Photograph By Mark Thiessen
Photograph By Mark Thiessen

Doctors’ orders: watch what you’re eating when

From salmon burgers for breakfast to spinach for a headache, these M.D.’s say eating the right foods at the right times is the key to health.

This story appears in the January 2019 issue of National Geographic magazine.

Michael Roizen, M.D., is chief wellness officer at the Cleveland Clinic and a New York Times best-selling author. Michael Crupain, M.D., is chief medical officer for television’s The Dr. Oz Show. Both have long linked a better diet with better health. But their new book, What to Eat When, says emerging science confirms that “when you eat is as essential as what you eat for maintaining a good weight, preventing and curing some diseases, and living a long, energetic, and happy life.”

Ahead of the book’s December release, the doctor-authors answered questions for National Geographic.

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What to Eat When: A Strategic Plan to Improve Your Health & Life Through Food, published by National Geographic Books, is available December 31, 2018, where books are sold and at

Tell us about the science behind your approach to eating.
We’re all familiar with our biological clocks—the circadian rhythm that sends out chemical signals at certain times to help us wake, sleep, and do other activities. Well, we also have a food clock with a similar purpose: to sync our consumption of food with chemical reactions in the body.

According to these clocks, the optimal way to eat is to consume more energy earlier in the day and less energy later in the day. But our bodies crave the opposite—more calories at the end of the day and fewer in the morning. This is a holdover from a time when humans’ food supply was unreliable and storing energy was an advantage. Today this schedule of eating has negative effects on our health: Studies in animals and humans have associated it with weight gain, chronic disease, and premature aging.

So we need to override that craving-driven schedule and eat in a way—more early, less later—that aligns food patterns with our internal clocks. We call this chrononutrition.

What are the guiding principles in this system of eating?
A few adjustments to existing eating habits will help sync up our internal systems. Two are especially important:

1) Limit your eating to when the sun shines—a window of about 12 hours or, better yet, fewer. That means cutting out nighttime refrigerator raids.

2) Eat more in the morning and midday and less later on. Your body will work best—and be healthier—if you preload calories rather than save them until later in the day, as many of us do.

We help people learn how to do this day to day. But we also define “when” another way. Our bodies change depending on what’s going on in our lives—so we may need to adjust what we eat to be at our best. In the book we list common situations, suggest the best things to eat to prepare for them, and explain the science behind our advice.

What benefits do you tell people they’ll derive from eating this way?
The endgame is that this way of eating will extend your own endgame. It will help prevent disease and, in some cases, even curb or reverse disease. Following this plan can result in all kinds of improvements to your quality of life: better sleep, more energy—and just overall better health.

This interview was edited for length and clarity.