Big dieting names like Atkins, Ornish and Weight Watchers have long competed in the battle of the bulge. But researchers saw little difference when it came to weight loss after one year.
For years, people seeking to shed weight have heard conflicting messages about the best route: Low fat? Low carb? Low glycaemic index?
The analysis, published in the Journal of the American Medical Association, suggests it doesn't matter much.
Across 48 clinical trials of more than 7,000 people on diets like Atkins, Zone, Ornish and South Beach, researchers found minimal differences in average weight loss.
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Instead, experts said, the old-fashioned advice to cut calories, rather than specific nutrients, seems key to success, as is burning more calories through exercise.
And ultimately, the "best" diet for any individual is the one he or she can live with long-term, according to Linda Van Horn, a professor of preventive medicine at Northwestern University Feinberg School of Medicine in Chicago.
"The most relevant issue is to choose one that you can stick to indefinitely, since weight loss is only half the battle," said Van Horn, who wrote an editorial published with the study.
The ultimate victory
Unfortunately, maintenance is also the hardest part. Even though people in those 48 studies typically lost weight, they also started gaining some back by the one-year mark.
"People who follow either a low-fat or low-carb diet lose about 8 kilos, on average, over six months," said lead researcher Bradley Johnston, of the University of Toronto and McMaster University in Ontario, Canada.
By the one-year point, though, they'd gained back 2 to 4 pounds. People on more "moderate" diets – like Weight Watchers, Jenny Craig and Nutrisystem – lost slightly less weight, and also gained back a similar amount.
"We're not saying there were no differences among the diets. But the differences were minimal, and not enough to matter to the individual trying to lose weight." Johnston said.
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Johnston agreed that with such small differences, the best weight-loss choice is the one you think you can stick with.
"Choose the one that gives you the fewest challenges as far as adherence," he said.
But as far as researchers are concerned, Johnston said, "What we really need to understand is, how can people best maintain the initial weight loss?"
For the study, Johnston's team analysed data from the clinical trials testing various diets, sometimes in combination with exercise and behavioural counselling.
Some studies included people who were obese but healthy, in others, people had obesity-related ills, like type 2 diabetes or heart disease.
Many of the studies focused on lower-carb diets, like Atkins, South Beach and Zone, where people were told to get, at most, 40 percent of their calories from carbs.
Low-fat diets, like Ornish and Rosemary Conley, required people to get no more than 20 percent of their calories from fat, and about 60 percent from carbs.
At six months, people in those trials lost a few pounds more than people in studies of Weight Watchers and other more moderate diets, which capped fat intake at about 30 percent of daily calories, and carbs at 55 percent to 60 percent.
Across the studies, people generally lost a few extra pounds if the programme explicitly told them to exercise, or offered behavioural counselling at least twice a month for the first three months.
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According to Van Horn, the findings show there is nothing "magic" about cutting carbs or fat, or adding protein. "The laws of thermodynamics still apply," she said. "Weight loss happens when you consume [fewer calories] than you need. Increasing physical activity helps to lose that weight more steadily, but only if you do not compensate by eating more."
But while shedding pounds is healthy for obese people, it's not everything. "Our study looked only at weight loss," Johnston pointed out. "So we're not necessarily talking about what's best for your health."
Van Horn agreed. "Good health is more than weight loss or weight control," she said.
People need to eat a variety of foods – including fruits, vegetables, fibre-rich grains, lean proteins and unsaturated fats (from sources like vegetable oil, fatty fish and nuts) – to get the nutrients that support good health, Van Horn said.
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