"At some point you come back to energy balance -- where your output and input are the same -- so you stop losing weight," Keith explains. "That's the sticking point or plateau a lot of people talk about."

If losing more weight is your goal, you'll have to either exercise more or restrict your calories more, but be wary -- if you're already at a healthy weight, be honest with yourself in analyzing whether you truly need to push to lose even more. There's no viable reason to take your weight loss to the extreme.

Among all these fallacies, there is one truth we've all heard before: As we age, our metabolisms do slow down. "Typically, we use a number like 20%—25% [to describe how much your metabolism decreases] from the time you're 22 to the time you're 75 or so," Keith explains. So if you had a RMR of 1,500 calories at age 20, then at age 75 your RMR may be 1,200 calories. Part of the decline in metabolic rate is hormonal.

"Your hormones aren't as geared up as they once were when you were younger," notes Keith. But if you're able to preserve your muscle mass, that will help offset the drop somewhat. And remember, fat-free mass is the greatest predictor of metabolic rate. Since you're reading muscle & fitness hers, you probably put in some time in the weight room, and your efforts won't go metabolically unnoticed.

All in all, it can be said that metabolism is the great equalizer. The body likes to defend its weight. "If you eat too much and gain weight, your metabolism goes up trying to fight the weight gain," Niemen says. "If you eat too little, the metabolism slows down below normal to help the body conserve what it has so it doesn't lose as much as expected."

"Your body is going to make adjustments to protect itself," adds Keith. "At some point you come back to what we call energy equilibrium -- energy intake is equal to your energy output -- and you stop losing weight." Ultimately, the metabolism wants to maintain balance and, in the grand scheme of things, that's a function that actually makes a lot of sense.