Dr. Amy Shah on Circadian Fasting (aka Intermittent Fasting)

Intermittent fasting was the most-Googled diet of the year in 2019, and all over the internet it’s touted as a magical way to lose weight and cure disease. But as Dr. Amy Shah explained to us, the internet’s definition of intermittent fasting is overly broad and often way too extreme — still, real research shows that specific forms of intermittent fasting can provide huge health benefits. Dr. Shah, who is double board-certified in immunology and internal medicine and author of the forthcoming book I’m So Effing Tired, cleared it all up for us, including what intermittent fasting science says, why circadian fasting is the best approach, how to determine the right eating window for you, and more.
*This is a short clip from our interview with Dr. Amy Shah. Click here to watch the whole thing.*
You can also listen to an audio version of my interview with Dr. Amy Shah on The WellBe Podcast.

Dr. Amy Shah’s Personal Turning Point

Like many of the health practitioners we’ve interviewed, it was a personal health crisis that led Dr. Shah to reevaluate what she’d been taught in medical school and reconsider how she wanted to practice medicine. She was in her 30s, fresh out of residency and in the middle of opening her own practice, when her mental and physical health began to break down. 
She was bloated, gaining weight, and constantly exhausted. She was plagued with anxiety day and night, and couldn’t sleep because she was so stressed out about everything that could possibly go wrong in her life. She was also wracked with guilt, feeling like she didn’t devote enough time to either her children or her practice. “I didn’t know what was wrong with me, and I didn’t have anybody to turn to,” Dr. Shah remembers. 
That’s when the shortcomings of what she’d been taught in medical school began to crystallize. “What I realized is that none of us are given the tools to manage our mind and our body, because if you can’t find anything in the lab work, Western medicine is pretty much at a loss, right?”
So Dr. Shah sought insight and advice from elsewhere, turning to both the internet and the nutrition knowledge she’d gained in undergraduate education. She read studies and articles, finding dozens of different approaches and diving headfirst into each — and failing, again and again. But eventually, she found a few things that stuck, and which made a huge difference in her overall health. From that point on, she approached patients in a wholly new way. “I felt like if I could heal myself, then I needed to share that with others,” she says.
Today, Dr. Shah still sees patients in private practice, but she views her patients through a holistic lens and uses an integrative approach. She also has a robust virtual platform through which she shares courses, blogs, and other content that helps equip people everywhere with actionable knowledge to improve their health. One of her main focuses on that virtual platform is also one of the key changes she made when she went through her own personal health transformation: intermittent fasting. 

Circadian Fasting: What Is It and Why Does It Work?

If you’ve gone on the internet at all in the past couple years, you’ve no doubt encountered the notion of intermittent fasting. As Dr. Shah explains, the term “intermittent fasting” is quite broad, and can apply to basically any approach to eating in which you restrict not what you eat, but when you eat. “Intermittent fasting basically means any kind of rest from food for any length of time,” she says. While many types of intermittent fasting can have benefits, Dr. Shah focuses on a specific subset called circadian fasting, which has a clearer definition and has been consistently tied to desirable health outcomes. 
In a nutshell, circadian fasting means aligning your eating with the body’s natural circadian rhythms — aka, being active during the day and resting at night. In terms of food, that means not eating from evening until morning. “Dinner to breakfast, shouldn’t we all be taking a break anyway?” says Dr. Shah. “I feel that it is intuitive, and something that we should recommend to pretty much everyone. However, it has not reached the medical mainstream and most people are still clueless about what circadian fasting even is and why anyone would want to do that.”
Dr. Shah first came across circadian fasting when she was doing research to solve her own health issues. As she scoured studies, she saw the concept of circadian rhythms come up again and again. She learned that these rhythms, which most of us associate only with sleep, actually play an integral role in regulating our hormone health, gut health, and brain health. “Circadian rhythms literally help our body get balance,” she says.
She began to see that the way a person eats can have a negative or positive effect on their circadian rhythms. It can either work with the natural flow of energy and bodily processes that we need, or confuse the body and throw things out of whack. She was struck by the notion that 100 years ago, people were basically forced to eat in accordance with circadian rhythms, because there weren’t really late-night eating options — no refrigerators or TVs or packaged snacks to tear into at 10PM. 
“It was basically, once the evening came around, you turned down the lights, you didn’t have refrigerators and you were kind of done eating for the evening,” she says. The studies she read showed that the body thrives on this kind of natural pattern of eating and fasting, leading to improvements in everything from gut health to immunity to brain and hormone health simply by following a natural circadian fasting cycle. 

