Keep in mind that the scientific evidence on this is still quite limited. Scientists still don't know which diets that alter meal frequency or sharply restrict calories will work best for which people.
But the findings from the handful of human and animal studies exploring "intermittent energy restriction" seem to support one fundamental idea: that exposing our cells to this "mild stress results in adaptive changes that protect against more severe stress," according to Mattson's paper.
This makes sense, since humans (and other animals) have fasted intermittently for much of our time on Earth. As the paper notes, "The most common eating pattern in modern societies, three meals plus snacks every day, is abnormal from an evolutionary perspective." Thousands of years ago, we began fasting to express religious devotion, and to this day all of the world's major religions encourage it at least once a year.
But fasting may seem extremely daunting if you've never attempted it. The healthy among us have enough fat stored to survive, on average, for about 40 days without eating (don't try this one at home). But we get all kinds of signals from our bodies and the environment to eat at least twice a day. And it's those biochemical and psychological messages that we have to override if we're going to disrupt that cycle of habitual eating, however briefly.
Back in the spring, when I did the yeast-beer experiment with two colleagues, I fasted on four Sundays – skipping both lunch and dinner – so that I could "test" the effects of alcohol when combined with yeast and yogurt on an empty stomach.
While I'd had moments of weakness when I doubted the endeavor and was seriously tempted to grab a snack, in hindsight, I found that I actually enjoyed forgoing food for most of a day. I felt liberated, lighter, clearer of mind. And I really appreciated food when I finally was "allowed" to eat again.
Lately, I've been pondering the secrets to successful fasting, since a fast might be exactly what I'll want to do this month and next to counter holiday gluttony.
My first question: How long, and how, to do it? Well, turns out that the answer depends on your objective. If you're shooting high for big perks – anti-aging, immune system regeneration and disease fighting – a mini-fast may not cut it, according to Valter Longo, a gerentologist at the University of Southern California who studies the health benefits of fasting.
Longo argues that four to five days is the minimum amount of time you have to go with little to no food to see real changes and improvements in the immune system, for one. Anything less than that isn't a true fast, he tells me.
But since that's impractical (and potentially unsafe without the supervision of a doctor) to do regularly, Longo recommends skipping lunch instead. "I think skipping a meal – lunch or dinner — is probably the best thing that everybody can do. I skip lunch because I love dinner, and I want to be able to have pasta," says the researcher, who's from Italy.
Holding out for a big bowl of tagliatelle may be enough to keep Longo away from the cafeteria at noon, but some of us may need another strategy.
One approach is the 5-2 diet, which involves eating a low-calorie (around 500 calories) diet of lean protein and very few carbs twice a week. As my colleague Allison Aubrey reported, that approach is gaining traction in the U.K. and here in the U.S.
Susan Roberts, a senior scientist and director of the Energy Metabolism Laboratory at the Human Nutrition Research Center on Aging at Tufts University, offers a different view on practical fasting.
Roberts designed the "I" or Instinct Diet, with hunger management as its principle objective. She recommends a modified fast of 600 to 800 calories per day for one to two days a few times a year to people on the diet.
She says the fast can be useful for people who've learned how to curb cravings for cakes or doughnuts, but who feel the temptation to gorge creeping back, she says. This is especially likely to happen around the holidays, Roberts says, when huge, decadent meals have the potential to knock people back into their old ways.
"If they go through the holidays eating garbage, they're going to get back some of the temptation for unhealthy food," she says. "Doing a modified fast in January to zap away the cravings again can be helpful for some people."
And in the long run, Roberts tells me she thinks one of the principle benefit of doing a modified fast a few times a year is that it can make you more insulin sensitive. "Someone who is more insulin sensitive won't have blood glucose swings, which means they'll have a better ability to balance the amount of calories they take in and regulate their weight," she says. In other words, being more insulin sensitive means being less hungry, and less likely to overeat.
But to get there, you'll have to endure some hunger during a fast, Roberts says. Soldiering through it is essential to getting the benefit of insulin sensitivity.
So how do you get through those hunger pangs?
- Fasting is easier with a buddy. I had the bulwark of my fellow fasting colleagues to steel me against the hunger pangs that occasionally gnawed at me during our mini-fasts. And the faithful who fast for Ramadan and Yom Kippur of course rely on peer pressure to keep them out of the kitchen.
- On a mini-fast, choose the food you do eat carefully. Roberts recommends high-protein, high-fiber foods, with some carbohydrates as well. Avoiding refined carbs and sugar, which will spike blood sugar, is best, she says.
- To minimize temptation, stay out of the kitchen and away from food, if you can. Take a walk; get outside.
- Try a pattern of weekly intermittent fasting for at least a month. The PNAS paper notes that studies have shown that a long-term lifestyle change (and the benefits associated with it) is more likely for people who can stick with the diet for at least a month. And tolerating some hunger gets easier the longer you do it.
- Don't be surprised if there are some side effects. Longo cautions that caloric restriction may cause some people to have trouble sleeping or have gastrointestinal issues because of how it affects the circadian clock. "It would not be surprising for some people to be getting GI issues, because the system needs the meal, when it gets going. It may be that meal buffers the acids in the stomach and without the meal, the acids are released."
- Experiment with different intermittent fasting patterns. The PNAS paper proposes several possibilities: eating nothing for 16 hours, skipping breakfast, eating three small meals and skipping breakfast and lunch or dinner.