by Kathleen Donnelly for MSN Health & FitnessHigh on any  list of weight loss mistakes you'd expect to find one of the original  seven deadly sins: gluttony. And if gluttony means regularly consuming  portion sizes that might be best measured in bushels, many of us are  going to have more than one problem when it comes to fitting through the  Pearly Gates. 
"It's  not the food, it's how much you're eating," says Jane Kirby, a  registered dietitian and author of the recently revised guide, "Dieting  for Dummies" (John Wiley & Sons, 2003). "People on a low-carb diet  might think, 'I can eat all the ham and Swiss cheese rollups dipped in  mayonnaise that I want.'" She sighs. "No, you can't. It's portion,  portion, portion."
We've  known about at least one deadly sin of weight loss for a long, long  time: Eating too much too often is not a winning strategy. It doesn't  matter that ham and cheese is low in carbohydrates, or, for that matter,  that bagels are fat-free. You cannot eat more calories than you expend  day after day and expect to have a happy experience on the scale. 
This brings us to a second big mistake many dieters make:
Trusting in a miracle diet.  We all want weight loss to be quick and easy, and if it means six weeks  of eating nothing but somebody's secret recipe for slimming soup, we'll  do it. Unfortunately, it's not taking off the pounds that bedevils many  dieters, it's keeping them off. That's why many weight loss researchers  prefer the term "weight management" to "dieting." Staying fit and  healthy – and at a reasonable weight – is not something you do for six  weeks. It's a lifelong commitment. And no one can eat that much soup.
Not counting calories.  At its most basic, losing weight is a matter of taking in fewer  calories than you expend. So as tedious as it sounds, Kirby says  calories do count, and counting them can help you stay on track. For  example, ignoring the nibbles and sips you take each day can foil your  weight loss plans. An energy bar and a sugary sports drink – even if you  consume them at the gym – both count toward your daily calorie total. 
Eating too little.  If eating too much is bad, shouldn't sealing your lips to everything  but leafy greens and an all-purpose vitamin work? Not necessarily.  Eating too few calories may slow your metabolism, the process your cells  use to burn food and create energy. Researchers vary on how few  calories it takes before you slip into starvation mode and begin  conserving calories, and the number depends on your own body and  activity level. But as a general rule, Kirby says, going below about 800  calories a day may be counterproductive. 
Expecting too much.  How much weight do you want to lose, and how fast do you want to lose  it? If the answer gets you back to your junior high weight in a week or  two, it's probably not a reasonable goal.  Keep in mind, says Kirby, that losing just 5 to 10 percent of your body  weight – that's 10 to 20 pounds if you weigh 200 pounds – can provide  health benefits as well as make you feel like a winner.* Once you attain that goal, you can always set another.
Skipping exercise (also known as sloth).  Let's revisit the calories in vs. calories out concept: Exercise burns  calories. Therefore, add exercise to your routine and the "calories out"  part of your equation jumps up. But just as important, says Cedric  Bryant, Ph.D., chief exercise physiologist for the American Council on  Exercise, is what exercise does for your resting metabolism. It takes  more energy to maintain lean tissue than it does to maintain fat. So, by  building your lean tissue, exercise helps you burn calories even when  you are not moving. Because of this, Bryant adds, research shows that  people who are physically active are more likely to keep weight off once  they lose it. 
Neglecting your exit strategy.  No matter how you choose to lose weight – especially if you opt for a  "miracle diet" – make a plan for keeping the weight off. You've come too  far to go back to your old eating and exercise habits, which led to  weight gain in the first place. 
"One  big deadly mistake is thinking that you are going on a diet, and when  it's over, that's it," says Kirby. "If people can think more about doing  something good for themselves, as opposed to denying themselves, I  think they'll find it a more successful strategy."
Correction, September 16, 2005:   This article originally stated that losing 5 to 10 percent of your  body weight would equate to "10 to 15 pounds if you weigh 200 pounds"  which is inaccurate. This error has been corrected. Return to the corrected sentence.