"It's about accountability, knowing what you're eating and how much, and how that all adds up compared with your calorie goal for losing weight," she tells WebMD.
The study appears in the latest issue of the Journal of the Academy of Nutrition and Dietetics.
Journal keepers lost more weight
The study included 123 previously inactive, overweight, postmenopausal women in Seattle enrolled in a weight loss study.
Over the course of a year, the women followed a restricted-calorie diet with the goal of achieving a 10% reduction in weight in six months. Half the women were put on an exercise program and the other half were not.
All the participants were asked to record the foods they ate daily in seven-day diaries provided weekly by dietician counselors.
During the study, the women also completed a series of questionnaires designed to assess their individual eating-related behaviors and strategies to achieve weight loss.
At the end of the year, both the diet-alone and diet-and-exercise groups had lost an average of 10% of their starting weight.
Meal skippers lost less
Among the specific findings:
Women who consistently filled out the food journals lost about 6 pounds more than those who didn't.
Those who skipped meals lost an average of 8 fewer pounds than those who didn't.
Women who ate in restaurants at lunch at least once a week lost an average of 5 pounds less than those who ate out less.
"Eating out may be a barrier for making healthful dietary changes because it usually means less individual control over ingredients and cooking methods, as well as larger portion sizes," the researchers wrote.
Journal keepers avoid mindless eating
The study is not the first to find that keeping a food journal helps people shed pounds.
A 2008 study found that dieters who kept food diaries at least six days a week lost twice as much weight as those who kept the journals one day a week or less.
Keeping a food diary helps increase awareness of mindless, distracted eating, says nutritionist and diabetes educator Megrette Fletcher, RD, who is also co-founder of The Center for Mindful Eating.
"We know that when people keep food journals they are more aware of what they eat and in what quantities," she tells WebMD. "Whether the goal is to lose weight, keep diabetes under control, or just to avoid eating when you are not hungry, food journals can help."
Most experts recommend writing down the foods you eat as soon as you eat them, rather than waiting until the end of the day.
Some other tips:
It may also help to write down what you were doing when you were eating and how eating made you feel.
Record your level of hunger along with the foods you eat.
Be honest: Keeping a journal will do you no good if you only do it when you are being virtuous. Record the food slips along with the food triumphs.