Fasting: Is it safe?

As far as diets go, it sounds like a pretty good deal: eat whatever you want except for two days a week, when you power through a part-time fast designed to stoke your body’s fat-burning furnace

Damien Bennett

It’s a trend that’s poised to have as much staying power as the topknot: the 5:2 diet. Also known as intermittent fasting, this on-again-off-again approach to dieting asks you to fast (well, actually, you eat a meagre 2100kJ to get through the day) twice a week. On the other five days, you can eat what you like.

Fans of the diet claim you’ll drop about half a kilogram every week – and there’s no need to obsessively count kilojoules or measure out your portions before every meal. Intermittent fasting could even strengthen your defence against diseases like diabetes, heart disease and cancer – and call time on the ageing process.

Here at Women’s Health, we’d never recommend an unsafe crash diet. Yet we were intrigued by the research we’d seen on this method of eating. Here’s what we discovered when we took a closer look…

Though the buzz on intermittent fasting has only recently reached fever pitch, there’s nothing new about the notion of fasting for health. Praised by the likes of Socrates and Plato for its power to clear both the body and mind of toxic influences, fasting has long been central to spiritual traditions like Ramadan and Lent. Hippocrates frequently prescribed fasting for all manner of illnesses and, in the early 20th century, doctors such as Frederick Madison Allen (a physician at the Rockefeller Institute for Medical Research, US) and H Rawle Geyelin (an endocrinologist and professor of medicine at Columbia University, US) began devising fasting plans to treat chronic conditions like diabetes and epilepsy.

By the 1930s, the ongoing development of medicines had effectively blunted the popularity of therapeutic fasting, but in the mid-1940s scientists began exploring the concept of intermittent fasting, with preliminary studies in mice and other lab animals hinting that periodic, short-term fasts could lengthen life span.

A modern spin

Fast forward to August 2012 in the UK, when the BBC airs a special called Eat, Fast and Live Longer. In the program, medical journalist Dr Michael Mosley tries intermittent fasting and successfully loses more than 6kg and seven per cent of his body fat in just five weeks.

More than 2.5 million British viewers tune in to watch the program, and millions more around the world see it when it’s posted online. A trend is born. Within a few months, numerous books about intermittent fasting hit the shelves (including Mosley’s bestselling The Fast Diet). Each takes a slightly different approach, but most of them revolve around the idea that by eating a mere 2100kJ (2500 for men) – about 20 per cent of your body’s normal energy needs – on just two days a week, you can otherwise eat how you like and still lose weight, improve your cholesterol levels and possibly even prevent cancer.

Devotees say it’s easier to stick to a part-time diet than it is to count kilojoules every day for weeks on end. Another bonus – this is one of the few weight-loss plans where you’ll actually save money. Fans of intermittent fasting also say it results in greater fat loss than you’re likely to achieve in conventional dieting. Since fasting is thought to promote ketosis (a state in which the body burns fat for energy), intermittent fasting is touted as a way to accelerate the fat-zapping process and burn off flab instead of your hard-earned muscles.

A new crop of research is helping fuel these claims. In a 2007 report published in the American Journal of Clinical Nutrition, researchers from the University of California at Berkeley, US, sized up the available science on alternate-day fasting and found a good deal of animal-based evidence that the approach could help keep blood sugar in check, cut cholesterol, lower heart rate, bring down blood pressure and stave off cancer.

There have been several human trials, including a 2012 study published in the journal Nutrition & Metabolism, in which 54 obese women were placed on an intermittent fasting program for 10 weeks. By the end of the study, they showed a significant drop in weight, waist size and fat mass – and improved their cholesterol and inflammation levels.

As for fasting’s potential anti-cancer benefits, it has to do with a hormone called insulin-like growth factor 1 (IGF-1) that prompts your cells to grow, divide and reproduce.

“The problem with IGF-1 is that once we’re fully grown, we don’t need a lot of the new cells it’s telling the body to make,” explains Dr Valter Longo, a fasting researcher and director of the University of Southern California’s Longevity Institute in the US. “So we have all these cells constantly in ‘go’ mode, which causes them to age much more rapidly. It’s like if you tried to race your car all the time – how long will that car last?”

Research from Longo’s laboratory and other centres has found that fasting can drive down levels of IGF-1, which may help keep cells youthful and fend off ageing-related disease. Who doesn’t want that?

Not so, erm, fast…
If intermittent fasting sounds like the perfect diet, keep in mind there’s still a lot that researchers don’t know. For one thing, there’s not much science to support the claim that eating 2100kJ actually counts as fasting. However, “It’s a modification that could make it easier for people to stick with the diet in the long term, which is always a good thing,” says Dr Leonie Heilbronn, a University of Adelaide associate professor who’s headed up several studies on alternate-day fasting.

Experts also haven’t agreed on the best way to divide up those 2100kJ. Should you eat three small meals, and we mean very small (see “What 2100kJ looks like” below); or should you have all your food at breakfast and go without for the rest of the day? Heilbronn notes it’s possible to reach ketosis through intermittent fasting, but going food-free for just a few hours won’t do the trick.

