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The short answer: It doesn't really matter. If you have extra weight or blood sugar problems, just pick something that works with your life, and try it. If you don't have extra weight, you're a woman under 30, or you have a fast metabolism, approach with caution.
The long answer: We don't really know at this point what is the "best" system (if there is one), nor can we measure many of the features we're curious about. We have small studies in humans and many studies in all sorts of animals. But those animals have a different metabolic rate than humans - fasting a mouse for a day is like fasting a human for over a week - so you can't transfer day-by-day results, and it's quite likely that given all the variation in humans, a 3-day fast for one person may jumpstart as much cellular housecleaning as a week-long fast for another.
Time-restricted eating, that is, eating in a smaller window than all-day grazing, such as 6 or 8 hours, has a variety of demonstrated benefits for people who need it, and possible pitfalls for people who don't. It seems like common sense not to fast a child or teen of normal weight, a pregnant woman, or a nursing mother. On the other hand, someone with an extra 20 pounds and an average blood sugar creeping upward could really benefit from the improvements in blood sugar that can come with a smaller eating window.
What happens when you eat in a smaller window of time? For one, you'll likely eat less. And if you eat a bit too much, or more sugar than you need, and it is stored in the liver and muscles as glycogen, you might just be able to use it up before you start eating again. And then when you eat again there's somewhere for that sugar to go - back into the liver and muscles as glycogen - instead of into your circulation. It also gives your body a break from producing insulin, and cells a break from shuttling in or blocking the sugar.
Longer fasts can dig deeper. After your glycogen is depleted in a day or two, your metabolism shifts to using fat as a fuel source, and breaking down old half-functional cells for the protein bits you need. This is the main idea behind fasting for cancer prevention and to optimize cancer treatment, and fasting for longevity. Not only do cancer cells generally not function well without sugar, there are many stages of imperfection before a cell ever becomes cancer. With regular removal of imperfect cells, broken down when needed for raw materials, there should be fewer cells that could take the path to cancer. This "cellular clean out" is thought to be part of the reason that fasted animals live longer, sometimes much longer.
If you're a woman over 30, not pregnant or nursing, time-restricted eating can be a tool to balance blood sugar and re-sensitize your cells to insulin. And fasting for one or more days can be a tool to dig deeper and perhaps clean out cellular debris. Just be aware that young women of normal weight may tolerate fasting and time restriction less than men, and need to use a gentler set of guidelines in how they approach it.
Even the traditional religious fasting (2 small snacks + 1 real meal) was difficult for me in my 20s. My metabolism was too fast, or my ability to use fat as fuel was poor; I would reliably run a fever by the end of the day, before I sat down to my "full meal" dinner. I thought this happened to everyone since it was it was such a consistent response to me, and a friend said no, she never felt feverish when fasting, and I should probably be less restrictive. Her common-sense assessment was a revelation and a relief.
In my 30s this started to change. I didn't always wake up ravenous and need to snack between meals. Sometimes I ate very little for dinner. Sometimes, to my surprise, I could WAIT an hour or two after waking up to eat breakfast. This makes me feel like a real adult, and at this point I consider the metabolic slowdown well worth the trade off of being able to go longer without eating.
My ideal version of time restricted eating would be to eat between 7 or 8 am and 6 pm, because it's easy for me to eat less in the evening. This would assume a bedtime of 8:30, which of course doesn't happen most nights. And I'm nursing a toddler who takes nutrients at night, and doesn't it seem like that would count for a couple extra hours of not eating? Someone should research what happens to mom's blood sugar with overnight nursing, compared to matched women not nursing.
There are various modified systems of fasting which put calories under 1000 for the day, and I think this is the easiest form to practice as a mom. Who hasn't had a day when they barely eat? Now you can think of it as an asset instead of a liability. Fasting on alternate days over a period of time has demonstrated benefits, and a low-calorie fast has many benefits as well. A day of moderate amounts of fruit, vegetables, and soup (for the salt) would accomplish this without buying any special program or tracking what you eat. The family might not even notice.
Whether you find it simpler to eat nothing or something, and however you time it, there are benefits to the reset that happens when we take a break from eating our usual amounts. Consider whether the occasional fasting day, vegetable day, or eating within a smaller time frame might improve your health, and start small with your experiments.
