There's a switch inside every cell in your body. Flip it on and you're in growth mode. Your cells start dividing but in the process, they make a lot of junk like mis-folded proteins, which help create the conditions for our biggest diseases (including cardiovascular, Alzheimer's and the big C). Flip the switch off, though, and your cells literally take out the trash leaving them clean, renewed, effectively young.

We know how to flip the switch. The trick is figuring out when. Because leaving your body in cleanup mode for too long can also be extremely bad for your health, in the much shorter term. Doing so has been the cause of anxiety, misery and disorder, for decades. It's also known as starvation.

The delicate dance of food consumption is at the heart of The Switch, a new book about new body-energy science and how it can help us live longer. Author and research scientist James Clement studies people who reach the age of 110; Harvard's David Sinclair, who recently wrote a groundbreaking book on the end of aging, is his mentor. As Clement's book hit shelves, an unrelated study in Nature confirmed its premise: mTOR (your genetic "on" switch) cannot coexist with autophagy (trash removal), and that is "implicated in metabolic disorders, neuro-degeneration, cancer and aging," the study said.

In other words: We age faster, get sicker and harm our brains when we fill the hours we're awake with food, day in and day out. Organic beings need more of a break than just a good night's rest in order to properly take out the trash. We're the opposite of automobiles. We break down eventually unless we run out of fuel. (Glycogen, which is what the body converts food into, is our gas.)

These revelations shed a new spotlight on fasting, the main way to induce autophagy (you can also kickstart it with intense exercise on a mostly empty stomach). But this is where we run into problems, and not just because autophagy literally translates to "eating yourself." (It can be hard for scientists to explain that this is actually a good thing and that all living things do it, from simple yeast all the way up to primates; we were designed to work this way by millennia of feast and famine.)

The problem isn't the science, it's the culture. For most of history, fasting was locked into human lives at a steady, healthy pace in some form of ritual, religious or otherwise. But in the modern world, we make our own rituals, and they easily shade into obsessions. This happens a lot with new diets: We get the zeal of the convert. We bore our friends to death with the particulars. And we take it too far, which in the case of fasting can be dangerous.

In a column published this week, the New York Times' veteran health columnist Jane Brody came around to the value of intermittent fasting. But she sounded a personal note of caution: "For people with a known or hidden tendency to develop an eating disorder, fasting can be the perfect trigger, which I discovered in my early 20s. In trying to control my weight, I consumed little or nothing all day, but once I ate in the evening, I couldnt stop and ended up with a binge eating disorder."

Something similar, at least to the first part of that story, seems to have happened to Twitter CEO Jack Dorsey. Last year Dorsey boasted about fasting for 22 hours, eating just one meal at dinnertime, and skipping food for the whole damn weekend. "I felt like I was hallucinating," he enthused, boasting of his increased focus and euphoria.

But as many withering articles pointed out, Dorsey's words would have triggered concern if they came from the mouth of a teenage girl since focus and euphoria can also be early signs of anorexia and bulimia. Clearly there is a tangled set of gendered assumptions at play here. "Its both remarkable and depressing to watch Jack Dorsey blithely describe a diet that would put any woman or any non-wealthy man into the penalty box of public opinion," wrote Washington Post columnist Monica Hesse.

That's not what The Switch is about. Clement doesn't endorse Dorsey's extreme approach, since the research shows benefits diminish after 16 hours of fasting. "I have friends who are bulimic, I know how serious a problem it is," he said when I raised the issue. "The kind of fasting that I'm talking about is just making sure your mTOR and autophagy are in balance."

Indeed, The Switch is a very balanced book, with plenty of nuanced suggestions for how you can make your food situation just a little bit better without making too many radical changes. (That probably explains why it hasn't taken off on the diet book media circuit, which tends to favor rules that are extreme, unusual, and headline-friendly.)

Here's a breakdown of Clement's advice.

Like most medicine, the mTOR switch is good for you if used at the correct dose, and poison at high doses. There's a reason it exists: It's your body's way of saying "times are good, let's grow muscle and fat!" Fat isn't inherently bad for you, either on your body or in your diet. Indeed, the good fats are what Clement suggests we consume the most fish, avocados, plant-based oils and nuts, macadamias especially alongside regular greens, most legumes and a little fruit.

If you're cutting down the amount of time you eat, then the content of your meals matters more. Clement himself gets good results from a meatless version of the ketogenic diet, which he says makes him less hungry but he doesn't rule out other diets that focus on good fat and fiber.

At the very least, be sure to avoid the stuff that spikes blood sugar. It will make you too hungry too soon, which will make autophagy impossible. You didn't think this whole Switch thing was going to give you permission to snarf on soda and hot dogs, did you?

