North America’s weight-loss market was estimated at more than $66 billion US last year, including the Atkins, South Beach and Zone diets, but researchers say it’s unclear how well the diets promote sustained weight loss and improve cardiovascular risk factors.
In Tuesday’s issue of Circulation: Cardiovascular Quality and Outcomes, published by the American Heart Association, cardiologist Dr. Mark Eisenberg of Montreal’s Jewish General Hospital and his team reviewed 10 randomized control trials that were at least 12 months long.
"I was shocked to find how little evidence there was to support these interventions," Eisenberg said.
"These diets are modestly effective, so they do work, but they are not a panacea for weight loss."
In the two trials that involved head-to-head comparisons between the popular diets and usual care, no diet really stood out, Einsenberg said. He defined "modestly effective" as the loss of less than five per cent of body weight in one year.
In trials comparing Weight Watchers to usual care, Weight Watchers dieters lost an average 3.5 to 6.0 kilograms after one year compared with 0.8 to 5.4 kilograms with usual care. But at two years, the weight lost was partially regained.
Usual care refers to traditional methods to promote weight loss such as low-fat diets, behavioural weight loss intervention, nutritional counselling, or self-help materials.
There are individuals who commit to lose weight and keep it off through a combination of diet and more physical activity, but Eisenberg said that at a societal level, a more holistic approach could help.
"We've seen a lot of public health interventions for smoking cessation over the past many decades and I think that may come over the future decades in nutrition."
He pointed to:- Massive advertising that is hard to escape.
- The ubiquity of fast food and processed foods.
- The high percentage of caloric intake in adolescents from soft drinks.
- Design of many urban spaces that doesn’t promote exercise or walking.
- The lack of availability of nutritious foods in inner city areas.
Previously, Eisenberg and his colleagues have suggested taxing junk food and changes to agricultural subsidies, which he thinks promote obesity in North America.
A broader lifestyle intervention that involves doctors and other health professionals may be more effective than popular diets alone, he said.
The research was funded by the Canadian Institutes of Health Research.