The study, from the University of Wisconsin-Madison, followed 1,700 overweight people over six months, and revealed that those who wrote down everything they ate lost an average of 13lb.
"Recording what you eat can really help you lose weight, because a lot of the time, you're not conscious of your behavior," Times Online quoted Azmina Govindji, a consultant nutritionist and author of The Hot Body Plan, as saying.
"Some eating habits are situational - for example, having tea and a chocolate biscuit while watching your favorite television programme. We do it on autopilot. If you know that you actually have to record every item you eat, it gives a split-second pause for thought. You think, 'Do I really want to write this down.'
"Through the use of a food journal, we often become aware of patterns that are self-destructive.
"We become more authentically honest with ourselves, and we start asking the question 'What's eating me?' instead of 'What can I eat?' " Govindji said.
Even people who believe that they are resolutely healthy can get a shock when they see everything written down in black and white.
"Once someone spots a pattern, then they can do something to change it," Govindji said.
The most common patterns she uncovers are in those who eat naughty foods because they are bored, hormonal or depressed.
The study is published in the American Journal of Preventive Medicine.