Nope, the strategy of so-called 5-2 diets is to endure two days a week of mini-fasting.
This doesn't mean starving yourself. Rather, it entails reducing your calorie intake during two days of the week down to somewhere in the range of 500 to 1,000 calories.
The idea of intermittent mini-fasting seems to be gaining traction. One version of the diet is being popularized by Michael Mosley, a British physician and journalist who's written a best-selling book called The Fast Diet and produced a documentary.
And there are other popular tomes on the topic. Take for instance, The 5:2 Diet Book, which claims you can boost your brain power and transform your health by feasting for five days and fasting for two.
With so much hype, I was skeptical so I dug a little deeper into the science behind these diets.
My first stop was Baltimore, where I visiting the National Institute on Aging. It's here that researcher Mark Mattson has conducted a lot of pioneering animal studies. He's interested in how limiting calories may fend off aging-related diseases.
He's found that intermittent fasting in rodents seems to improve their blood sugar levels, boost performance on cognitive tasks and help keep them lean.
"The bottom line is that the intermittent fasting does a good job in allowing [the animals] to maintain a low, lean body weight," Mattson told me.
Mattson has been following a version of the fasting diet himself for several years. His intent is not to lose weight, since he's already very lean. Rather, he says he fasts for about 18 hours a day, several times a week, because it makes him feel better.
"My mind is more focused," he told me. He says he's more productive.
So, what's the evidence that this diet works in people who are trying to lose weight? The most convincing evidence comes from a study that was done in Manchester, England, a few years back.
About 100 women, all of them overweight, were asked to follow a diet in which, for five days a week, they followed a Mediterranean diet pattern of eating. The other two days, they ate a low-calorie diet of lean protein and cut out almost all carbohydrates.
At the end of the study, the women on this 5-2 diet lost significantly more weight compared to women who'd tried to restrict calories all week long. And in addition, their insulin resistance improved.
"The hardest time was the first [low-carb 'fasting'] day, midmorning," says Jane Whyatt, one of the participants in the study.
She told me that before the diet started, she'd developed a few bad habits and was eating out a lot since she had a job that required a lot of travel.
"I don't think I was a massive overeater, but I had actually put on about 28 pounds over a short period of time," she says.
The 5-2 diet helped her shed about 14 pounds over four months. Years later, she's kept the weight off and is still following this pattern of eating.
"To be honest, you actually want to carry on because you do feel quite refreshed and healthy," says Whyatt. "This is something you can do, really, for life."
Michelle Harvie, a research dietitian at England's Manchester Breast Centre who directed the study, says lots of people seem to be able to stick with the 5-2 strategy.
"We know that people struggle with daily diets," she says. "Hence, our interest in two-day diets."
And she says once dieters acclimate to the two days of low-calorie, low-carb eating, they seem to get into the habit of eating less overall.
She's hoping to do more research to study the long-term effects of the diet. And she's published more about the results of her research in a new book, The 2-Day Diet.
DAVID GREENE, HOST:
Now, if you are trying to jump-start weight loss, the idea of intermittent fasting might be on your radar. There are several popular books on the topic, and the idea is gaining traction, because most days of the week, you don't have to count calories or change your diet in any major way. The strategy hinges on what you do just two days a week, which is essentially a partial fast, cutting back dramatically on how much you eat.
NPR's Allison Aubrey was curious about the science behind this so-called 5-2 diet.
ALLISON AUBREY, BYLINE: We're not talking about a detox diet here or complete fasting where you starve yourself. Instead, this is a staggered approach to eating. Most days you eat normally and the other two days you eat just a little.
Mark Mattson, a neuroscientist who studies calorie restriction, has been following a version of this diet for years. And when I caught up with him at his office in Baltimore at the National Institute on Aging, it was nearly lunchtime, so the smells wafting from the Institute's cafe made me kind of hungry.
But Mattson's taking me to his lab. He's not about to stop to eat.
DR. MARK MATTSON: Actually, I go in this direction and go up the stairs instead of taking the elevator.
AUBREY: Skipping the elevator and the cafeteria entirely.
MATTSON: That's right.
AUBREY: Mattson is very lean, and he hasn't eaten for nearly 17 hours.
Are you hungry?
MATTSON: Not too hungry.
AUBREY: Today, Mattson won't eat until later in the afternoon. It's a mini-fast, and his motivation to keep it up? He says it makes him feel better.
MATTSON: My mind is more focused.
AUBREY: He says he feels more productive.
Now, the studies he's doing here in his lab with rodents suggest there is something to this way of eating. He says when you deprive animals of food on some days, it seems to improve their blood sugar, boost performance on tasks and keep them lean.
MATTSON: The bottom line is that the intermittent fasting does a very good job in allowing them to maintain a low, lean body weight and maintain lean mass.
AUBREY: So what's the key here? And does it work for people too? Mattson thinks it does. He says when we eat three meals and a couple of snacks every day, our bodies don't need all the leftover fuel that gets stored in our livers.
It takes several hours of fasting before we begin to draw on this reserve of stored energy, and we've got to use it all up before our bodies will start burning fat.
So if you're like Mattson, who hasn't eaten for more than 12 hours...
MATTSON: OK. So, at this point, I've pretty much depleted the stores in my liver.
AUBREY: So now, he says the fat-burning switch turns on.
MATTSON: That is, you tap into the potential energy in the fat cells.
AUBREY: Now, the best evidence that this strategy works for people who are struggling to lose weight comes from a study that was done in Manchester, England a few years back.
It included about 100 women, all of whom were overweight. One participant was Jane Whyatt, who we reached on her cell phone. She told me that before the study began, she'd been burning the candle at both ends, traveling for work and eating out a lot.
JANE WHYATT: I don't think I was a massive overeater, but I had actually put on about 28 pounds over a short period of time.
AUBREY: And she was determined to get it off. Jane says when she started the diet she was told that five days a week, she should follow a Mediterranean style of eating, but not count calories. And the other two days, the more challenging part, she'd severely restricting calories by eating mostly lean protein and very few carbs, which she thought would be tough.
WHYATT: The hardest time is probably on the first day, midmorning.
AUBREY: Now, the hunger pangs do set in. But Jane says if you push through those two diet days, you feel much better.
WHYATT: To be honest with you, you actually want to carry on because you do feel quite refreshed and you feel that you are really making a difference.
AUBREY: Jane not only lost 14 pounds over four months, which she's kept off; her blood sugar improved.
And it turns out the whole group of women in the study who followed the 5-2 pattern ended up losing significantly more weight compared to women who just tried to stick to a low-calorie diet every day.
MICHELLE HARVIE: We know that people really struggle with daily diets, hence our interest in two-day diets.
AUBREY: That's Michelle Harvie, who directed the study. She says this is a strategy that many people can stick to. And she's hoping to do more research to study the long-term effects.
Allison Aubrey, NPR News. Transcript provided by NPR, Copyright NPR.