the liver from glycerol (itself a product of fat breakdown). This still leaves a deficit of about 10 g of glucose per day that must be supplied from another source; this other source will be the body's own proteins.
This is the biggest reason I've never done more than toy with ketosis is the body consuming muscle for protein to supply that remaining 10g of glucose per day. Esp. because 1 lb of muscle only 1,800 Cal vs 3,500 Cal / lb. Fat.
https://en.wikipedia.org/wiki/Starvation
Phase one: ... After that, the body begins to break down fat and protein. Fatty acids are used by the body as an energy source for muscles, but lower the amount of glucose that gets to the brain. Another chemical that comes from fatty acids is glycerol. It can be used like glucose for energy, but eventually runs out.
Phase two: Phase two can last for up to weeks at a time. In this phase, the body mainly uses stored fat for energy. The breakdown occurs in the liver and turns fat into ketones. After fasting has gone on for one week, the brain will use these ketones and any leftover glucose. Using ketones lowers the need for glucose, and the body slows the breakdown of proteins.
Your point seems be an objection to the use of the word starvation because it doesn't progress to the later stages due to maintenance of available fatty acids. And that it can be useful and therefore the word starvation is inappropriate. Does this sum it up?
Yes, I've noticed this as well. Personally I fast for 17 hours every day and I've noticed I feel the benefits without negative consequences. Once in a while I'll go for a 3 day fast but aside from the personal challenge accomplishment I don't notice a major gain. My experience is all very non scientific. For the more rigorous science, I usually look to Peter Attia[1]