But rising obesity rates are more than endless grazing. Timing is also important, some experts believe. We eat when we shouldn’t and don’t give our bodies a break long enough.
We didn’t evolve to eat day and night, says neuroscientist Dominic D’Agostino of the University of South Florida. Until the beginning of agriculture about 12,000 years ago, we made a living from hunting and gathering and often had to do these activities with empty bellies. “We’re hard-wired,” says D’Agostino, “to fast intermittently on a regular basis.”
Additionally, people now eat at times of the day they would historically have slept, says Satchin Panda, a circadian biologist at the Salk Institute for Biological Studies in La Jolla, California who penned an overview of the timing of eating in the 2019 Annual Review of Nutrition. For thousands of years, he says, our night fasts probably started much earlier than they did in those days of nighttime television.
Although research is still mixed, timing of eating seems to be important to body weight and health. Studies suggest significant potential benefits of fasting every other day or so – or only eating daily when we are normally awake, within a time window of 12 hours or less – a practice known as limited-time eating.
Such practices – collectively known as “intermittent fasting” – can help ward off obesity. More speculatively, there is evidence in animal studies that the therapies could improve athletic endurance and cognition, alleviate diabetes, and possibly even help fight other diseases.
How Americans became a nation of snackers is complicated, but part of the problem is studies that began in the 1960s that found obvious benefits from eating many small, nutritious meals throughout the day. Nibble cues became a health mantra, based in part on the belief that eating more often speeds up your metabolism and causes the body to burn more calories. This could be one of the reasons why “a lot of people now eat 16 hours a day,” says biochemist Valter Longo of the University of Southern California.
Recent research in humans finds no support for the assumption that endless nibbling increases the metabolic rate, says Antonio Paoli of the University of Padua in Italy. A 2017 study found that people who ate three or more times a day gained more weight per year than people who ate just one or two meals a day.
Could Breaks Between Meals Really Be Good For Your Health? Studies conducted as early as the 1940s reported that regimens similar to intermittent fasting improved the health and lifespan of animals such as rats. However, it was unclear whether this was due to the breaks or simply because the animals were consuming fewer calories.
One of the earliest indications that meal breaks could be healthy comes from a 2003 mouse study conducted by neuroscientist Mark Mattson at the time at the National Institute on Aging. He and his colleagues compared a group of mice that were fed every other day with mice that were fed every day. These groups ate essentially the same amount of food. The researchers also followed another group who ate 40 percent less.
After 20 weeks, the mice in the first two groups weighed approximately the same. Strikingly, however, both the mice that fasted on the second day and the group with fewer calories (including lower blood sugar and insulin levels) were healthier than the daily eaters.
The study also suggested that intermittent fasting might improve brain function, which makes sense from an evolutionary standpoint, says Mattson, who is now at Johns Hopkins University School of Medicine. When an animal is hungry, the brain needs to stay sharp so it can find food.
However, studies replicating Mattson’s animal findings in humans are difficult. Studies where people eat less or nothing every other day or every few days consistently show weight loss and improved cardiovascular health. But you can get similar results just by cutting down on calories. It’s been hard to prove that taking regular breaks from eating has additional benefits.
In the longest human study to date, nutritionist Krista Varady of the University of Illinois at Chicago and her team randomly assigned 100 otherwise healthy overweight people to one of three groups: Group 1 ate just 25 percent of their normal intake every other day (and 25 percent more than normal on days in between); Group two ate 75 percent of their caloric needs each day; and group three ate normally.
The study, published in 2017, found that both groups one and two lost the same body weight on average (about 7 percent) and showed similar measures of risk factors for heart disease and diabetes. That intermittent fasting offered no additional benefits beyond traditional calorie restriction was “pretty disappointing,” says nutritionist Courtney Peterson of the University of Alabama in Birmingham.
Meanwhile, another area of research suggested that timing of meals is also important to health. A 2009 rodent study led by sleep researcher Fred Turek of Northwestern University showed that mice that were fed a high-calorie diet during the day (when the nocturnal animals were normally sleeping) gained much more weight than mice fed on the same diet became the night even though the animals consumed the same number of calories.
