Intermittent fasting - fasting every other day - is guaranteed to lose weight in the short term, because it's a crash diet. 

But like lots of other fad diets, the people selling books about it are basing their speculation on animal models and an unrealistic amount of optimism. In biological reality, intermittent fasting impairs the action of sugar-regulating hormone, insulin, which may increase diabetes risk.

Findings presented in the spring at at the European Society of Endocrinology annual meeting, suggest that fasting-based diets may be associated with long-term health risks and careful consideration should be made before starting this fad program - or any fad diet. Energy balance is the only known way to lose weight.

Type-2 diabetes is a growing global problem that is often attributed to poor diet and a sedentary lifestyle that results in obesity. Blood sugar is partially regulated by the hormone insulin, which is produced by the pancreas, if insulin levels are too low, or the body becomes resistant to its effects, type-2 diabetes results and high blood sugar levels can cause serious health issues, including heart, kidney and eye damage. In addition to medical strategies used to treat type-2 diabetes, patients are also advised to make lifestyle and dietary changes to lose weight.

Recently, intermittent fasting diets have gained general popularity but evidence on their success has been contradictory and there is a lack of knowledge and some debate on their potentially harmful long-term health effects. Previous research has also shown that short-term fasting can produce molecules called free radicals, which are highly reactive chemicals that can cause damage to the body at a cellular and may be associated with impaired organ function, cancer risk and accelerated aging.

In order to investigate whether an intermittent fasting diet could also generate damaging free radicals, Ana Bonassa and colleagues, from the University of Sao Paulo in Brazil, examined the effects of fasting every other day on the body weight, free radical levels and insulin function of normal, adult rats, over a 3-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, with the presence of increased levels of free radicals and markers of insulin resistance were also detected.

Ana Bonassa commented, "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."