Eat very little for a couple of days, then eat whatever you'd like for the rest of the week, and lose weight… Sounds easy, but if you're seeking weight loss, does intermittent fasting stack up? Read more in our Hot Topic.
What is it?
The idea of reducing your kilojoule (energy) intake from food to lose weight is nothing new, and while fasting has been a part of many religions for centuries, doing so on some, but not all days of the week for health is a relatively new concept.
Popularised by diet programs like the -5:2 diet,' a typical week of intermittent fasting might involve two (non-consecutive) days of eating around 25 per cent of your energy requirements, then eating normally on the remaining five days of the week. Other programs are more arduous, and suggest -fasting' every second day.
Energy requirements are individual, based on factors like height, age, gender, and activity level. But in general, a fasting day involves eating and drinking around 2,000 – 3,000 kilojoules worth of food and drinks. To put this in context, two boiled eggs, a slice of wholemeal toast, and an apple amount to around 1,000 kilojoules, so it's clear to see a fasting day wouldn't involve much food.
What does the evidence say?
Supporters of intermittent fasting point to weight loss, and improvements in risk factors for chronic disease, like reduced cholesterol, as reasons to follow the diet. But while the current body of evidence tells us that while intermittent fasting diets may lead to weight loss, it's unlikely they're superior to a standard balanced reduced-kilojoule diet.
Research presented at DAA's 2016 National Conference compared the 5:2 diet to a standard reduced kilojoule diet, amongst a small group (24) of obese male war veterans. After six months, researchers reported both groups lost weight, and reduced their waist circumference, but there was no significant difference between the two groups.
Interestingly, weight loss slowed at the three-month mark for both groups, which was when dietitian follow-up tapered out, showing support may be the key element in continuing success.
Another recent study, published in the Journal of the American Medical Association (JAMA), compared alternate-day fasting to standard kilojoule restriction, to assess whether the former was more effective in weight loss and maintenance. The randomised control trial divided 100 obese adults into one of three groups - an alternate day fasting group, a kilojoule restriction group, or a control group with no intervention.
The group was followed for 12 months, and researchers found similar results for weight loss amongst the alternate-day fasting group, and those in the kilojoule-restriction group. There were also no significant differences between the intervention groups in blood pressure, triglycerides, fasting glucose or insulin, and insulin resistance.
Interestingly, the alternate-day fasting group also had a high dropout rate, and struggled to meet their recommended intake, while the kilojoule-restricted group generally met theirs.
Why do it?
Evidence shows intermittent fasting is as effective (but not better) than a standard reduced-kilojoule diet for weight loss, and may be useful for weight maintenance. Daily dieting is challenging, and for some people, the idea of -watching what you eat' for only a set number of days, or hours in a week is mentally more appealing than other alternatives. The process of doing so may also encourage a more -mindful' approach to eating, with followers being more attuned to hunger and fullness cues.
Why shy away from it?
Intermittent fasting focuses on kilojoule-counting (quantity), rather than the kinds of foods eaten (quality). While being a healthy weight for you is one measure of health, making small tweaks to your current diet, like focusing on eating more fruit and vegetables, can lead to big changes in your health. Given that less than seven per cent of Australians currently meet the target of five serves of vegetables each day, there are clear improvements we can make to our everyday eating patterns – and for most Australians, this should be the focus.
It's also not suitable for everyone. Children and teenagers, athletes and highly-active people, those with diabetes, pregnant women, and people with eating disorders should all steer clear of fasting diets. Intermittent fasting diets may also lead to some unpleasant side effects, like fatigue, feeling foggy-headed, affected mood, feeling -hangry,' constipation, and headaches.
The bottom line:
In reality, any -diet' that encourages an energy deficit – that is, means you take in fewer kilojoules than you expend through exercise and daily activities, will result in weight loss. But for lasting, long-term health benefits, it's best to find an eating pattern that you enjoy, and can stick with.
We're all different, and there is no one-size-fits all approach when it comes to eating. While intermittent fasting may suit some people, it's not a magic bullet to improved health.
For nutrition advice tailored to you, see an Accredited Practising Dietitian. They're passionate about translating the latest evidence on healthy eating into practical, everyday advice and tips, and can support you on your journey towards better health.