Do we really know what we think we know about eating — and dieting?

Nutrition researchers are pondering whether breakfast really is the most important meal of the day and if time-restricted eating (a form of intermittent fasting) actually leads to weight loss.

Key questions include how many meals a day one really needs and whether restricting food intake to certain hours can lead to weight loss.

The latter is an appetizing idea and one that’s gained attention since a mouse study of time-restricted eating and a few small human studies suggested eating only in a six to eight hours window helps with losing weight.

But a year-long study detailed this week in the New England Journal of Medicine found those who rigorously restricted eating times didn’t shed more pounds than those who ate when they wanted — as long as calorie intake was roughly the same.

For a year, researchers from Southern Medical University in Guangzhou, China, tracked two randomly assigned groups of people (139 total) who each consumed the same number of calories a day — 1,200 to 1,500 for women and 1,500 to 1,800 calories for men. One group ate when they felt like it, while the other group ate only between 8 a.m. and 4 p.m. Both groups kept food diaries and photographed everything they ate.

The researchers considered not only weight loss, but changes in waist circumference, amount of body fat and body mass index. They also wanted to measure metabolic risk factors like blood glucose levels, insulin sensitivity, blood lipids and blood pressure.

Both groups lost an average of 14 to 18 pounds. But when they ate appeared to make no difference. Nor did the researchers find significant changes in the other factors they measured.

“Among patients with obesity, a regimen of time-restricted eating was not more beneficial with regard to reduction in body weight, body fat or metabolic risk factors than daily calorie restriction,” the researchers wrote.

Dr. Ethan Weiss, a diet researcher at the University of California, San Francisco, told the New York Times that he had followed the pattern of restricting the hours he ate for seven years and had been surprised when his own short-term study found little benefit to the practice of eating only during a certain time period each day. The research from China was just further proof.

“I was a devotee,” he said. “This was a hard thing to accept.”

Others told the Times that some might benefit from limiting their consumption window, especially if they had trouble counting calories, as it would afford them less time in which to eat. And several called for further studies.

What about breakfast?

Many experts believe breakfast is vital.

What, when and how much you eat matters, though, as WebMD reported. “Many studies have linked eating breakfast to good health, including better memory and concentration, lower levels of ‘bad’ LDL cholesterol and lower chances of getting diabetes, heart disease, and being overweight,” wrote Lisa Hill.

“It’s hard to know, though, if breakfast causes these healthy habits or if people who eat it have healthier lifestyles,” she added.

Breakfast literally takes its name from breaking the overnight fast, as BBC’s Jessica Brown noted.

Dietician Sarah Elder told Brown that “the body uses a lot of energy stores for growth and repair during the night. Eating a balanced breakfast helps to up our energy as well as protein and calcium used throughout the night.”

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Still, breakfast’s ranking atop of the good-eating hierarchy isn’t clear.

A 2017 study in the Journal of Nutrition found people who ate one or two meals a day saw their body mass index reduce, compared to those who ate three meals, while those who snacked and ate three meals had greater body mass index. It also found, though, that body mass decreased for those who went 18 hours a day without eating, compared to those who went 12 to 17 hours a day without eating.

And breakfast eaters lost body mass compared to those who skipped breakfast.

“Our results suggest that in relatively healthy adults, eating less frequently, no snacking, consuming breakfast and eating the largest meal in the morning may be effective methods for preventing long-term weight gain,” those researchers concluded. “Eating breakfast and lunch 5 to 6 hours apart and making the overnight fast last 18-19 hours may be a useful, practical strategy.”

“For most people, there is a benefit to eating breakfast even when not hungry,” Rahaf Al Bochi, spokesperson for the Academy of Nutrition and Dietetics and owner of Olive Tree Nutrition, recently told HuffPost. “Breakfast helps to break the overnight fast and helps to kickstart and fuel your day.”

Christina Meyer-Jax of Northwestern Health Sciences University agreed, noting people should eat something within a couple of hours of waking up. She cited benefits including providing energy, supporting metabolism and keeping blood sugar levels in check. Breakfast contributes to a healthy weight and is good for the heart, cognitive function and mental health, the article said.

“But most of these benefits are ultimately dependent on choosing the right foods,” Meyer-Jax told HuffPost. “That means protein, like lean meats and eggs; healthy fats, like nuts and seeds; complex carbohydrates, like berries and other high-fiber fruits; and whole grains with no added sugar.”

Researchers and clinicians will likely continue to chew on food-related findings. What remains clear, however, is that how much someone eats and its nutritional value are important to a healthy diet and maintaining a healthy weight.

David Levitsky, a professor at Cornell University, recently told the BBC that he eats one meal a day.

“There’s a lot of data showing that, if I show you food or pictures of food, you’re likely to eat, and the more frequently food is in front of you, the more you’re going to eat that day,” he said.

He said hunger is often psychological. “When the clock says 12 p.m., we may get feelings to eat, or you might be conditioned to eat breakfast in the morning, but this is nonsense. Data shows that if you don’t eat breakfast, you’re going to eat fewer calories overall that day,” he said. “Our physiology is built for feasting and fasting.”

That doesn’t apply to people with diabetes, he emphasized.

People used to eat when they had food, since there weren’t refrigerators or grocery stores. That meant one meal a day and probably around midday, as food historian Seren Charrington-Hollins told the BBC.

In the same article, Emily Manoogian, clinical researcher at the Salk Institute for Biological Studies in California, and author of a 2019 paper entitled “When to Eat,” offered advice on timing. She believes not eating for 12 hours at a time lets the digestive system rest.