A new study suggests that while fasting diets might help with weight loss, they could increase your risk of diabetes – and the 5:2 author has responded


There has been conflicting evidence on the long-term effects of intermittent fasting.
  • Intermittent fasting has rapidly grown in popularity in recent years.
  • The jury’s still out in the science world as to the long-term health effects of the regime.
  • A new study on rats suggests that while fasting may help to 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, author of the popular 5:2 fasting diet, has penned a response to the findings.

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

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

Looking at adult rats, the study analysed the effects of fasting every other day on body weight, free radical levels (highly reactive chemicals that can cause damage to cells in the body), and insulin function for three months.

They found that while the rats lost weight overall and ate less, the amount of fat around their tummies actually increased. The insulin secreting cells of the pancreas also showed damage, with the presence of increased levels of free radicals and markers of insulin resistance – an “early warning sign of heading towards diabetes” – also observed.

Ana Bonassa, lead author of the study, 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.”

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 regimes such as the 5:2 and the 16:8. Many followers of the regime claim they eat less, lose weight, and have more energy, among many other short-term benefits.

But the jury’s still out as to the long-term effects of the regime in the science world as there has been conflicting evidence on the benefits and disadvantages. Some evidence has even suggested fasting could help to reverse diabetes or reduce your risk of developing it.

The scientists say they now plan to investigate how fasting impairs pancreas and insulin function, and concluded that more research is needed to assess how people may be affected, particularly those with existing metabolic issues.

Bonassa warned: “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 author had to say

Michael Mosley, author of the 5:2 – arguably the most well-known intermittent fasting regime – wrote a response to the findings in the Mail on Sunday.

He pointed out 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. This differs to that of the 5:2, under which followers eat either 500 calories for females/600 for males in a day for two days per week and eat regularly for the other five.

Mosley added that since publishing his 2013 book “The Fast Diet,” he now recommends a “more generous” 800 calories a day as well as eating a healthy Mediterranean-style diet on both fasting and non-fasting days.

Regarding the increase of fat tissue around the rats’ abdomen, he went on: “This is a surprising finding because it contradicts so many other animal and human studies of intermittent fasting. 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.

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

Mosley pointed to examples of other human studies in which men and women who followed the 5:2 principles achieved their goal weight faster, and witnessed 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. 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,” he said.

“Scans of the pancreas and liver showed they were far healthier than at the start of the trial.”


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