Intermittent Fasting Science and Health Benefits

Speaking of studies, Dr. Shah had a lot to share with us about intermittent fasting science. While it’s a relatively new concept, an overwhelming amount of research has come out in the past decade or so, all of which supports the benefits of intermittent fasting with near unanimity.
First, Dr. Shah points to a breakthrough study from the Salk Institute from 2012. In the study, researchers separated mice into two groups and gave them different diet regimens. Both groups ate exactly the same kind and amount of food, the only difference being that one group was allowed 24-hour access to food, and the other was only allowed to eat during an 8-hour daytime window. After 12 weeks, they found that the mice who were allowed to eat at all hours of the day got all the diseases of modern life: obesity, diabetes, hypertension, heart disease, brain disease. The other group, despite eating the exact same food, experienced none of these issues. 
The researchers were shocked at this outcome, so they repeated the study again and again, and each time they found the same result. When they did a similar study on humans, they found the same thing: people who were restricted to eating during a 10-hour window — so fasting for 14 hours a day — stopped getting the chronic diseases of Western life. 
The body of intermittent fasting science extends to a huge range of health issues, including cancer. Dr. Shah cites a study done with breast cancer survivors, in which half the participants fasted for 13 hours per day. The researchers found that, compared with those who were using circadian fasting, the non-fasting group had a 36% higher chance of breast cancer recurrence.
Dr. Shah also brings up a recent study from the New England Journal of Medicine which shed some light on how and why circadian fasting is so beneficial. The study showed that when you stop eating in the evening, your body senses that there’s no more food coming in, so it turns on various rest and repair processes, all of which need fuel in order to happen. 
“Then, there’s this amazing switch, this fuel switch that happens during a fast that is totally unused by so much of our population” says Dr. Shah. What happens is that, once your body uses up all of its preferred sources of fuel — first blood sugar, then sugar stored as glycogen in the liver — it begins using fatty acids for fuel. (It’s important to note that it generally takes more than 12 hours for the body to use up all of this stored fuel, which is why fasting is necessary to reap this benefit.) 
“It’s a fuel switch from sugar for fuel to fatty acids for fuel,” says Dr. Shah. “Once you get into using fatty acid for fuel, and then in the morning you break your fast and you’re back on glucose for fuel, the switch back and forth turns on all of these genetic processes that we think are evolutionarily conserved, so better mitochondrial biosynthesis.” As she explains, we want more mitochondria, because that means cells that are more stress-resilient, which in turn means less inflammation in the body, which has tons of downstream health benefits, including increased longevity and anti-aging effects. Essentially, fasting is a hormetic stressor (just like exercise or a sauna), meaning it creates a short-term stress on the body, which leads to long-term benefits. 
Other health benefits that the intermittent fasting science shows include: lower risk of diabetes, longer lifespan, and protection against heart disease, among many, many others.

Determining Your Optimal Eating Window

So circadian fasting loosely means not eating from dinner until breakfast, but to get the health benefits documented in these studies, you need to get a little more precise than that. Specifically, you need to set a certain “eating window” for the day, during which you get all your calories — and it matters a lot when and how long that eating window is.
As the research shows, and Dr. Shah’s work corroborates, it takes a minimum of about 12 hours to see any benefits from fasting. The breast cancer benefits came in at around a 13-hour fast, the Salk Institute studies used a 14-hour fast, and the New England Journal of Medicine study used an 18-hour fast. Though all of these numbers are different, Dr. Shah simplifies the key takeaway: “Twelve hours is where things start to change, and with each hour after that, you just keep getting more and more benefits.” 
So given the above, it’s clear that your eating window should be no longer than 12 hours, and ideally somewhat shorter (say, 10 or 8 hours). But what’s interesting is that, because of the same principles that underpin the idea of circadian fasting, it really matters when your eating occurs — i.e., you won’t see the same benefits if you eat all your calories between midnight and 5AM, even though your eating window is only 5 hours long. “Eastern and Western medicine can agree on one thing,” says Dr. Shah, “And that’s that the middle of the day, between 12 and 5, is your peak digestion time. The more calories you can fit into peak digestion time, the better it is.”
The reason for this is because, due to the evolutionary effects of our natural circadian rhythms, we have very good insulin sensitivity in the morning. That means that if you eat the same food in the morning that you eat at 10PM, it will be processed differently at night. “We know that late-night eating has huge effects on our hormones,” says Dr. Shah. “We’re very insulin-resistant late into the evening, and so what I tell people is when you intermittent fast, timing does matter.” She suggests that people eat as many of their calories as they can during the peak digestion hours, then smaller meals later in the evening, and then stop eating at least two to three hours before bed. 
The precise eating window that works for you will be unique to your body and lifestyle, and Dr. Shah is quick to point out that it doesn’t have to be the same every day. As the research shows, you get benefits at 12 hours just like you get benefits at 18 hours — the shorter fast may not provide the same amount or degree of benefits, but it still helps. 
“So what I realized is that you can alternate. You can do just 12 hours on some nights, and then 16 hours one day when you’re really feeling it,” she says. “It doesn’t have to be all or nothing, just like anything in life. People who say they can’t fast, that the diet’s not for them, if they can just fast periodically, a couple days a week, it’s better than nothing. The health benefits of intermittent fasting are so powerful that even a few days makes a difference.”
Also, circadian fasting doesn’t mean skipping breakfast. It just means shifting the timing of breakfast so that it fits into the eating window you set for the day. For instance, if you stopped eating at 8PM the prior evening, you can eat breakfast at 10AM and still get the benefits of a 14-hour fast. Dr. Shah says that this is vastly different from the habits of the American population at large, who eat 15 to 16 hours a day, on average. She attributes our skyrocketing rates or chronic diseases like diabetes and heart disease, at least in part, to this expansive eating window. 
But with the flexibility of eating windows, and the ability to use circadian fasting differently on different days, she believes that it’s something that nearly everyone can take advantage of with just a few small lifestyle tweaks. “This is something that is not a huge shift to most people’s lives if you’re really willing to put your mind to it, and it’s going to give you small, incremental benefits, both now and in the future,” says Dr. Shah. 

