People who sign up to an online weight loss program may find the only thing that gets lighter is their wallet, a Choice survey says.
Choice put five of Australia's most popular online diets to the test but found many were not only expensive but unrealistic.
The Ashy Bines Bikini Body Challenge, SureSlim, Weight Watchers and The Biggest Loser Club - which demand fees of up to $83 a week - were all put to the test.
But many eating plans were not based on scientific evidence, were overcomplicated and did not fit with a healthy, balanced diet.
In the Ashy Bines program, the listed nutritionist isn't even a registered dietician, Choice says.
"For the amount of money you're spending, you'd be better off seeing your GP or an accredited dietician," Choice spokeswoman Kate Browne said.
Many online diets champion "crash weightloss" instead of long-term health, she said.
The Michelle Bridges program, costing $199 for 12 weeks, fared best, but the survey noted customers might not be able to keep up with the prescribed exercise regime without a personal trainer of their own.