"One of the factors which I credit most strongly with helping me to lose weight was keeping a food diary. Jim's written on Diet Blog before about how to keep a food diary and why, but I suspect there are plenty of dieters who think it's not worth the effort. Here's my five top reasons to write down what you eat:
Account for all those 'little extras'
Many people eat healthily at mealtimes, but snack poorly in between. If you're aiming to lose a pound a week, just a few extras (eg. a can of coke, a cookie and a packet of crisps) during the day prevents you from seeing results on the scales.
Writing down everything you eat demonstrates the cost of those 'occasional' nibbles...
Know when you can afford to treat yourself
Conversely, keeping track of your food intake over the course of a day or a week gives you the freedom to enjoy a treat once in a while - guilt-free. If you know you've got calories to spare for the day, and no chocolate has passed your lips for six days, you can indulge yourself!
Be aware of when you're eating
Keeping a food diary highlights patterns, showing if you overeat at particular times. Perhaps you binge late at night, because you've been eating too little all day? Or perhaps you graze constantly? If you don't keep a food diary because you have no hope of remembering everything you eat ... you may need to change your habits.
Fight 'portion creep'
Those of us who've been dieting or maintaining for a long time often get used to "eyeballing" portions rather than weighing everything out. But if your weight loss has plateaued, or if those pounds are edging back on, keeping a diary means you need to weigh your foods. That "medium" portion of pasta or rice might be bigger than you think...
See your habits changing
It can be motivational to look back on a food diary from a few months or even years ago and see how your nutritional choices have changed. Perhaps you've curbed your chocolate habit, or maybe you now eat proper meals instead of junk-food snacks. And if you're having a bad day, flicking back to a "perfect" week in your diary is encouraging: if you did it once, you can do it again!
Boost your self-control
Knowing you have to write down everything you eat makes you think twice about that donut, or that second plateful at a buffet. Even if you're the only person who'll see your diary, recording your food intake is a very easy way to improve your self-control."
This is one of the many articles/people who have said that it is a great way to see what you eat in order to not eat so much after all.
I believe it more now because when I used to do it for WW the writing down of food made me realize that if I ate food with less fat/calories and more fiber/protein it would fill me up faster.
Also seeing every day/week what I had to eat I would try to make healthier choices rather than eating the wrong foods.