Fasting every other day could be the secret to losing weight while staying healthy because it mimics humans' caveman diet, a new study suggests.
A trial showed that people who ate no food at all for 36 hours then anything they felt like for 12 hours lost more than half a stone within a month.
Crucially, their immune systems remained stable, even after six months, in contrast to many diets which aim to restrict calorie intake consistently each day.
Scientists at the University of Graz in Austria believe the strength of alternate-day fasting (ADF) may lie in its adherence to hunter-gatherers' patterns of eating thousands of years ago, when food was not available every day.
However, they warn that it may not be suitable for everyone and that further studies need to prove its safety over the long-term.
Published in the journal Cell Metabolism, the study recruited 60 participants who were enrolled either into an ADF group or into a control group where they were were allowed to eat whatever they wanted.
The ADF group were required to fill in food diaries and also underwent continuous glucose monitoring to ensure they stuck to the routine.
The scientists found that, on average, the dieters ate normally during the 12 hours they were at liberty to eat an unlimited amount.
Overall, they reached an average calorie restriction of around 35 per cent and lost an average of 7.7 lb or 3.5 kg after four weeks of the programme.
"Why exactly calorie restriction and fasting induce so many beneficial effects is not fully clear yet," says Professor Thomas Pieber, head of endocrinology at the Medical University of Graz.
"The elegant thing about strict ADF is that it doesn't require participants to count their meals and calories: they just don't eat anything for one day."
His colleague, Professor Frank Madeo, added: "The reason might be due to evolutionary biology.
"Our physiology is familiar with periods of starvation followed by food excesses."
A further 30 participants were put on ADF for six months to assess the safety of the diet over a longer period, with positive results.
Previous studies had suggested that consistent calorie-restrictive diets can result in malnutrition and a decrease in immune function.
In contrast, even after six months of ADF, the immune function in the participants appeared to be stable. They had a reduction in belly fat, which is increasingly linked to a higher risk of cancer.
The group also showed lower levels of the hormone triiodothyronine, which has been associated with longer life-spans in previous research.
The new study is likely to shift the ongoing debate in favour of intermittent rather than consistent dieting.
Many people find consistent calorie restriction difficult to sustain and often succumb to "yo-yo" eating, where they end up consuming more than they otherwise would have done.
Despite the apparent benefits, the researchers say they do not recommend ADF as a general nutrition scheme for everybody.
"We feel that it is a good regime for some months for obese people to cut weight, or it might even be a useful clinical intervention in diseases driven by inflammation," said Professor Madeo.
"However, further research is needed before it can be applied in daily practice.
"Additionally, we advise people not to fast if they have a viral infection, because the immune system probably requires immediate energy to fight viruses.
"Hence, it is important to consult a doctor before any harsh dietary regime is undertaken."