Restrictive low-calorie ‘crash diets’ have long been criticised as ineffective at managing weight, but new medical research challenges the existing recommended dietary guidelines.

Whether you lose weight quickly or gradually has no bearing on your likelihood of putting weight back on, according to a new study released by the University of Melbourne this week.

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So, could the findings challenge the way obesity is treated around the world? And can a restrictive diet of 800 calories per day be healthy?

Very low calorie diets are safe to treat obesity, according to new research. Image: ShutterStock

Are low calorie diets safe?

The study’s lead researcher, Melbourne University medical professor Joseph Proietto, says low calorie diets have a stigma attached to them but are a healthy way to treat obesity.

“I think there’s a lot of that residual memory of those badly formulated diets that for years and years dieticians were taught to stay away from,” Professor Proietto says.

“Our diet is very well formulated to ensure that the patient receives all of the micronutrients and vitamins that are required to be healthy.”

Dieticians Association of Australian spokesperson Professor Claire Collins agrees there is a place for restrictive diets but warns they come with health risks.

“People do need to be monitored because some of the risks are things like gall bladder disease, constipation, headaches and nausea,” Professor Collins says.

“It’s not nutritionally adequate. Generally the person will need to take multivitamins.”

Not for everyone

Sydney University health professor and weight management expert Louise Baur warns the diet would be unsafe for people in a normal weight range.

“This is for people struggling with obesity. It’s not something to be adapted to healthy weight or overweight people. That’s not the message the study is giving,” Professor Baur says.

According to Professor Baur, obese patients may have been receiving unhelpful advice.

“I think a lot of clinics tended to be rather conservative with the advice given. We don’t recognise the level of support people struggling with obesity need to make some change,” Professor Baur says.

“What the research shows is that rapid weight loss overseen by a dietician in a healthy way is just as safe as traditional slower methods.”

She says the study challenges the idea that people who lose weight more quickly are more likely to regain weight later.

“There has been anxiety in the past that more rapid weight loss may lead to greater rebounds and poorer longer-term outcomes, but this study showed this is not the case.”

Rapid weight loss is effective but regular weight people shouldn’t try such restrictive diets. Image: Shutterstock

Can anybody do it?

Experts warn such restrictive low calorie diets should only be used for weight loss in people who are morbidly obese. In the case of the medical study, all the participants weighed over 100kg.

While the average weekly weight loss of 1.5kg was great for study participants, this would be unhealthy for someone in an overweight or normal weight range.

Professor Collins says a diet of 800 calories per day should only be used in certain cases, like when someone needs to lose weight in order to have life-saving surgery.

“For most of us, taking away packaged foods and alcohol is the best place to start in terms of reducing kilojoules.”

“For somebody who really want to lose weight, my advice is to have a kilojoule target and work towards achieving that.”

Rapid weight loss a motivator

The study also found people placed on very low-calorie diets were more likely to reach their goal weight than those who lost weight slowly.

Professor Proietto says people who lose weight quicker were more likely to stick to the diet program because they were motivated by the weight loss they saw.

“You can lose weight gradually, the traditional lifestyle way, but it’s a little bit harder than if you did it with a very low energy diet,” he says.

“The reason why is, for one, people are losing weight rapidly because they can see the weight coming off more obviously.”