Thirteen hours into his self-imposed exile from eating, Jake Nyberg insisted this was no starvation diet.

The time was noon — lunchtime for most people. But for Nyberg, who still had seven hours to go on his fast, it was a good time to explain why he skips food once in a while.

At first, he did it for the challenge. A trainer at a local gym recommended intermittent fasting (known as IF) to help jump-start his metabolism. Nyberg survived his first 20-hour fast, consuming only water and black coffee.

These days he fasts every so often for 20 to 24 hours at a time, to “reset” his body. When he fasts, he feels less pain running and more creative in his thinking. “You have a focus when you’re hungry that is a little different than normal. Beyond that, it hypertunes all your senses,” he said. “You’re not distracted by eating.”

Nyberg, 34, of Minneapolis, is riding a fresh wave of interest in fasting from two disparate camps: scientists testing how fasting affects the body and brain, and fitness enthusiasts who promote intermittent fasting to lose weight.

Recent studies proclaiming the health benefits of fasting in mice have rekindled the debate over whether the practice is helpful or harmful.

Scientists at the University of Southern California last month published findings from a study that showed fasting from food for two to four days over a six-month period generated essentially new immune systems in mice. Fasting appears to flip an internal switch, causing stem cells to make new white blood cells, researchers observed.

“The fasting cycles are able to bring it all back, brand new,” said Valter Longo, one of the study authors. “It’s really striking that you can do that with such an important system of the body.”

Built for hunger?

Eating three meals a day plus snacks is a way of life for most of us, but some health researchers question whether our bodies were built for constant consumption.

Mark Mattson, a researcher at the U.S. National Institute on Aging who has conducted fasting studies on animals and humans, blames our current eating patterns for the obesity and diabetes epidemics.

“Now we have food available constantly. It wasn’t always that way,” he said. “When you eat meals regularly, every time you eat, then it goes into your liver and it’s stored as a glycogen. That’s always used first, and then you never tap into the energy supplies in the fat cells. So the fat cells get bigger and bigger.”

Fasting forces the body to deplete the glycogen stored in the liver and shift to using fatty acids from the fat cells, he said.

Other studies have shown that taking a break from eating also improves learning and memory in the same way that vigorous exercise does, Mattson said. That may explain why the philosopher Plato said he fasted for greater mental and physical efficiency.

But there’s disagreement about what constitutes a fast.

Long says there’s a difference between the kind of fast used in his research at USC and popular fasting diets that promote eating one meal of about 500 calories a day for two days a week. The results observed in his study — new white blood cells, creating a robust immune system — haven’t been proven in fasting diets, he said.

Surviving on one low-­calorie meal for a day or two a week isn’t really fasting, Long argues. It takes about two to three days in a row of fasting before your body and brain make that key internal switch that produces the health benefits observed in the USC study, he said.

Critics warn of dangers

Not everyone is sold on the idea of fasting for better health.

Dr. Robert Cima, a professor of surgery at the Mayo Clinic, tells his patients to eat the day before a surgery — a departure from the usual pre-surgery drill. A well-nourished body is more likely to recover quickly from the trauma of surgery, he argues.

He is equally skeptical of the intermittent fasting trend.

“I’m not sure it resets anything,” he said. “The main issue in our country today is we’re eating too much.”

A better approach than skipping meals for a day or two, he suggested, would be to eat less throughout the week.

Even fasting supporters acknowledge it’s unhealthy for some people. Consider: pregnant women, frail and elderly people, children, Type 1 diabetics and anyone prone to eating disorders.

Nyberg says he dropped 30 pounds and lost 5 percent of his body fat during the first six months of intermittent fasting. But he was quick to point out that he was also working out three times a week. These days, he fasts whenever he feels like doing it. He said taking a day off from food from time to time makes him more mindful about how much he eats and what he is putting into his body.

“As much as everyone in our culture loves eating and cooking and restaurants, far too often people don’t think about how often we’re eating and how much we’re eating,” he said, adding, “When you’re done fasting, you’ll be hungry but you can’t eat as much.”

When it’s all over, he prefers to break his fast with something simple: a burger and a beer.