To burn more calories, heed your body’s clock
You might be tempted to eat a breakfast like this in the wee hours of the morning, but experts say you might pay the price. A new study suggests that the most basic operations of the human body throttle back their caloric needs by about 10 percent at that time when compared with late afternoon or early evening. corresponding to somewhere between 4 and 5 a.m. — our core body temperature dips to its lowest point and our idling fuel use reaches its nadir. From that point, at first quickly and then a bit more slowly, the body’s “resting energy expenditure” rises until the late afternoon/early evening. After reaching its peak at roughly 5 p.m., the number of calories we burn while at rest plummets steadily for about 12 hours.
And then, just as surely as day follows night, we start again.
These new findings are a reminder that no matter how 24/7 our schedules have become, our bodies were built for a slower, simpler world in which humans moved around all day in search of food, ate while the sun was up and slept when the sky was dark.
Today, our appetites and the all-night availability of tempting food might induce us to eat well after sundown. And our jobs might demand that we sleep during the day and wait tables, care for patients or drive trucks through the night. But our bodies still adhere to their ancient, inflexible clocks.
The study’s findings also come with an implicit warning: When we disregard the biological rhythms that rule our bodies, we do so at our peril.
Resting energy expenditure accounts for the majority of the minimum calories we burn in a day. Just to spend a day eating, sleeping and breathing uses up 60 to 70 percent of our “resting energy expenditure.” So a serious mismatch in the time when calories are consumed and the time when most of them are burned could prompt the body to make decisions — like storing calories as fat — that aren’t necessarily healthy.
The new study adds to a growing body of evidence suggesting that a good 12-hour fast, when aligned with darkness and our bodies’ nocturnal response, could be a way to prevent or reverse obesity. In lab animals and a growing number of people, Salk Institute researcher Satchin Panda has demonstrated the impact of dietary obedience to our circadian rhythms.
Others have demonstrated the power of timing by showing how readily it can be disrupted.
In a 2014 study, 14 lean, healthy adults agreed to turn their days upside-down over a six-day period. Fed a diet sufficient to maintain their weight, the subjects quickly adapted by turning their internal thermostats down. Compared with the baseline readings taken upon their arrival (when they were awake by day and asleep eight hours at night), the subjects burned 52 fewer calories on Day 2 of their swing-shift schedule, and 59 fewer calories on Day 3 of that schedule.
Do that for a couple of days and you might feel a little off. Do it for months, years or a lifetime, and the result could be too much stored fat and metabolic processes that go haywire.
“One take-away is indeed that for optimal health, including metabolic health, it’s best for us to have a regular schedule seven days a week — getting up and going to bed at the same time and eating our meals at the same time,” said senior author Jeanne F. Duffy, a neuroscientist and sleep specialist at Brigham & Women’s Hospital in Boston.
“We have these powerful clocks in ourselves and they’re prepared to deal with certain events — eating and sleeping — at particular times every day. So we want them to be optimally prepared for that.”