Should You Try Intermittent Fasting?

Dr. Mark Hyman explains everything you need to know

Katie Couric
Nov 13 · 5 min read
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It certainly seems like one of the biggest diet trends right now is intermittent fasting. Everyone on Instagram’s talking about it, how-to guides are being published online, and some outlets are sharing its benefits — or warning about its potential dangers. So… what’s the deal? Is intermittent fasting safe? Should you try it? And what’s more, does it even work?! Luckily we were able to turn to our friend Dr. Mark Hyman, author of the new cookbook Food: What the Heck Should I Cook?, who told us everything we need to know…


Katie Couric: Before we go into the health aspect of this practice, can you tell us what exactly intermittent fasting is — and how it’s different from a full fast?

Dr. Mark Hyman: Fasting implies not eating anything. Emerging research points to the regenerative, health, and anti-aging benefits of various types of fasting including time restricted eating (eating during an 8 to 12-hour window), alternate day fasting, long-term fasts for reversal of disease, and even fasting mimicking diets or short-term calorie restriction. All these have similar benefits for health. In fact, many fast without even realizing it.

The most basic kind of intermittent fast, better referred to as time restricted eating, is a 12 to 14-hour break taken between dinner and breakfast. Many people eat after dinner but our bodies and cells need time for waste removal, regeneration, and repair.

The term intermittent fasting can also describe other variations of when we eat. A 16:8 intermittent fast means extending your nightly fast, taking 16 hours between dinner and breakfast. The 5:2 approach means eating only about 25% of caloric needs during two separate days each week.

Other approaches include fasting one day a week, or longer-term fasts of up to three weeks for reversing diseases like diabetes. Much research has been done on fasting mimicking diets eating about 800 calories a day for 5 days to reset your biology.

So why is intermittent fasting such a big diet trend right now?

There is much interest in performance, energy, weight loss strategies, and longevity and these types of fasting address all these issues. Intermittent fasting is trendy, but not new. We often went through periods of food scarcity and are well adapted to it. We have over 200 genes that protect us in times of starvation. The intentional practice of intermittent fasting matches our evolutionary needs to renew, repair, and regenerate our cells and organs. Fasting has also been an important part of religious and spiritual practices for centuries. Our culture is one of constant consumption, snacking, and late eating, and it has undermined our collective health. The science of anti-aging regenerative medicine has made dramatic advances in our understanding of biological repair mechanisms that are activated by various versions of calorie restriction. The side effect is not just longevity but renewed energy, weight loss, mental focus, reduction in inflammation, and more. That is why this trend is growing.

Are there actually any benefits to restricting the time you eat meals each day?

Time restricted eating, intermittent fasts, fasting mimicking diets, and even ketogenic diets all promote healing processes in the body. Restricting when we eat can activate weight loss, reverse insulin resistance, and promote the loss of dangerous inflammatory belly fat; it can reduce insulin levels and blood pressure, improve insulin sensitivity, and reduce the risk of diabetes; and it helps the brain function better by clearing out waste products, and increases brain-derived neurotrophic factor, a natural compound that acts like miracle grow for the brain and enhances memory and learning.

It helps us clean out old cells and boost mitochondrial function, the body’s cellular energy powerhouse. It also reverses the effects of aging. It has hormonal benefits, increasing human growth hormone and testosterone and improving cholesterol levels. And fasting activates a family of proteins called sirtuins, responsible for the death of aging cells and the production of age-fighting antioxidants. That’s a lot of benefits! Eating in harmony with our natural circadian rhythms and not eating at night creates further benefit.

And what are the negative aspects of this practice?

Everyone is different genetically and metabolically. Some do well with intermittent fasting, others might not, especially those who are at a low weight or underweight. Some people report headaches and fatigue or mood swings. Some people get what’s called the “keto-flu”: When you fast your body is pushed into a state of ketosis, it starts using stored fat for fuel instead of glucose, which also happens on the high-fat low-carb keto diet. This can be averted by increasing fluid intake, and increasing salt and magnesium. When people are new to fasting or the ketogenic diet they may initially get flu-like symptoms, such as weakness, nausea, headaches, or aches and pains, as their body learns to adapt. People with diabetes should work with a practitioner before any kind of fasting as well as those on blood pressure medications or who have heart disease because there can be rapid positive changes in blood sugar and blood pressure that may require medication adjustment.

There’s been talk that intermittent fasting could affect a person’s DNA. Can you tell us what exactly that means, and how that might happen? Or is this something that’s still being studied?

As we age — or are exposed to excessive stress, poor sleep, junk food and toxins — it accelerates inflammation and oxidative stress (free radicals) that damage our cells and DNA, accelerating aging and promoting disease. Most of what we see as aging is abnormal aging and the result of inflammation and oxidative stress that can be addressed through lifestyle and diet. We can support our cells, our mitochondrial health, and DNA in many ways, and intermittent fasting is an important tool in the toolbox for creating health. It cleans out the senescent or old cells, reduces oxidative stress and inflammation, and supports the regeneration of our own stem cells to help the body heal and stay healthy. More research is emerging daily on the benefits across all aspects of our biology including DNA repair and stem cell production.

If a person is curious about trying out intermittent fasting, what should they know? Do you have any tips to make sure they’re doing this in as healthy a way as possible?

A 12 to 14-hour overnight fast is simple and available to everyone with no risk. Longer fasts are often helpful. However, if you have any medical conditions, are pregnant, or underweight, it’s best to consult with a Functional Medicine doctor or nutritionist. It’s important to listen to your body and listen to what it tells you.

Mild discomfort is sometimes part of the adaptation process, especially since so many of us are used to eating much more often than we need to. Keeping a journal is one helpful way to track your progress. Start with a diet of real, whole nutrient-dense foods first, before even venturing into fasting. It will improve your overall health and nutritional status and allow you to better tolerate longer fasts. When you’re balanced from eating real whole foods it’s easier to adapt to intermittent fasting and avoid sabotaging cravings.

Follow Dr. Mark Hyman on Instagram, Twitter, and Facebook, and check out his books on Amazon or your local books retailer.

This originally appeared in Katie Couric’s Wake-Up Call newsletter. Subscribe here.

Katie Couric

Written by

Founder, Katie Couric Media. Newscaster: Wake-Up Call. Podcaster: Next Question. Doc filmmaker. @SU2C founder.

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