What exactly is a fad diet? These are weight loss plans that are marketed as quick ways to lose the extra pounds. Year after year, the most popular diets change, but the promises they make don't.  

"Most fad diets go something like this: Take a few foods, give them 'magic' power, and set a plan to convince people that eating this way and only this way will promote weight loss," said Alexandra Caspero, RD, a nutritionist based in Sacramento, California.

So, here's the skinny on some of the most well-known diets, and why they don't work... in the long haul.

1.       Gluten-free diet

Unless you're suffering from celiac disease or doing it to preserve your health, then by all means, continue to do so. But if you're doing it to lose some weight, you're probably wasting your time. Aside from having no research to support the weight loss claim due to a gluten-free diet, the only reason why you're losing weight when you stay away from cake, muffins, chips, or cookies is that you're cutting away junk calories.

2.       The Cabbage Soup Diet

Aside from being zero fun, this grandmother of all fad diets lacks in protein, which is needed to preserve lean body mass. How does this diet go? For one week, you consume a couple bowls of cabbage soup per day, plus a prescribed set of low-calorie foods. Example, today you may only eat cabbage soup with a serve of veggies or bananas and skim milk. Bonnie Taub- Dix, owner of Betterthandieting.com and author of Read It Before You Eat It, says, "Cabbage soup is a wonderful soup, but as a diet, it's a bogus."

3.       Low-Fat Diet

Surprised? While going on a low-fat diet is better than just lounging on your couch doing nothing to lose the pounds, it's not actually the best strategy. In a study led by Deirdre Tobias at the Harvard School of Public Health, a team of researchers analyzed 53 experiments, each lasting at least one year, that placed participants in different kinds of diets, like low-fat, some high-fat and low-carb, and so on.

The researchers found out that low-fat diets recommended to study participants did not result in greater weight loss over time in contrast with other approaches executed with the same intensity (which points to added effort that includes meetings with a dietician, counseling sessions, cooking lessons, etc.). "There is no good evidence for recommending low-fat diets," said Tobias. "We don't eat calories per se - we eat foods," Tobias added. For an effective weight loss, "the focus needs to shift away from specific nutrients - carbs and fats - to a discussion of healthy foods and eating patterns."

4.       Alkaline Diet

While celebrity Kelly Ripa said that the diet changed her life (have you seen her abs?), it actually requires a lot of effort. Fans of the diet say that foods included in this diet (nuts, seeds, fresh fruits, and veggies) can change the pH levels of your blood and urine, which in turn, affects everything from weight management to cancer risk. However, according to Christopher N. Ochner, Ph.D., director of research development and administration at the Mount Sinai Adolescent Health Center, your body is incredibly efficient at keeping your pH levels where they need to be, so cutting out on sweets, caffeine (!), wheat, meat, and processed food really won't affect your body's pH. On a positive note, eating less processed foods and munching more produce can help you shed pounds, but it has nothing to do with your pH levels.

5.       The Master Cleanse

No question about it, Beyoncé did look amazing when she reportedly used this diet to lose 20 pounds for her Dreamgirls role. This diet has been around for a long time, and pops up occasionally, especially around New Year's when everyone's feeling a bit guilty about their holiday binge. It involves consuming a mixture of lemon juice, maple syrup and cayenne pepper mixed in water throughout the day. If you need energy to do, well, anything, then this diet will be difficult to follow. Common side effects include dizziness, nausea, fatigue, and dehydration. You'd be feeding your body diuretics, and with so few calories and none of the nutrients you need to go about your day, you are going to lose muscle, exactly the kind of weight you don't want to lose, Caspero says. It may work short-term, but you'll gain everything back (and then some) when you start eating solid foods again.