By Eloise Parker (Daily News)
Updated: 2007-08-03 17:05

If all that good summer food has left your pants feeling tight, it's time to retune your diet and exercise plan.

Not sure where to start?

Don't fall prey to these common myths, warns Deborah Arneson, author of "Fries, Thighs, and Lies: The Girlfriends' Guide to Getting the Skinny on Fat."

MYTH #1: According to my scales, I'm not losing weight.

You're more than just a number. The issue is not how much your body weighs but how much your fat weighs. Muscle takes up to five times less space than fat, pound for pound.

So, if you're exercising but your weight is holding steady, you're probably building muscle and losing fat. Aim to get your body fat to a healthy 25% or less of your total body weight.

MYTH #2: Losing weight is too expensive.

Americans spend $40 billion a year on dieting, but if you go online or ask around, you can find great inexpensive resources.

People are happy to spend $7 on a glass of wine in a bar, or $300 or more every year to tune up their car, but spending money on good food and preventative care is priceless.

Forget about expensive diet pills. Multivitamins are a worthwhile investment, and the only other pills that I advise are three or four enzymes before bed, which will aid digestion. Look for natural products instead of synthetic ingredients.

MYTH #3: The latest diet trend is the key to weight loss.

Changing your diet is about changing your lifestyle for good, not cutting carbs for a few weeks.

Think of food in terms of the good, the bad and the ugly to help understand which fats, proteins and carbohydrates to cut back on and which to eat. There are a lot of good fats.

If there were three things I could tell people to look at in their diet, it would be healthy fats, fiber and water. You can't process food properly if you don't eat enough fiber and I've seen people lose 20 pounds in a year just by drinking more water.

MYTH #4: The more calories you cut, the faster you lose weight.

Women who undereat and overexercise wonder why they're grumpy, tired and have no libido. Cutting too many calories slows your metabolism and encourages your body to store fat.

Avoid skipping meals or cutting more than 500 calories from your recommended daily allowance.

A well-balanced meal should consist of protein the size of a deck of cards, half a plate of vegetables and a portion of starchy carbs about the size of a kiwi fruit.

MYTH #5: All exercise melts fat.

Any exercise is better than no exercise, but it's about how long you do it for, not how often.

It can take 30 minutes for your body to shift from burning carbs to burning fat, so doing 60 minutes three times a week is more effective than a short daily burst.

Work out at 45%-50% of your maximum heart rate, which is light enough to make you sweat, but low enough to allow you to keep going.

And mix it up - if you only use the StairMaster, your muscles will get used to it and it won't be as effective.