Intermittent fasting has rapidly grown in popularity in recent years.

The jury's still out in the science world about the long-term health effects of the diet regimen.

A new study on rats suggests that while fasting may help achieve weight loss, it may also damage the pancreas and affect insulin function, which could lead to diabetes.

More investigation is needed into how people may be affected, particularly those with existing metabolic issues, the researchers concluded.

Michael Mosley, who created the popular 5:2 intermittent-fasting diet, has written a response to the findings.

Researchers from the University of Sao Paulo, in Brazil, presented new research on the effects of intermittent fasting at the annual meeting of the European Society of Endocrinology in Barcelona, Spain, over the weekend.

In their study on rats, the researchers found that while fasting might achieve weight loss, it may also damage the pancreas and impair the action of the sugar-regulating hormone insulin, something that could lead to diabetes.

They analyzed the effects of fasting every other day for three months on the adult rats' body weight, insulin function, and levels of free radicals, or highly reactive chemicals that can damage cells in the body.

They found that while the rats lost weight overall and ate less, the amount of fat around their tummies increased. The insulin-secreting cells of the pancreas also showed damage, and the researchers observed increased levels of free radicals and markers of insulin resistance, an "early warning sign of heading towards diabetes," they said.

"This is the first study to show that, despite weight loss, intermittent-fasting diets may actually damage the pancreas and affect insulin function in normal healthy individuals, which could lead to diabetes and serious health issues," said Ana Bonassa, the lead author of the study.

The scientists urged people to take "careful consideration" before opting to follow a fasting diet.

Intermittent fasting has rapidly grown in popularity in recent years thanks to regimens such as the 5:2 diet and the 16:8 plan. Many people who follow such diets say they eat less, lose weight, have more energy, and see many other short-term benefits.

But the jury's still out in the science world on the long-term effects of intermittent fasting amid conflicting studies about the benefits and disadvantages. Some research has suggested that fasting could help reverse diabetes or reduce the risk of developing it.

The scientists say they now plan to investigate how fasting affects pancreas and insulin function, concluding that more research is needed to assess the effects on people, particularly those with existing metabolic issues.

Bonassa said: "We should consider that overweight or obese people who opt for intermittent-fasting diets may already have insulin resistance, so although this diet may lead to early, rapid weight loss, in the long term there could be potentially serious damaging effects to their health, such as the development of Type 2 diabetes."

Here's what the 5:2 diet creator had to say

Michael Mosley, who created the well-known intermittent-fasting regimen known as the 5:2 diet, responded to the findings in the Daily Mail on Sunday.

He said that in this study, the rats were put on an "absolute fast," meaning they ate nothing at all every other day during the three-month period. In contrast, under the 5:2 plan, people eat 500 to 600 calories a day for two days a week and eat regularly for the other five.

Mosley, who published his book "The Fast Diet" in 2013, said he now recommended a "more generous" 800 calories a day, as well as eating a healthy Mediterranean-style diet on both fasting and non-fasting days.

He called the researchers' finding of increased fat tissue around the rats' abdomens surprising.

"It contradicts so many other animal and human studies of intermittent fasting," Mosley said. "I wasn't given detail about what the rats ate on non-fast days, but if they were allowed to gorge, that would undoubtedly skew any result."

He added: "I would not, anyway, recommend an absolute fast every other day as you need adequate levels of protein to maintain muscle mass."

Mosley pointed to other studies in which people who followed the 5:2 principles reported achieving their goal weight faster and witnessing improvements in blood pressure and blood fats.

"I would point to a really important randomised controlled trial of 298 Type 2 diabetics published a few months ago in The Lancet," he said. "Those allocated to an 800-calorie diet every day for 12 weeks not only lost large amounts of abdominal fat but nearly half were able to come off all diabetes medication. Scans of the pancreas and liver showed they were far healthier than at the start of the trial."