Story highlights There are 9 calories in every gram of fat

Gallons of water won't compensate for eating too much

30% of your calories should come from fat

Most every woman has a food angel and devil resting on either shoulder, one giving permission to indulge even as the other advises against it.

"We've come to label foods as 'good' and 'bad,' so we feel guilty when we eat something we believe we shouldn't," says Harley Pasternak, celebrity trainer (to Lady Gaga and Katy Perry) and author of the book "The Body Reset Diet."

But we've also become masters at rationalizing what we put into our mouths, which can lead to overeating, dubious food choices and even weight gain.

Evelyn Tribole, a registered dietician and nutritionist in Newport Beach, California, says, "Let's get rid of the guilt! Women need to remember that having foods they love won't make or break their diets as a whole."

Readers bravely let us into their heads to hear how they justify dining decisions, then experts shared some eye-openers. Bet you can relate!

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"As long as it's a 'good' fat, like the kind in avocados or almonds, I can have as much as I want," -- Stacy Rogers Sharp, Austin, Texas.

Reality check! For sure, certain fats are beneficial.

"Monounsaturated and polyunsaturated fats help reduce cholesterol and lower the risk of heart disease and stroke," says Dr. Walter Willett, professor of epidemiology and nutrition at the Harvard School of Public Health.

Still, they are just as fattening as the bad-boy saturated kind found in cheese and red meat. There are 9 calories in every gram of fat, generally twice the density of proteins and carbohydrates, points out Caroline Kaufman, a registered dietician nutritionist in San Francisco.

Nutrition guidelines to keep in mind: 30% of your calories should come from fat, with less than 10% from the saturated kind.

In other words, favoring heart-healthy fats like the ones in nuts, avocado and olive oil: good. Treating them like an all-you-can-eat buffet: bad.

"I tell myself that if I have mostly salad all week, I can pig out on the weekend," -- Elizabeth Upchurch, Jackson, Mississippi.

Reality check! Enjoying an indulgence is fine, says Willett, "but a whole weekend of pigging out may undo progress made during the week."

Weight control boils down to basic math: There are approximately 3,500 calories in a pound of fat, so unless you burn more calories than you consume, you're likely to gain weight.

Many people kick off their food fiestas on Fridays, notes Tribole, co-author of the book "Intuitive Eating." That means they're overdoing it 156 days a year, not just the 104 of weekends. "Overeating promotes a disconnect between you and your body," she continues. "You should be focusing on hunger, fullness and satisfaction anytime you eat."

And don't fool yourself about sticking to salads, adds nutrition pro Pasternak: "The reality is that most salads are far from healthy, loaded with calories from dried cranberries, bacon bits and dressing."

"Of course I can have two slices of pizza at lunch -- I'm going to the gym tonight," -- Hope A. Rising, Clearwater, Florida.

Reality check! Props for working out, but sadly, calories burned at the gym do not necessarily cross out calories consumed, says Pasternak. Two slices of pizza, on average, pack close to 800 calories.

A 130-pound woman burns roughly half that calorie amount during an hour of high-impact aerobics.

Some days, says Tribole, we're hungrier than others; it's often due to sleep deprivation. "Think about what your here-and-now body needs," she says. "If it's a slice of pizza you want, have it. You don't have to justify it." Just don't rationalize having the whole pizza.

"When I have PMS and my body is craving salt and sugar, all bets are off. Either I eat or I kill someone," -- Liz Seccuro, Alexandria, Virginia.

Reality check! There's a scientific reason your body yearns for potato chips and cookies when you're PMS-ing: It's the bliss fix.

Studies show that production of serotonin -- a hormone that regulates mood and weight --slows down during PMS; starchy foods tend to boost it, improving your mood ... but not your waistline.

One MIT study showed women ate about 1,100 more calories per day during that time of the month.

Your best pick: complex carbohydrates with a little protein (it inhibits the production of serotonin).

"By eating rice, pasta and oatmeal, women in our studies felt better," says researcher Judith J. Wurtman, co-author of the book "The Serotonin Power Diet."

And when only a super sugary treat will do, "have your special food -- just don't buy a whole box of it," says Melinda Manore, professor of nutrition and exercise at Oregon State University. "Get one cookie and eat just that."

"Popcorn is one of those 'free' foods, so I can eat as much of it as I want," -- Cynthia Fabian, Somerset, New Jersey.

Reality check! With all the low-carb, low-fat choices out there, nutritionists say many clients mistakenly deem them "free" foods.

"I see some women do this with frozen yogurt and diet gelatin," says Tribole.

Low-fat foods are particularly deceptive, adds Brian Wansink, a researcher in food psychology at Cornell University: "People often believe that low-fat foods have 44% fewer calories than they do, but when companies remove fat, they add sugar, so these alternates typically have only about 10% less calories."

Popcorn may taste like air, yet 3 cups (without anything on it) is the caloric equivalent of a slice of bread. But, he says, don't think too hard about produce: "If you start counting calories in fruits and vegetables, you'll talk yourself out of eating them -- and you shouldn't."

"If I eat too much, drinking lots of water will flush it out of my system," -- Diana Lynn, Woodland Hills, California.

Reality check! Although guzzling glass after glass of water will help whoosh out toxins from your body, it won't do much for the massive plate of spaghetti carbonara you feasted on.

When you eat, your body breaks down the food and shuttles the nutrients off to your cells for energy or stores them, explains Willett: "Even drinking gallons of water won't compensate for eating too much."