The study, by researchers at the Duke University and Northeastern University in US, offers a sober insight about the complexities of weight loss and potential limitations of an app-based approach.
The inexpensive and easily accessed tool was aimed at tech-savvy adults ages 18 to 35.
"Thirty-five per cent of this age-group is overweight or obese, and that's a huge public health problem," said lead author Laura P Svetkey, professor of medicine at the Duke University School of Medicine.
The randomised study included 365 people ages 18 to 35 who were overweight or obese.
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One group of participants used a free Android app called CITY (Cell Phone Intervention for You), which was designed exclusively for the study.
Like many commercially available cell phone apps, CITY could be used to track calorie intake, activity and weight loss goals, and also offered weight loss tips and opportunities to connect with other users for social support.
In a separate arm of the study, participants received personal coaching from a weight loss coach - a model of behavioural intervention that some studies have shown to be more effective but costly, Svetkey said.
Coaches met with participants weekly for six weeks, and then followed up with monthly phone meetings.
Members of the coached group lost more weight on average than both the control group and the cell phone group - about 8 pounds after 12 months, compared to about 5 pounds in the control group.
All participants were permitted to download other commercially available weight loss apps such as MyFitnessPal during the two-year period.
But there was no significant difference in weight loss between those who used the CITY app and those who added commercial apps, Svetkey said.
The study was published in the journal Obesity.