Well, it does, actually just very occasionally.

Clement brings a lot of science on protein to the table, and the bad news is you're probably eating way more of it than you think you need. Animal protein flips the mTOR switch into high gear (which is why Clement is into mostly vegan keto). Sadly, so does regular dairy, and as a milk fan I found the new studies on this particularly hard reading.

But it makes evolutionary sense. Cow milk is designed to make calves grow many sizes in a short space of time, and the way you do that is by activating the mTOR pathway. So it's hard to switch into autophagy if you're chugging milk all the time. (Non-cow milks and cheeses seem to be fine, mTOR-wise.)

Which isn't to say you can't have meat and milk at all. This isn't one of those fundamentally restrictive diets we always break. Clement suggests dividing the week or month or year into growth and fasting phases. You might decide to eat as much as you want for three months of the year (which takes care of the holidays problem), say, or try doing the fasting thing for five days a month.

Whichever way you do it, the sweet spot seems to put you in growth mode around 20 percent of the time. But that's not a hard and fast number, because again, this isn't one-size-fits-all. (It certainly doesn't apply to kids, who need to grow more like calves.) I told Clement that after reading the book I was thinking of only allowing myself meat or milk on the weekends; he enthusiastically endorsed the idea.

Ready to turn on autophagy for its disease-fighting benefits? Ready to avoid doing it too much? Ready to eat more nutritious food when you break your fast? Then it's time to figure out how long you want to fast for and you'll be surprised about how little time it takes to see the effects.

The math varies from human to human, but "you only have about six to 10 hours worth of glycogen stored in your body at any given time," says Clement. "So you can actually burn through those overnight if you didn't load up with carbs in your evening meal or 11 o'clock snacks."

That provides one particularly effortless way to fast for those of us who don't wake up hungry (and if you're eating the right stuff, you generally won't). Let's say you ate your last bite at 9 p.m. and wake up at 7 a.m. Congratulations, you're already out of glycogen and in autophagy! Now the question is: how long is it comfortable for you to stay foodless, bearing in mind you don't want to go past a total of 16 hours? (In this example, that would be 1 p.m.)

You'll definitely want to hydrate immediately, of course: Sleep literally shrivels your brain. You might want to drink some coffee, which enhances autophagy (the all-time Guinness World Record oldest human, Jeanne Calment of France, took no breakfast but coffee, and died at 122). If you can stand to do so, this would be a great time to work out. Exercise seems to act like an autophagy power up; one study suggests working up a sweat might boost our cells' trash-cleaning effectiveness all the way up to the 80-minute mark.

So if you went from 9 p.m. to 1 p.m., or whatever 16-hour period suits your schedule (7 p.m. to 11 a.m. seems to be a popular one for fasters who don't make late dinner reservations, and it is easily remembered as "7-11"), then congratulations. You just did the maximally beneficial fast. Take that, Jack Dorsey.

But if you didn't? No sweat. If you only made it until 10 a.m., or 8 a.m. before needing food, your entire body still got a boost of cleanup time. And if you needed an immediate breakfast, that's fine too. Fasting doesn't have to happen every day; in fact it's imperative that it doesn't. Every morning is an opportunity to listen to your body and see if it's ready for a quick restorative food break.

Everyone who's ever tried to diet knows the terrible guilt that comes after grabbing obviously bad food, Don't stress over it, says Clement. Don't be maniacal. The whole point is to be in balance. We all need mTOR-boosting feasts from time to time. "It's fine to have one pepperoni pizza on a Sunday, or whatever," he says. So long as you're eating well most of the time and fasting every now and again, you'll see positive effects.

And if you can't fast at all and can't stop snacking? No worries, just change what you're eating. "If you switch over to snacking on either very low glycemic veggies like broccoli tops or carrots, or nuts, then you're not going to be replenishing your glycogen stores," Clement says. Stick a small bowl of almonds and blueberries in the kitchen and you'll be surprised, over time, at how little it takes to satisfy supposedly giant cravings.

That was what I learned, not from Clement's book, but from David Sinclair's. The Harvard geneticist and Clement mentor doesn't focus so much on lengthy fasts, although he takes a number of fast-mimicking supplements. His dieting approach is to simply eat less, to "flip a switch in your head that allows you to be OK with being a little hungry." For some of us, such small moves may be more effective than going all-out on a new diet.

If youd like to talk to someone about your eating behaviors, call the National Eating Disorder Associations helpline at 800-931-2237. You can also text NEDA to 741-741 to be connected with a trained volunteer at the Crisis Text Line or visit NEDA's website for more information.

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