Fasting every other day to lose weight impairs the action of the body’s primary sugar-regulating hormone, insulin, which may increase diabetes risk, according to data presented in Barcelona at the European Society of Endocrinology annual meeting.

Type-2 diabetes is caused by a mix of genetic and lifestyle factors. The major lifestyle factors attributed to diabetes risk are poor diet and a sedentary lifestyle. These recent findings suggest another aspect to lifestyle risk: fasting-based diets.

Intermittent fasting diets have gained recent popularity for weight loss, however, evidence of their success has been contradictory. What research exists points to potentially harmful long-term health effects; previous studies have shown short-term fasting can produce molecules called free radicals, which are highly reactive chemicals that can damage the body at a cellular level.

In this latest study, Ana Bonassa and colleagues from the University of Sao Paulo in Brazil examined the effects of fasting every other day on body weight, free radical levels and insulin function of normal, adult rats, over a three-month period. Although the rats’ body weight and food intake decreased as expected over the study period, the amount of fat tissue in their abdomen actually increased. Furthermore, the cells of the pancreas that release insulin showed damage; increased levels of free radicals and markers of insulin resistance were also detected.

“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,” says Bonassa, the study’s lead author.

The researchers now plan to investigate exactly how this diet impairs pancreas and insulin function. But Bonassa and team hope the findings can already inform medical advice to people going forward.

“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,” Bonassa says.

The team stresses that more investigation is needed to assess how actual people may be affected by alternating days of fasting, particularly those with existing metabolic issues. But it is a promising step in clarifying all of the lifestyle underpinnings of diabetes, a task that has taken on some urgency, given recent reports that predict a quarter of all people globally will have diabetes by 2045.