The WellBe Takeaway on Circadian Fasting

Intermittent fasting can feel like a confusing topic, especially with all of the information floating around the internet and social media, but the key principles are straightforward. Here’s what to remember about intermittent fasting and how to use it for your health:
  • Intermittent fasting is a general term that can encompass a wide range of approaches to eating. All that it means is applying some sort of time restriction to when you consume food.
  • Circadian fasting means limiting your eating to daytime hours, with a fast between dinner and breakfast. It is based on the idea that up until very recently, people didn’t have a lot of late-night eating options, so our bodies are optimized to eat and digest during the middle and earlier parts of the day. 
  • A broad body of research has consistently found powerful benefits of circadian fasting. The studies show benefits including less chronic disease (like diabetes, high blood pressure, and obesity), improved longevity, and even a protective effect against cancer, among others.
  • It generally takes at least 12 hours to begin seeing the benefits of fasting. At this point, your body has worked through the stored sugar in your blood and liver, and begins using fatty acids for fuel. This switch from sugar to fatty acids — and then back to sugar when you break your fast — is what turns on certain genetic processes that improve the resilience of cells.
  • When determining your eating window, remember that it should be 12 hours or shorter, and include your peak digestion time, which is from noon to 5 P.M. 
  • You don’t need to use the same circadian fasting schedule every day to get benefits. You can fast for 12 hours on certain days, 16 hours on other days, and even skip days.
Have you tried intermittent or circadian fasting? Tell us about your experience in the comments below!
Watch our full interview with Dr. Amy Shah to learn how a serious car accident jump started her own personal health journey, the “energy trifecta” that controls women’s health, what she recommends to people who have schedules that make intermittent fasting difficult, why getting sunlight in the morning matters so much for your health, how fasting schedules change along with the seasons and menstrual cycles, how to fit in exercise with a fasting schedule, why intermittent fasting can actually fix a broken metabolism, who should not try intermittent fasting, and much more.  
You can also listen to an audio version of my interview with Dr. Amy Shah on The WellBe Podcast.


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  1. Wilkinson, M. et al. Ten-hour Time-restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome. Cell Metabolism. Volume 31, Issue 1, p. 92-104. January 07, 2020.
  1. Cabo, R. et al. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med 2019; 381:2541-2551.
  1. Belkacemi, L. et al. “Intermittent Fasting Modulation of the Diabetic Syndrome in Streptozotocin-Injected Rats”, International Journal of Endocrinology, vol. 2012, Article ID 962012, 12 pages, 2012.
Dr. Amy Shah is a nutrition expert and physician who is double board-certified in internal medicine and immunology. She received a B.A. in nutrition and graduated Magna Cum Laude from Cornell University, where she also went on to get her M.D. She has training from Harvard and Columbia in addition to her education at Cornell, and has expertise in hormones, inflammation, and gut health as well as nutrition and intermittent fasting. You can learn more about Dr. Amy Shah here
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