If you're anything like me, you have no idea how much food you inhale on a day-to-day basis. Thanks to multitasking, grabbing grub on the go, parking myself in front of the TV while munching, and various other weight-loss crimes, I often barely register that I'm eating. Take last week: I was totally oblivious that I was popping jelly beans into my mouth until my nephew complained that I was about to polish off his bag (sorry again, Jake). It's tough to watch every mouthful you eat, even if you're an expert. Researchers at Louisiana State University asked dietitians to estimate their daily caloric intake — and even the professionals lowballed the number by 10 percent. That may explain why it's so hard to shed pounds, no matter how good the plan is.

But there's a simple solution: Keep a food diary. Studies show that a journal doesn't just aid weight loss — it turbo-charges it. When researchers from Kaiser Permanente Center for Health Research followed more than 2,000 dieters who were encouraged to record meals and snacks, they found that the single best predictor of whether a participant would drop weight was whether the person kept a food diary. It trumped exercise habits, age, and body mass index. The number of pounds people lost was directly related to the number of days they wrote in their log. (It's no coincidence that Weight Watchers, one of the most successful diet programs, asks participants to track what they eat.)

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Here are five more reasons to start a journal today — even if you swore off diaries in middle school.

1. You'll get a reality check about how many calories you consume.

Americans typically underestimate their daily intake by about 25 percent, reports nutritionist Carrie Latt Wiatt, author of Portion Savvy. The situation gets even more complicated when you eat out. In a 2006 study led by Cornell University, 105 diners in fast-food restaurants were asked how many calories were in their orders. For the small items, almost everyone guessed correctly; for the larger orders, the diners underestimated the calories by a whopping 38 percent. It may be an issue of perception, says study author Brian Wansink, Ph.D. The larger the quantity, the harder it is to make an accurate guess — the same is true for distances and heights.

Even professionals can be tricked by hefty portion sizes. When an NYU researcher asked 200 dietitians to estimate the calorie count of four popular restaurant dishes, the experts lowballed the number for each by a whopping 250 to 700 calories.

So stop assuming and start calculating. A good manual, like the classic Calorie King Fat & Carbohydrate Counter, makes it easy by giving you the calorie counts and fat grams for a huge number of foods. Or try nutritiondata.com, which also lists the stats for many foods.

2. You'll cut back on between-meal munching.

Make a mental checklist of what you ate yesterday. Sure, you can probably remember breakfast, lunch, and dinner — but what about that mini Snickers you snatched from your coworker's stash? Or that spoonful of mashed potatoes you took off your husband's plate?

It's easy to overlook bites, licks, and tastes (known as "BLTs" to professionals). But that's a huge mistake — there are 25 calories, on average, in each mouthful. Translation: Six little bites a day add up to around 15 extra pounds a year.