“You switch to ketosis in order to preserve glycogen (the body’s prime source of stored energy), and that doesn’t happen until about 18 hours into a fast,” she explains. “If this is how intermittent fasting works for weight loss – and we don’t yet know that it is – then fasting for less than an entire day may block that response.”

For this reason, Heilbronn suggests that dieters may want to polish off those 2100kJ at the start of the fasting period – then limit their intake to kilojoule-free water and tea for the rest of the fast.

But Mosley argues that the most important thing is for dieters to find an approach they’ll stick with – so they should split up the 2100kJ however it works best for them. “On my fasting days I’ll have a breakfast of eggs and ham, drink lots of fluids throughout the day, and then for dinner have a small piece of meat and some vegetables,” he says. “It really all depends on how you cope and what feels comfortable to you.”

Another concern that both Longo and University of Manchester, UK, research dietitian Dr Michelle Harvie point out is that the brief bouts of fasting recommended in some intermittent-fasting plans won’t have a huge effect on IGF-1. “There seems to be a minor drop in IGF-1 levels after a full day of fasting, but going for two days in a row will cause a much bigger decrease,” says Longo.

He’s also concerned that intermittent fasting could sap your health by throwing off your body clock.

“Just like if you were to sleep normally one night and then not sleep at all the next, constantly shifting from eating to fasting is going to be very confusing for your body,” Longo says.

But since less frequent, longer periods of fasting have been found to offer a host of benefits – including increased protection against heart disease, diabetes and cancer – Longo recommends trying a four-day-long fast once or twice a year.

“One of the big advantages of periodic fasting – apart from reprogramming your body in a very powerful way that clears out the junk and switches you over to anti-ageing mode – is that it detaches you from the idea that you need food all the time, which can have a long-term impact in terms of improving the way you eat,” he says.

Lastly, there’s a possibility women’s bodies could respond differently to intermittent fasting than men’s – the same with overweight women versus slim ones. “More research is needed,” says Heilbronn. She conducted a study with eight women and eight men who had BMIs in the normal range and found that intermittent fasting caused a bigger spike in insulin levels in the women. (Insulin controls how your body uses glucose, so these results offer preliminary evidence that your cells might behave differently when you feast and fast, compared to a bloke.) What’s more, Heilbronn cautions that intermittent fasting might mess with your reproductive health.

“We know that severe energy restriction can lower your chances of getting pregnant, so women who are trying to conceive should probably avoid any type of fasting,” she says.

Because of these unknowns, some institutions – including the National Heart Foundation – have warned against the 5:2 diet altogether, deeming it a fad diet with no proven benefit and little regard for nutritional needs. And even those who approve of the diet agree that you shouldn’t try it if you have a history of an eating disorder or are pregnant. If you have a chronic health condition (abnormally low blood pressure or irritable bowel syndrome, for example) you should consult your GP first.

The bottom line

Some of the science is sketchy, but the diet might be worth a try if you’re looking to drop a jeans size or two. “From the studies we’ve seen so far, there does seem to be some weight-loss benefit and maybe even some health benefits to intermittent fasting,” says GP and Women’s Health medical advisor Dr Ginni Mansberg.

Cautioning that studies on intermittent fasting’s long-term health effects are currently lacking, Mansberg says going a day or two without eating won’t be dangerous or mess up your metabolism if you’re otherwise healthy. Be aware, though, that many dieters report irritability, fatigue, light-headedness and difficulty sleeping.

If you do decide to give it a go, tuning in to your body’s response is key to thriving on an intermittent-fasting plan.

“The hardest part is the first couple of weeks, because your body’s not used to this type of diet,” says Mosley. “Eventually, you’ll find you can tolerate hunger much better than expected.”

Although some versions of the 5:2 diet claim you can eat whatever you want on your non-fasting days, Mansberg and Mosley suggest a more mindful approach. “If you think you can fast for a day or two and then have carte blanche to binge for the rest of the week, that’s probably a mistake. You still have to avoid overeating, watch your nutrition and get plenty of exercise for this to work,” Mansberg says. “Interestingly, in most of the studies we’ve seen so far, participants were quite well behaved on their days off from fasting,” she adds.

This has been Mosley’s experience, too. “After being on the diet for about a month, my food preferences shifted and I was more likely to satisfy my hunger with vegetables instead of unhealthy snacks,” he says.

You may want to save your arse-kicking boot camp class for a non-fasting day, however. This doesn’t mean you can’t get out there and move at all, though, says Mansberg, “Just stick to moderate intensity exercise on your fast days,” she says. “Make sure even though you’re eating very little, you’re still drinking enough water. It can be easy to dehydrate, especially if the weather is warm. If you feel light-headed, nauseous or unwell while you’re fasting, see your GP.”

Ultimately, the biggest benefit of intermittent fasting could be the way it changes your relationship with food and reconnects you to your body’s hunger cues. “When we were living back in the caves, we’d be lucky to get one meal a day, since we’re actually not that great as hunters,” Mansberg says. “Now we’re surrounded by food and can eat well all the time.” Mosley agrees. “So many people have told me they used to fear hunger and would overeat to prevent it. But getting hungry is OK – it’s normal, in fact – and it passes.”