Lately we've heard the only thing that matters to your waistline is how much you eat. But there's a growing body of research that says when you eat really does make a difference in how much you weigh.
"Your body is more prone to burn fat at certain times of day and store fat at other times," said Satchin Panda, associate professor in the Regulatory Biology Laboratory at the Salk Institute in La Jolla, Cali.
New studies reveal that to burn the most fat, you need to go 12 hours without eating—say, from 8 p.m. to 8 a.m. So it's smart to time your calorie intake accordingly. Read on for the science-backed rules that will help you use the clock to shed excess pounds.
To keep pounds off, don't eat after dark
Before electricity and all-night diners, we humans used to spend a long stretch every night without food passing our lips. "Staying up and eating late is a very recent phenomenon in human history," Panda noted. So our metabolisms are hardwired to expect a nightly fast, which is a key time for your body to burn fat.
Here's how that works: During the day, your brain and muscles use some of the calories you eat for fuel, and the rest gets stored in the liver in the form of glycogen. At night, your body converts that glycogen into glucose and releases it into your bloodstream to keep your blood-sugar levels steady while you sleep. Once the stored glycogen is gone, your liver starts burning fat cells for energy. Yes, you read that right—you burn fat while you sleep.
The catch: "It takes a few hours to use up the day's glycogen stores," Panda said. So if you snack until midnight and sit down to your breakfast of oatmeal or eggs at 7 a.m., your body may never get the opportunity to burn any fat before you start reloading your glycogen stores again.
It doesn't help that you're also likely to overeat when you're up late—indeed, night owls consume an average of 248 calories more per day than those who go to bed earlier, and most of those excess calories rack up after 8 p.m., according to a 2011 study published in the journal Obesity.
"Willpower is lower when you're sleepy," explained study author Kelly Baron, a clinical health psychologist at Northwestern University's Feinberg School of Medicine in Chicago. "So if you're eating in the middle of the night, you're more likely to overeat and make poor food choices."
On the other hand, Panda said, "eating only between 8 a.m. and 8 p.m., and then not eating for at least 12 hours, should give your body enough time to burn all of the stored glycogen plus some fat every night."
This could have major consequences for your weight and health—and mean you can snack more and weigh less. In a study just published in Cell Metabolism, Panda's research team found that mice that ate a high-fat diet spread out over the day and night became obese and diabetic, while mice eating the same diet but only over an eight-hour period didn't gain any weight and remained healthy. "Fasting at night can even override most of the negative effects of an unhealthy diet," Panda said, "including weight gain."
Eat your calories at roughly the same time every day
You've probably heard of circadian rhythms: the internal clock that tells you when to wake and when to hit the hay. But did you know that nearly every organ in your body has its own circadian rhythm or clock, too? For example, our digestive organs—particularly the liver—are programmed to perform most efficiently during the day, Panda explained.
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The influence of these internal clocks may be one reason why shift workers—people who work (and eat) at night—have a higher body mass index (BMI), on average, than day workers, according to research published in the International Journal of Obesity. "In theory, shift work should be fine because your circadian rhythms can adjust just like they do when you travel abroad," Panda said. "The problem is that in order to be social, shift workers bounce back to a daytime schedule on weekends, so every organ clock gets disrupted."
It takes about one day to shift our internal clocks one hour. So if you have a night job and your bedtime jumps from 8 a.m. during the week to midnight on the weekends, your internal clocks never get the opportunity to reset themselves, which can set you up for weight gain if you're eating while your digestive organs are snoozing.
"Maintaining a regular sleep/wake schedule may be one of the most powerful ways to prevent the harmful effects of shift work," Panda said. Even if you're not working the night shift (or up with a baby or studying for final exams), sticking to a regular sleep and meal schedule will help keep your metabolism in peak form.
Have breakfast within an hour of waking up
Research has found that people who eat eggs at breakfast weigh less. Other research has found that people who down a bowl of cereal at breakfast weigh less. And still other research has found that people who chase their breakfast with a slice of chocolate cake weigh less. The common denominator? You guessed it: They all ate breakfast.
When you wake up in the morning, sunlight tells your brain that the day has started. Eating breakfast—breaking your night's fast—sends that same signal to the circadian rhythms in your body. Chowing shortly after you get up synchronizes these clocks and, as a result, delivers a powerful metabolic jump start, Panda said. That means you'll more efficiently use nutrients all day.
There's one caveat: If you eat a big dinner at 11 p.m. one night, it's actually smart to skip the early morning meal in order to fit in a 12-hour fast. "The idea that we need to have breakfast every single day has become so ingrained in people's minds that for late-night eaters, it means they end up eating around the clock," Panda said. If your last meal was a plate of bar fries at midnight, push your first meal the next day to noon. (Consider it lunch if you're returning to a regular schedule or breakfast if you expect a repeat late-night eating event.)
How big should your a.m. dish be? It depends whether you're counting calories. "Having a big breakfast can lead to feeling less deprived overall, but you have to take total calories into account," Baron explained. A recent study from Tel Aviv University looked at women on a strict 1,400-calorie-a-day plan and found that those who had a 600-calorie breakfast high in protein and carbs that included dessert (like chocolate!) not only lost more weight on average than the ones who ate a 300-calorie low-carb breakfast, but they also had fewer cravings and were better at keeping off the weight.
However, if you're not tallying your total daily calories—balancing a big breakfast with a smaller lunch and dinner—eating a lot first thing in the morning could cause you to take in too many calories, and subsequently put on weight.
Good advice for all: Simply include protein (like yogurt or nuts) in your morning meal. Several studies have found that higher-protein breakfasts can help you feel full and make you less likely to overeat all day—and into the night.