Boost your immunity and rejuvenate your metabolism… in 36 hours or less

I’ve been talking about the benefits of fasting for years—way before it was on everyone’s radar. And I’ve personally experienced how beneficial it can be… in more ways than one.

So I really can’t think of a more urgent time than now—as we continue facing the many challenges of stay-at-home orders in the face of a global pandemic—to remind you of one of fasting’s most astonishing and life-saving benefits. And I’ll give you all the details on that in just a moment.

But first, let’s do a quick recap on what I mean when I talk about fasting.

Find your fasting style

There are a few different methods of fasting to choose from. The most common are intermittent fasting (IF) and alternate-day fasting (ADF).

The beauty of IF is that there’s no one set way to do it. For example, you can limit your daily meals to specific eating windows—say, between noon and 6 p.m., like I do. Or you can use the 5:2 approach, where any two days in a single week are your fasting days.

Meanwhile, ADF, as the name would suggest, essentially means fasting every other day. But that doesn’t necessarily mean going without food altogether. Many people who do ADF simply eat fewer than 500 calories on their fasting days. On non-fasting days, you may eat without caloric restrictions. (Of course, that doesn’t mean you should eat whatever you want. It’s always imperative to be mindful of what you’re putting into your body—so continue following a healthy, balanced diet full of fresh, whole foods.)

Sound simple? That’s because it is.

But it also works. In fact, according to a new study, ADF may be a particularly powerful form of fasting.

Lower BMI, less belly fat, better metabolism

As part of this recent study, researchers recruited 30 subjects, all of whom had been following ADF for at least six months. They compared these subjects to 60 healthy controls—half of whom were randomly assigned to ADF for four weeks, and half of whom ate whatever they wanted.

In this instance, subjects following ADF—both the long-termers and the new recruits—would alternate between 36 hours of fasting and 12 hours of eating whatever they wanted.

Ultimately, body mass index (BMI) among the new fasters dropped by more than a point… accompanied by a 14.5 percent reduction in belly fat. Total weight loss came in around seven pounds within four weeks—a result that, according to the lead study author, probably wouldn’t be achieved with shorter fasting periods.

And, even more importantly, apart from weight loss, ADF subjects’ metabolic profiles also shifted for the better.

Reverse aging with autophagy

The ADF subjects also showed significant drops in a wide range of markers linked to heart risk and aging: inflammation, blood pressure, LDL (bad) cholesterol, and short-chain fatty acids.

In addition, their bodies generated ketones (the driving force behind using fat for energy) continuously, even on non-fasting days. (This is why fasting is an important tool for anyone following a ketogenic diet, like my A-List Diet.)

And more importantly, fasting achieved these metabolic benefits without any adverse effects. But of course, the safety of IF has always been a given, as far as I’m concerned. After all, you won’t find a better way to mimic how our hunting and gathering ancestors ate and survived.

When you think about it, our bodies are meant to go through frequent periods of hunger, punctuated with periods of food “excess.” Because in nature, it truly is feast or famine.

Despite the United States Department of Agriculture (USDA) urging people to eat small meals throughout the day, periods of starvation actually serve an important restorative purpose to our bodies.

For one thing, they trigger autophagy, which is the process by which your body “eats up” damaged cells and generates newer, healthier replacements. This turbocharges your cells’ metabolic function within just 36 hours.

And it’s an anti-aging benefit you’ll never achieve through textbook “dieting.” In fact, continuous calorie restriction has the exact opposite effect—resulting in a loss of nutrition and depressed immune function.

These researchers theorize that continuous calorie restriction actually blocks the onset of age-reversing autophagy. ADF, on the other hand, promotes this protective process. Which comes as no surprise to me.

After all, cavemen and women didn’t have refrigerators, much less 24/7 convenience stores or fast food restaurants. And they certainly weren’t able to go out and purchase food and drinks at midnight.

But we can—and sadly, do—now. And the modern consequences, mainly in the form of chronic disease, have been dire.

Reboot an aging immune system, too

Now, let me pivot to the real reason I wanted to share all of this with you today: Researchers think that fasting may be the secret to regenerating a weakened immune system, too.

Recent studies show that undergoing several two- to four-day fasting cycles, staggered over the course of six months can flip a switch that puts your immune system into “turnover” mode.

That’s because, when you’re fasting, your body attempts to conserve energy by cutting the fat—literally and figuratively. It burns through fat and sugar stores for energy, while killing off worn out white blood cells that aren’t pulling their weight anymore.

At the same time, your levels of an enzyme called PKA start to lower. And this eventually sends signals to your body to start producing new, healthy immune cells. (It’s worth mentioning that prolonged fasting also lowers levels of IGF-1—the same hormone implicated in tumor growth.)

The end result is a rehabbed, rejuvenated immune system. Without even lifting a finger!

And this strategy could have some pretty amazing applications. In fact, one team of scientists found that prolonged fasting reduced levels of circulating PKA—and put the brakes on chemotherapy-related immunosuppression and mortality—in mice.2

Which would mean nothing, of course, if the results didn’t happen to coincide with the outcome of a previous pilot study.

That Phase I clinical trial showed that prolonged fasting in humans—in this case, during the 72 hours prior to chemotherapy—can protect white blood cells and ward off toxicity as well.

So just imagine what it might do for someone who’s simply trying to strengthen their defenses against viral infections.

What a difference a day makes

Obviously, this is still developing research, focused mainly on mice—and there are still a lot of blanks to fill in when it comes to prolonged fasting in humans. So I’m not about to start recommending 72-hour fasts.

But luckily, you don’t need to jump off the deep end. Because published clinical studies on actual humans show that there’s plenty to be gained from perfectly safe fasting periods of 36 hours or shorter.

Over the last decade, science has uncovered a long list of impressive benefits you can reap from IF in any form. (And not just as a weight-loss strategy—as I explained above, fasting can curb high blood sugar, lower cholesterol and blood pressure, boost cognitive function, and cut inflammation, too.)3

So whether or not this strategy can indeed bolster your immune system and spare you a potentially devastating illness, I’m still a big proponent of responsible, properly planned fasting. Nothing drastic—just 24 to 36 hours, with plenty of water.

I do this at least once a month myself, in addition to my time-restricted eating windows, and I always feel refreshed and energized afterwards. And I find that most people can manage this approach relatively easily.

So if you can, you should. Because ultimately, fasting isn’t some fad diet—it’s a way to harness your body’s own natural rhythms to boost longevity and prevent disease.

And if you ask me, it’s about time mainstream nutrition “experts” got the memo.


1. Stekovic S, et al. “Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans.” Cell Metab. 2019 Sep 3;30(3):462-476.e6.

2. Cheng CW, et al. “Prolonged fasting reduces IGF-1/PKA to promote hematopoietic-stem-cell-based regeneration and reverse immunosuppression.” Cell Stem Cell. 2014 Jun 5;14(6):810-23.

3. de Cabo R, et al. Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine, 2019; 381 (26): 2541.