albedo No problem, I've gotten to be a small expert in the subject from doing so many that it's better to distill it anyhow. What I'll talk about comes from the latest from Longo, and from doing fasts at the True North Health center in California which has been doing this for decades. Disclaimer: I am not a medical doctor, consult yours or the good folks at True North before doing a fast.
First fasting has become popular recently and there's all sorts of boutique approaches, many of them which aren't true fasts (such as 'juice fasts'). Typical kind of thing, something gets popular then the crowd makes it complicated and gets it mostly wrong.
The ideal fast is water only (no medications (definitely do a medically supervised fast if you need medications), no teas, nothing except water) for greater than 3 days - 4 is typical for buffer. Two days before the fast you ideally lower your carbohydrate intake and eat only fruits and vegetables - the reason for this is to reduce the glycogen in your body so as to enter ketosis sooner (it's the opposite of 'carbo loading' that athletes do).
- If you are 'ideal' - you eat vegan or maybe pescatarian, no dairy, no meat otherwise, and are an athlete then twice a year for 4 days is all you need for a 'tune up'.
- If you have no health issues but aren't 'ideal', then do it quarterly (4 times/year)
- If you have health issues, then do it monthly until you correct the problem
- During the fast drink LOTS OF WATER. Keep track, and ideally it's steam distilled water (you are very sensitive to taste - tap is usually awful when you fast). Bathe your tissues in pure steam distilled water you get from the health store or your own distiller.
- Come out of the fast slowly, eating only plant foods for at least a couple of weeks, and ideally forever.
If you have health issues you should do a medically supervised fast at True North. If you are ideal or otherwise very healthy, then doing it yourself at home is usually fine (see disclaimer). For reference, I am vegan, fit the definition of a tri-athlete (exercise every day for about an hour), take no medications and my numbers are all ideal - so I do a twice a year tune up. I've done fasts up to a week, but now a days find 4-5 days good enough as that's what the research shows. Otherwise I overnight fast for > 12 hours. During the in between time I eat within my diet (all whole food non animal foods), and I pretty much eat as much as I want. The research shows it doesn't really matter, as long as it's not bad foods like milk and meat. The effects of a fast on mTOR, IGF1 etc all last for about three months after a fast, so that's why Long recommends the above schedule for people of varying health.
The boutique fasts (one day/week, two day/week) kind of things have benefits, but you don't get the immune senescence therapy from those, plus the other benefits. At around the 3 day mark your body realizes it's getting serious (your body doesn't know you'll start eating again in a few days) and it starts ripping through your system, killing off and consuming about half of your T-cells, it starts going after senescent cells, and obviously your fat stores, all to feed the ketosis engine which is bathing every single cell in your body with ketones instead of glucose. From this, it shows that the re-feeding stage is critical, you want to slowly come out of it, eating only fruits and vegetables for the following week (and ideally forever - including nuts, seeds, etc later). This is because it's on re-feeding that your body creates the stem cells to replenish your immune system with new, baby (unprogrammed) cells. The concept here is 'tear down - build up'. You want to 'clean house', tear down the junk that has accumulated in your body, then rebuild with new, fresh cells in the vegan re-feeding. Only during a water only fast for > 3 days does your body do this. And your body is so desperate for calories, and it wants to 'protein spare', meaning save your muscle tissues, that it goes after any cellular junk you have as FOOD. This is not starvation, this is spring cleaning.
I believe this is the best approach to immune senescence therapy and much more. It's natural - our systems are designed to handle it, it's free, it's easy to do at home and there's no lingering questions as to how effective it is, since it affects every cell in the body. Drugs all have side effects and some degree of efficacy. If you consider, it's actually the root cause of most of the developed worlds health problems. The problem is too many calories all the time - too much energy, and we are designed for alternating energy. We need a 'feast and famine' cycle - we don't want chronic calories restriction as some do. That reduces our hormones, energy level and has other issues. We are truly designed for alternating feat and famine (as long as the feasting is healthy feasting). Our bodies only repair themselves in the fasting state. So it's better, much better, and easier than chronic calorie restriction, with the data to back that up. So what's the catch?
It's not pleasant. Being in ketosis doesn't feel good, though if you go on long enough people usually experience euphoria. For three days it feels like a weird flu. Also there are all sorts of mental things that go on. It induces your brain to repair too and you start generating new brain cells (presumably your body is trying to make you smarter so you can go find a way to get some food). No wonder that religious people use fasting - it's a different experience. I come out of a fast with new insights and understanding of whatever problems are bothering me. But, along with that, your body is saying "you're killing me!" (which isn't true, but your body wants to get back to an energy rich lifestyle) so all sorts of mental funnies occur. I 'enjoy' a fast in a way, it gives me a chance to not worry about food and just focus on myself. I bundle up, sit at my piano or computer, and think and do the things I like to do. But many people just won't do it, so for them a pill is the next best thing.
tl/dr - long post, but I hope that was helpful, and I have the short formula to follow up above for ideal results. Basically in my experience fasting is the most important modality you can take to longevity, next to what you put in your body, as it's the only time you give yourself a chance to heal and regenerate.