Fasting diets might be in fashion, but are they healthy?

By CASSIE WHITE.

Not so long ago fasting was a nutrition no-no. But a growing number of us are abstaining from food for short periods, and experts say it’s not such a bad thing.

We’ve long been told slow and steady wins the race when it comes to weight loss, with nutrition experts cautioning against skipping meals and drastically cutting kilojoules.

These days fasting is in fashion. Not only is drastically cutting kilojoules supposed to help you to lose weight, there are also claims it reduces your chances of developing some chronic diseases and may even extend your life. But are fasting diets just another nutrition fad?

Associate Professor Amanda Salis, from the University of Sydney’s Boden Institute of Obesity, Nutrition, Exercise & Eating Disorders, says research has found some forms of fasting really can help some of us to lose weight. In particular, there is strong evidence behind the popular intermittent fasting (IF), where you fast a few days of the week, then eat normally on other days.

“About 30 studies have looked at IF diets in healthy overweight or obese adults and they’ve consistently found that if you restrict energy intake periodically over time then you do lose weight,” she explains.

"The thing about the research with these diets is that there are so many variations of IF that are used and are successful, so there's no one best way – it's trial and error. But popular fasting diets are 5:2 (eat normally five days and fast for two) and alternate day fasting."

The 5:2 diet advocates limiting your kilojoule intake to about 25 per cent of that normally recommended for healthy weight adults – about 2100 kilojoules (500 calories) for women and 2500 kilojoules (600 calories) for men – for two non-consecutive days a week. On the feasting days, you can eat whatever you want. As the name suggests, alternate day fasting involves eating whatever you want one day and consuming no or limited kilojoules (e.g. the same as 5:2) the next.

Not best for everyone

But before you clear out the fridge, Salis says fasting is not necessarily going to work for everyone.

"There's no such thing as one-size-fits-all when it comes to weight management and some people suit a slow and steady approach with mild energy restriction over a longer period, while others prefer a short, sharp shock tactic," Salis says.

"Studies have looked at the effects of intermittent fasting regimes versus reducing kilojoules overall and the benefits of both strategies appear to be equal," she explains.

"We've looked hard to see if there's an extra magical thing about IF that somehow revs up your metabolic rate or makes you less hungry, but from the data so far there doesn't seem to be any evidence."

In fact, there have been some concerns that fasting can actually slow your metabolism, which can lead to weight gain when you start eating a normal diet again.

"A reduction in metabolic rate happens regardless of how you lose weight," Salis explains.

"The threshold is different for everyone and depends on how much fat you have on your body, how long you've been carrying it for, your genes, the types of foods you eat and how much exercise you do."

Most research on metabolic rate and IF has been done after 12 weeks on the diet, but Salis says some studies have found it happens after four.

Other health benefits

While the benefits of fat loss have a knock-on effect in terms of reduction in cholesterol and triglycerides (blood fats), which then reduce your risk of heart disease, Salis says most other supposed health benefits of intermittent fasting don't stack up.

"As for prevention of disease, cancer and helping us to live longer, there is some evidence in small animals like insects and mice, but as yet there's no evidence that it does these things in humans," she says.

"There haven't been definitive studies finding it helps people live longer because for those studies you need to wait about 20 years and count how many people die, and it's too early for that," Salis explains.

"There are indications that, like with weight loss, IF reduces risk factors for disease and death – therefore you'd expect it will prolong life. But we can't yet know for sure."

Preparing to fast

Salis strongly urges you see your GP or an Accredited Practicing Dietitian for guidance before starting any fasting regime.

Key points to keep in mind:

- Fasting day kilojoule restrictions in the 5:2 diet are 2500kJ (600 calories) for men and 2000kJ (500 calories) for women.

- Your intake can come from meal replacement shakes or small, healthy meals.
- You can spread your kilojoules out over the entire day or have them in one meal.
- It doesn't matter if you fast on consecutive or alternate days. The 5:2 diet recommends fasting on non-consecutive days, such as Tuesday and Thursday.
- Pregnant or breastfeeding women or people with underlying illnesses should not try a fasting diet.

You can eat one or two small meals while fasting, so long as these are less than the recommended kilojoules. But Salis recommends people try meal replacement products because it's so difficult to get all the nutrients you need on 2000kJ a day or less.

"I recommend using a meal replacement formula that contains the important micro and macronutrients you need in your diet and having three of those a day," she says.

Feasting days

Many advocates of intermittent fasting – including medical journalist and one of the creators of the 5:2 diet Dr Michael Mosley – say one of the benefits of this eating regime is that you can eat whatever you want on non-fasting days. There's also some research to show people can eat whatever they want on non-fasting days and still lose fat, which is a major drawcard for most dieters.

However, many experts are concerned with the notion of 'eat whatever you want on non-fasting days', and Salis warns intermittent fasting isn't a licence to eat burgers with wild abandon.

"It has been shown in clinical trials – which is different to the real world because people are very accountable in trials – that when people are told to eat whatever or as much as they want, they don't eat as much as was expected," she explains.

"It may also have something to do with that short, sharp shock of severe kilojoule restriction that resets appetite or makes you more sensitive to your body's fullness signal. But more research needs to be done on that.

"And when it comes to cardiovascular risk, studies showed that people who ate junk food on the non-fasting days were significantly worse off than those who didn't."

Psychological benefits

Salis does IF once a year for a month, and says fat loss aside, fasting has some unexpected benefits.

"One thing that I've noticed is that fasting makes you more able to handle life in non-food ways," she explains.

"It's such a habit to turn to food: you reach for something at work when you're stressed - it's so easy to grab chocolate or a biscuit or a milky coffee.

"Food becomes a crutch, but fasting teaches you other ways to cope with life's difficulties and I think that's a real pro."

Health risks

As with all diets, Clare Collins, professor of nutrition and dietetics at the University of Newcastle, says fasting can cause serious health consequences for some people.

"There have been concerns around risks for some people on certain medications or with particular medical conditions – fasting might make some conditions worse," she warns.

"If you're on medication for high blood pressure or type II diabetes, you may need a different medication regime on that day or a whole change of medication. So talk to your GP; don't try it alone.

"The other concern is that in some susceptible people, it can stir up their liver as the fat starts emptying out of it. This can actually make the liver more inflamed and trigger liver disease. A similar thing can happen in the gallbladder, too."

Types of fasts

Diagnostic fast: done before medical testing. You refrain from eating or drinking anything but water for 8-12 hours before the test. Tests that require a fast include those for diabetes and blood lipid levels.

Therapeutic fast: restricting kilojoule intake to approximately 25 per cent of what you need to maintain your body weight for a period of weeks, typically 8-16. This is medically supervised and is used for people who are obese or who are overweight with risk factors.

Juice fast: consuming nothing but fruit or vegetable juice for several days or weeks. "There's no reason why we need them to cleanse our bodies," Salis says.

This post originally appeared on the ABC and was republished here with full permission. 

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