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Mistake #6: Eating Like a Much Smaller Version of Yourself

Why it's wrong: For many reasons. First, consuming too few calories per day isn't sustainable, so you're setting yourself up to overeat at night and quit your plan sooner, says McDaniel. Second, "If you chronically under-eat, you slow down your resting metabolic rate, or how many calories you burn when you're doing nothing." Lastly, when you lose weight from calorie deprivation, you're more likely to lose lean tissue, says McDaniel, which means that if you do gain some weight back, it's most likely to be fat, not muscle.

What to do instead: A moderately active woman needs to eat somewhere between 1,400 and 1,800 calories per day to lose weight, says McDaniel. If you have an underactive thyroid or are on the short side, you could go down to 1,200, she adds. Talk to your doctor or a nutritionist about the number they'd recommend for you.