Intermittent fasting diets, like the 5:2 or 16:8 eating plans, have a steadfast reputation for enabling dieters to shed unwanted kilos. But according to a leading physician-scientist and nutrition expert, that’s only half the story.
Professor Luigi Fontana – one of the experts behind Dr Michael Mosley’s 5:2 diet – stresses that intermittent fasting, where you restrict calorie intake for a specific period of time, could help you to live a long life, free of illness and disease.
“There is now no doubt that the most powerful intervention to slow ageing and prevent the build-up of metabolic and molecular damage is calorie restriction without malnutrition,” writes Prof Fontana in his new book, The Path to Longevity.
However, Prof Fontana explains, there is one main issue with current fasting practices – many people, believing that a false set of rules apply to calorie restriction, are doing it wrong.
Here is some expert advice from Prof Fontana, director of The Healthy Longevity Research and Clinical Program at the University of Sydney, on how to practice healthy intermittent fasting the right way.
“There is now no doubt that the most powerful intervention to slow ageing and prevent the build-up of metabolic and molecular damage is calorie restriction without malnutrition."
1. Eat healthily, always
Unfortunately, the 5:2 eating plan can be misinterpreted as an intermittent fasting method that allows dieters to eat 500 calories during fasting periods and then chow down on whatever food they want, whenever they want on five non-fasting days.
But as Prof Fontana explains, that’s not how fasting for weight loss and longevity is meant to work. “Fasting is just one nutritional tool you have to lose weight and improve your health but it’s not a magical fix,” Prof Fontana tells SBS.
“You can’t eat junk food five days a week and then eat 500 calories a day for two days a week, and stay healthy and live longer.”
He says this is partly because highly processed foods can affect your gut microbiome and your metabolic health. That, in turn, can increase your risk of developing diseases, including cancer and many inflammatory and autoimmune disorders.
Instead, Prof Fontana advises dieters to always try to eat healthily – substitute processed foods with plenty of foods rich in vegetable fibre – during both fasting and non-fasting periods.
“What we are finding is that the composition of the diet is just as important as calorie restriction or fasting in [good health],” he tells SBS. “A healthy diversity of food – not in junk – is extremely important. The content of the proteins you eat is also very important.
Ideally, Prof Fontana says, people should always try to adopt a Mediterranean-style diet which includes lots of whole grains, vegetables, legumes, nuts and seeds. "They should also drastically reduce their consumption of meat, dairy and all highly refined foods.”
“What we are finding is that the composition of the diet is just as important as calorie restriction or fasting in [good health]."
2. Eat as many non-starchy vegetables as you like when you fast
Another misunderstanding about intermittent fasting plans is that dieters must stick to eating a set restricted-calorie amount when fasting.
But Dr Fontana says dieters don't have to worry about counting calories when eating non-starchy vegetables: you can eat as many non-starchy vegetables as you like during a fasting period. This will increase your feeling of fullness, maximise fibre intake and keep your calorie intake low, naturally.
Dr Fontana tells SBS he has led clinical randomised studies in the USA (currently unpublished) involving volunteers who fasted two to three times a week. He writes in his book that the results demonstrate that vegetable fasting works.
“Participants can consume non-starchy raw and cooked vegetables, dressed with a single tablespoon of extra-virgin olive oil, lemon juice or vinegar, at lunch and dinner,” Prof Fontana writes.
“We have calculated that by allowing unrestricted eating of different types of vegetables, daily calorie intake is never above 500 calories, which is equivalent to a weekly calorie restriction of about 20 to 23 per cent.”
In other words, by eating as many non-starchy veges as you like, it’s highly unlikely you’ll ever overdo it.
3. You don’t have to stick to eating 500 calories
Although consuming 500 calories a day when fasting will prove effective for some people following the 5:2 diet, Prof Fontana says, it may not work for everyone. People of different body masses will have varying starting points for weight loss and therefore different calorie restriction recommendations.
“It is also likely the calorie requirements needed to maximise health vary from person to person, and by factors like age, gender, levels of physical activity and genetic constitution,” he writes.
Five hundred calories of vegetables are also not equal to the nutritional content of 500 calories of fried food.
The nutritional content of food can always vary as well, even if it’s the same food. For example, Prof Fontana explains, an apple grown in Spain will not bear the same nutritional content as an apple grown in NSW because of differences in freshness and growing conditions.
So rather than aiming to eat 500 calories a day on fasting days, Prof Fontana recommends dieters drop 20-30 per cent of their usual calorie consumption per week. This is a more effective way to restrict calories and generate the weight loss and health benefits associated with fasting.
However, Prof Fontana warns against practising excessive, unhealthy calorie restriction.
“We [also] need to be careful not to overdo calorie restriction; it can be dangerous, especially if you don’t know what you are doing,” he writes in his book.
“…Recent experiments indicate we probably don’t need to undergo a severe regimen of calorie restriction to live a long and healthy life. A combination of other less drastic interventions may lead to similar or better results.”
It is always recommended to consult a GP before starting a diet and following medical advice.