Is fasting for five days a month the best way to slim?

It’s the latest twist to the diet everyone’s talking about. So why are some doctors still sceptical?

FEAST for five days, fast for two. Eat whatever you want and lose weight. Stay healthier, live longer. These are the kinds of claims being made for intermittent fasting — and the idea that you can eat with abandon, so long as you restrict food consumption for two or three days a week, is seductive.

Last month researchers revealed a new version of this idea: a five- day fast every month that apparently slashes the risk of cancer and heart disease and causes body fat to melt away.

But from every-other-day diets, to 5:2 fasts and regimens where you eat only within a four-hour window each evening, are the claimed benefits supported by any evidence? Or is fasting, as some experts believe, a quick ticket to osteoporosis and malnutrition?

The idea of intermittent fasting — or temporarily starving the body of the energy it needs — aims to replicate the known benefits of long-term drastic caloric restriction, where you eat 30-40 per cent fewer calories recommended every day, but still get all the essential vitamins, minerals and nutrients.

Studies on mice and monkeys suggest caloric restriction makes it harder for cancers to grow. It may also reduce the risk of diabetes, enhance brain function and redistribute fat away from the belly where it is most harmful.

The idea of using short, sharp bursts of starvation as an alternative to daily calorie counting initially came from studies of cancer development in mice in the early 2000s.

These suggested temporary periods of starvation might be just as good, if not more effective, at preventing cancer than daily dieting. The mice also lost weight.

Further studies in animals and humans, suggested intermittent fasting might replicate some other benefits of caloric restriction, too, including weight loss and a reduced risk of conditions such as diabetes.

But the idea didn’t really catch on until Michael Mosley presented his documentary Eat, Fast And Live Longer in 2012. His subsequent book became a bestseller. Today, a search of weight-control books and ‘fasting’ on Amazon returns hundreds of results.

Some of the first human studies of intermittent fasting involved people not eating for religious reasons, such as during the month of Ramadan when Muslims neither eat nor drink between sunrise and sunset.

The main problem found was a temporary reduction in cognitive function because of dehydration.

The studies also revealed that people participating in a religious fast do not lose weight — perhaps because they may eat an entire day’s worth of calories when breaking their fast each evening.

People consciously trying to lose weight might be different though. Indeed, Alexandra Johnstone, an appetite researcher at the University of Aberdeen, has found that although fasters consume around 20 per cent more calories in the first meal after a 36-hour fast, their food consumption returns to normal in the meals after that. This suggests intermittent fasting really could be useful for weight loss.

MOST studies directly investigating intermittent fasting’s effect on weight loss have monitored participants for only three to six months. They seem to work, but the weight loss is similar to what you lose through conventional calorie counting.

Alternate day fasting — where you consume very little for one day and eat normally the next, for instance — reduces calorie intake by an average 25 per cent a day. ‘This is very similar to calorie restriction diets,’ says Krista Varady, professor of nutrition at the University of Illinois in Chicago, and author of The Every-OtherDay Diet.

And weight loss for both methods is similar — 4 to 8 per cent of total body weight over two to three months, she adds — equivalent to 6-12lb for an 11st woman.

Most experts agree there’s nothing magical about how, or why, fasting diets work.

‘ I think it’s simply a way to manipulate the amount of calories you’re taking in,’ says Professor Varady, who insists fasting plans are still useful because people often fail to stick to regular diets.

In a year-long study where she compared alternate-day fasting with conventional dieting, Professor Varady found that alternate- day fasting ‘does a bit better’ at maintaining weight loss.

Mary Pegington, a research dietitian at the University of Manchester, has been involved in trials of a 5:2 diet — where people consume 650 calories for two days, and a Mediterranean diet rich in vegetables and wholegrains for the other five (eating as much of this as they want).

‘There seems to be something beneficial about intermittent fasting, but we need to get to the bottom of what length of time the restriction should be followed for, how low the energy intake should go on diet days and what the composition of those days should be,’ she says. Other unanswered questions include how often people should fast to reap the biggest benefits, and what should they be eating on the other five days. Perhaps the biggest myth around intermittent fasting is that you can eat whatever you want on non-fast days and still lose weight.

Most of the studies showing a beneficial effect of fasting have had participants following a Mediterranean diet on non-fast days. Of course, proponents of fasting diets would argue that it’s not only about weight loss.

Dr Mosley made much of the fact that his levels of insulin-like-growth factor (IGF-1) — a hormone that boosts cellular growth and may fuel certain cancers, including breast and prostate cancer — dropped in response to fasting.

Most other human studies have failed to replicate this finding, but a study Ms Pegington was involved in suggested a reduction in the related hormone, insulin, among 5:2 dieters.

‘Losing weight with a daily diet will reduce insulin levels, too, but intermittent diets seem to lead to greater reductions,’ she says.

High insulin levels seem to increase risk of diseases including heart disease, breast, liver and pancreatic cancer and can contribute to type 2 diabetes. It could be that we still haven’t hit upon the best form of fasting to reap all the potential health benefits.

Just last month, researchers at the University of Southern California published a study, in the journal Cell Metabolism of a so-called ‘fasting-mimicking diet’, which involved reducing people’s calorie intake by 34 to 54 per cent (eating 680-1,080 calories per day for the average woman) for five consecutive days each month; the rest of the time they ate as normal.

The 19 dieters seemed to experience a dramatic reduction in belly fat, as well as markers of cardiovascular disease, diabetes and cancer. Further studies in mice suggested the diet was prompting their tissues to undergo autophagy — a regenerative process where damaged cellular components are destroyed and then rebuilt.

But regenerating your tissues this way means you’re ‘basically losing bits of your body, then having to regrow them’, says Lynne Cox, who studies the molecular biology of ageing at the University of Oxford.

WHILE this could be beneficial in replacing worn-out components of brain, or muscle cells, it could also be dangerous. The Californian study found some older mice on the diet were dying prematurely.

‘For older people, or anyone with underlying disease, such as diabetes, heart problems, or a weak immune system, this diet is probably a really bad idea,’ says Lynne Cox, who studies the molecular biology of ageing at the University of Oxford.

Animal studies and observations of starvation in humans have revealed a greater susceptibility to osteoporosis and infections, disruptions to the menstrual cycle, reduced fertility and slower wound healing.

And although short-term studies of intermittent fasting have revealed no such safety concerns, participants had clear dietary instructions and were closely monitored.

Moreover, it is not enough to simply cut calories without ensuring you get all of the essential vitamins and minerals, including calcium for healthy bones, and limiting consumption of fats and sugar.

‘You’d probably increase your risk of heart disease, stroke, dementia, osteoporosis and some cancers,’ cautions Ms Pegington. So what’s the conclusion? First, clear long-term evidence on the benefits of intermittent fasting is surprisingly thin on the ground. Second, there are risks — in terms of bingeing on non-fasting days or missing vital nutrients — unless you’re sensible.

So while many may find intermittent fasting easier to stick to, as with all diets, it pays to keep a sense of perspective.