How to not pick up pounds during the holidays, and how to eat in general: Consume more in the morning, less in the evening

By Michael Crupain, Michael Crozien and Ted Spiker
National Geographic

Heading into the winter holidays, ’tis the time for eating — and plenty of it. But food can affect your body differently throughout the day, so let’s take a closer look at that internal timepiece.

Understanding your biological clock is the key to shifting your body to optimal function. The biological clock is your body’s automated energy conservation system, influencing behavior on a cellular level from sunset to sunset.

Some people live in opposition to their bodies’ natural instincts — and this is an important example of how food plays a role in the rhythm of life. About 15 million workers in the United States have shift jobs; they work nights and sleep during the day. Studies examining the health of these populations find that they have increased rates of both sleep issues and obesity. People who work the night shift tend to gain more weight than people with normal nine-to-five schedules.

Although we don’t completely understand the cause, the main suspect behind the disturbances in the health and metabolism of shift workers is that they are fighting their natural circadian rhythms — and their body’s instinctual notions of when to eat.

Our body clock and our food clock have a natural tension point: We crave food at night, but we function better when we eat earlier. Research has shown that in the absence of normal light and time cues, people are naturally the hungriest around the time that would correspond to 8 p.m. and least hungry at the time that would correspond to 8 a.m.

That basic instinct was an advantage in the early days of human existence, but in modern times it may be hurting us. One of the body’s most important hormones for dealing with food is insulin, which regulates the amount of glucose in the blood.

The body’s secretion of and response to insulin follows a circadian rhythm. Studies suggest that sensitivity to insulin is highest during active phases (when we are awake) and most insulin resistant during their typical sleeping hours. Mealtime has a big effect on what happens to your blood-sugar levels; if you eat the same meal in the morning and at night, your blood sugar will increase more in the evening than in the morning.

Fat cells also appear the most insulin sensitive early in the day, with a peak at noon; they are about 50 percent more sensitive midday than they are at midnight. This means that your body is primed to eat at certain times. In fact, eating at the “wrong” time can throw off everything. One study found that those who ate lunch earlier lost more weight than those who ate later. A related study found that those who ate later burned less energy.

Research has shown that our body’s natural rhythm is to want food later, even though it has a negative effect on our overall health. Why are our body’s food cravings out of sync with our circadian rhythm? During periods when we didn’t know when our next meals would come, the human body may have evolved the need for a food-storage mechanism. In that era, humans didn’t live long enough to experience the harms of late-night eating—and in any case, the body only cared about surviving the next day, not the next decade.

Today, we no longer need that extensive storage ability because food is plentiful. We have to consciously override our ancient instincts and make smart choices about when to eat—and that means more in the morning, less later on.

This excerpt is adapted from What to Eat When: A Strategic Plan to Improve Your Health and Life Through Food, originally published by National Geographic Partners, LLC, on December 31, 2018. Copyright © 2019 Michael F. Roizen and Michael Crupain. Compilation copyright © 2021 National Geographic Partners, LLC. All rights reserved.

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