If crash diets help patients lose weight, should we warn against them or not? And find out why it’s so incredibly hard to maintain weight loss long-term.

Doctors and health educators advise their overweight patients to make changes to their eating and exercise habits that will allow them to take off one to two pounds a week.

To lose more than that, patients must resort to drastic measures that can be risky in the short term and may prevent them from developing the exercise and eating habits they’ll need to keep the weight off. The faster it comes off, the theory goes, the faster it gets put back on.

But a group of Australian researchers is challenging the conventional wisdom. They compared obese patients who followed standard advice, eating a balanced diet with a reduced calorie count, to those who took in just 500 to 800 calories a day, entirely from low-carbohydrate meal replacement beverages.

More people in the “rapid weight loss” group successfully lost weight, and they were not more likely to gain it back within two years. The findings were published this week in The Lancet Diabetes and Endocrinology.

Eighty percent of the patients on the study’s rapid weight-loss plan of 500 to 800 calories per day met their weight-loss goals. Just half of their counterparts who followed conventional weight-loss advice did. The same percentage of patients in both groups gained their weight back within two years.

“There are many ways of losing weight, but the important thing of obesity management is keeping the weight off after,” said senior author Joseph Proietto, a professor in the department of medicine at the University of Melbourne.

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Many doctors and nurses strongly object to the idea that an extremely low-calorie diet, particularly one you drink from a can, is safe for dieters.

Proietto, who served for several years on the board of the company that makes the diet meal replacement used in the study, says that Americans have such a strong reaction because in the 1950s crash diets based on meal replacement beverages led to several deaths. Unlike those products, the beverage in his study is carefully formulated with a “long list of micronutrients,” making it safe, he argued in a previous paper.

“You can’t just eat anything, you’ve got to be careful,” Proietto said, cautioning those who would take his study as permission to engage in crash diets without doing their homework. He also notes that the study volunteers weren’t trying to lose weight for cosmetic reasons; their weight posed a serious health risk.

Katie Ferraro, a registered dietitian on the clinical nursing faculty of the University of California, San Francisco, is skeptical about relying long-term on a diet that offers 800 calories or fewer per day and is low enough in carbohydrates to induce ketosis. Ketosis is a metabolic state — normally temporary — in which the body more readily burns fat. Experts disagree about whether inducing ketosis for weight loss poses health risks.

“Ketosis is a dangerous condition if it’s for a prolonged period of time. You start messing with the pH-balance of your body. That’s not sustainable for a long period of time,” Ferraro said.

But even with those concerns, Ferraro said she might be less inclined to talk obese patients out of crash diets in the wake of the Australian findings.

“It’s eye-opening for me to say that for certain patients rapid weight loss might be a good option as opposed to just repeating this mantra which we always say, ‘Lose 1 to 2 pounds a week,’” she said. “There’s not a one-size-fits-all approach to weight loss.”

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Weight loss is a matter of months, while maintaining a healthy weight is a lifelong project.

The Australian study’s findings on maintenance are, dishearteningly, in line with other research. Among volunteers who lost weight on either diet in the study, 70 percent of them gained it all back.

For Proietto, the findings fall within a larger body of research that suggests permanently losing weight is so hard because biology and genetics fight against it. Proietto’s lab has pointed to the hormonal changes that occur during weight loss to make people hungrier and their metabolisms slower.

“[Having] good habits still doesn’t control the hunger that you get after,” he says. “Once you’ve lost the weight, it’s not the end of the road; it’s the beginning of the road.”

Traci Mann, Ph.D., a psychologist at the University of Minnesota who focuses on weight loss and body image, said it is “misleading” to conclude, as the Australian study does, that a very low-calorie diet works just as well as a conventional weight-loss diet.

“It works just as badly,” she said. “They both are terrible.”

Mann argues that dieting is a bad idea precisely because so many dieters regain the weight — and many gain even more weight.

“If health is what you’re interested in, you should measure health, not weight. You can improve somebody’s health whether or not they lose weight,” she said.

Ferraro isn’t as pessimistic about people being able to maintain their weight loss.

She pointed to the National Weight Control Registry, which analyzes the approaches of the minority of dieters who do manage to stay slim after a weight-loss program. Nearly all successful patients have adjusted both their diet and activity level, exercising on average for an hour a day.

In the success stories highlighted on the site, people talk about enjoying a healthy lifestyle, not just a smaller waist size.