Researchers discovered women who wrote down what they are lost six pounds more (2.7kg) than those who did not.
They concluded old-fashioned calorie counting was more important than the type of diet.
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Skipping meals was counterproductive, they also found, because it made dieters resort to unhealthy snacks instead. Those who skipped meals lost eight pounds less (3.6kg) than those who did not.
The American study team looked at dieting methods among 123 overweight woman aged 50 to 75.
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Dr Anne McTiernan, from the Fred Hutchinson Cancer Research Centre in Seattle, said: “For individuals who are trying to lose weight, the number one piece of advice based on these study results would be to keep a food journal to help meet daily calorie goals.
“It is difficult to make changes to your diet when you are not paying close attention to what you are eating.”
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The study is published in the Journal of the Academy of Nutrition and Dietetics.
Elaine Mealey, from the British Dietetic Association, agreed: “All the evidence shows that people who use a food diary not only lose more weight but also keep the weight off.”
More from the Telegraph.