If you've visited a bookstore this month, you probably weren't able to avoid the giant pyramid of diet books that magically appears right after Jan. 1. Whether you find those titles inspiring or just guilt inducing, one thing is certain: many of them sound too good to be true. And often they are. These popular weight-loss fads usually aren't all that new, but they come around every year with new names—and maybe even a fresh celebrity connection. Here are the six basic diet trends, why they don't work and why they may even be unhealthy. You won't find these exact titles on a shelf near you, but hopefully you'll be able to recognize the gimmick when you see it no matter how it's been tarted up.

(For more on weight loss, check out this story about diet tactics that are backed by science:  "What Works.")

1. The Fat-Free Diet
The theory: Eat whatever you want as long as it has no fat. If your diet contains no fat, you won't get fat.
Reality check: While it's true that extra fat in your diet adds calories, just sticking to foods touted as fat free doesn't necessarily help. Supermarket shelves are crammed with products advertised as fat free that are loaded with sugar and empty calories and that offer little in the way of fiber, vitamins or minerals. Check product labels before you buy.

2. The Snack-Pack Diet
The theory: Cookies and chips sorted into 100-calorie packs help limit the damage from an attack of the munchies.
Reality check: The dozens of 100-calorie snack pack foods on the market now may offer a lower-calorie alternative, but few of them are truly healthy choices, and they aren't likely to be very filling or fiber rich, which can send you running for another bag or something less healthy. Rather than knocking back a 100-calorie bag of Cheetos or Oreos, look for more nutritious alternatives, like half of a sandwich made with whole-wheat bread or a piece of fruit.

3. The Couch-Potato Diet
The theory: Who needs exercise? You can lose weight without working out! Cutting back on calories is enough.
Reality check: Who needs exercise? You do. Studies have shown that dieters who change what they eat and increase their regular activity are more likely to lose and keep weight off. Increasing activity has other health benefits as well, such as lowering your risk of heart disease, the leading killer of women. Even a brisk 20- to 30-minute walk most days of the week can make a big difference.

4. The Detox Plan
The theory: You'll lose weight when you clean out your insides by downing a concoction made from orange juice and molasses or some other bizarre mix.
Reality check: There's no evidence that purging your intestines of "toxins" makes you any healthier or more likely to lose weight. A high-fiber diet is all you really need.