That same year, neuroscientist Frank Scheer of Brigham and Women’s Hospital and a professor at Harvard Medical School reported that people were placed in a laboratory for 10 days on an artificial 28-hour day (so they often ate at times when she ate would normally sleep) led to increased blood pressure and blood sugar and insulin levels. “That,” says Peterson, “was a huge watershed study because it literally showed that the time you eat affects your health.”
If eating is unhealthy when the body should sleep, it may be healthy to limit eating to waking hours. This is what Panda’s team has shown in studies published in 2012 and 2014. They reported that mice that were on a high-calorie diet for a time window of eight to 12 hours at night (the animals’ active period) were protected from obesity and gained less weight than mice could eat at any time – although both groups received the same number Ate calories.
Restricting eating to just nine hours a night actually caused obese mice to lose weight. It also lowered blood sugar and improved glucose tolerance, suggesting that time-limited eating could help relieve diabetes.
However, it is unclear whether pandas mouse findings apply to humans – mouse studies often do not apply to humans. “It sounds too good to be true,” says nutritionist Michelle Harvie of the NHS Foundation Trust at the University of Manchester in the UK. A human study published in 2007 even suggested that restricting meal times too much could be bad. If people ate all the calories in a single meal between 4 p.m. and 8 p.m., blood sugar levels rose and glucose tolerance deteriorated, both signs of poor health.
A recent, randomized, controlled study in overweight and obese people also showed no improvement because meal times were limited to the eight hours between noon and 8 p.m.
Still, some researchers believe that the problem with such disappointments may be timing. Peterson suggests that in both studies, the last daily meal may have been taken too late when blood insulin levels fell too low to process foods properly.
In 2018, Peterson and her colleagues reported lower blood pressure and better controlled blood sugar levels in eight overweight prediabetic men who were asked to eat all of their food within a six-hour window – but with dinner before 3 p.m. If this was confirmed with more people, the study that would suggest that restricting eating to a window of less than 12 hours while we are awake, regardless of the calorie cut, may have health benefits while the window is not closed is late in the day.
And what is going on in the body to make it healthier to eat one time than another? Daily biological rhythms could play a central role here, says Dorothy Sears, an obesity researcher at Arizona State University. During the day, your body is best able to process food, says Sears, who wrote on the metabolic effects of intermittent fasting in the 2017 annual report. And just like the brain needs rest at night to do much-needed repair and cleanup work, so does the body, says Panda.
All of the unexplained points and lack of clear evidence from human trials have not stopped thousands of enthusiasts from experimenting with their own personal fasting plans, in large part because many find it easier to count hours than calories. And the hope is that things get even easier when what applies to mice applies to humans: panda’s rodent studies suggest that skipping weekends doesn’t ruin the temporary health effect.
“You think, ‘What about Saturday night if I go out for a late dinner?’ It’s fine with the mice, “says Sears.” It’s very encouraging because it seems you don’t have to ask people to be perfect every day of the week … and you never have to read a label. “
Andreas von Bubnoff is a freelance science journalist and professor for science communication in Germany. He drinks his morning latte in a beer mug and likes to have breakfast at lunchtime. This article was first published in extended form in Knowable Magazine.
What the experts do
The data is not exhaustive, but here is how some of the scientists studying intermittent fasting incorporate what they have learned into their own lives:
● Satchin Panda, a circadian biologist at the Salk Institute for Biological Studies in La Jolla, California, fasts 14 hours each evening and only eats twice a day: breakfast and then dinner before 6 p.m. an hour or two after waking up and three hours before Go to sleep.
● Dominic D’Agostino, a neuroscientist at the University of South Florida, follows a low-carbohydrate ketogenic diet and doesn’t eat until noon or afternoon on important work days – with the exception of a glass of water with lemon and black coffee to improve his focus. “I feel sharper and more creative in this state,” he says.
● Mark Mattson, a neuroscientist at Johns Hopkins University, fasts about 18 hours every night: he only eats after a late morning exercise session. He says it takes about a month to adjust to such a regime without getting hungry.
● Valter Longo, a biochemist at the University of Southern California, developed (and occasionally follows) a “fasting-mimicking diet” to be consumed for five consecutive days. That being said, Longo says he should watch out for the habits of certain centenarians who have a low-meat diet and regularly refrain from eating for 12 hours. “You really don’t see centenarians fasting 16 hours a day,” he says.