Do Crash Diets Really Work?

People desperate to lose weight in a hurry have always latched on to various fad diets, but even if they work, are they dangerous?

CBS News Medical Correspondent Dr. Jennifer Ashton shared some of the most popular crash diets and the pros and cons of each.

Special Section: Dr. Jennifer Ashton
Video Series: Dr. Ashton's Health and Wellness

This one has been around for decades and was originally known as The Hollywood Diet. The diet is based on the premise that grapefruit contains a special "magic" ingredient that helps the body to burn fat, but there is little to no evidence to support that claim. Basically, the diet is a low carb, high protein diet -- a half a grapefruit is added to meals, averaging between 800 and 1000 calories. The diet goes for quick weight loss, but unfortunately, weight loss is mostly from fluids and not fat, and results from cutting carbs and calories, not from the grapefruit. Bottom line: There's nothing wrong with eating grapefruit -- it's a very nutritious low-calorie fruit, loaded with Vitamin C and fiber, but it's really cutting the calories, not adding the grapefruit that will do the trick.

This diet is basically a "modified fast" and promises to help dieters shed at least 10 pounds in one week. With this diet, you're allowed bottomless bowls of cabbage soup along with some low-calorie vegetables. So, you can imagine, it does get a little monotonous. With this diet, you may see weight loss -- but it may not stick because much of the weight loss is from fluids, which will come back once you go back to your normal eating habits. You lose out on a lot of nutrients with this diet as well. Another word of caution: cabbages do cause gas, which could be uncomfortable.

This type of diet usually is shorter than other crash diets -- you can do a cleanse for three days. You basically drink your calories via specially-designed juices which can be ordered online or are offered by several companies here in the New York area. They run about at least $60 a day -- you replace meals with juices pressed from fruits, vegetables and nuts. Because you're taking in fewer calories, you will lose a little bit of weight. However, some of the makers of these juices say these cleanses are not all about losing weight, but detoxing the body -- which happens naturally through our body anyways. However, because of the shorter duration and the variation and convenience of juices (you're not just eating one thing and you don't have to make anything) may make it easier to stick to.

Allegedly this diet became popular after some Hollywood stars tried it. There are different versions of the diet, but the most popular is to eat 14 jars of baby food throughout the day in lieu of breakfast and lunch and then eat a normal dinner. The idea is that baby food is loaded with vitamins and is neatly portion controlled. Also, if you think chewing is too much work, you're in luck. But the key to many diets is to eat more bulky food that requires chewing to give you a sense of satiety and satisfaction. To go to pureed food for reasons other than health issues or being a baby won't work in the long run. Baby food is designed for babies and they don't have the nutrient power for adults. After eating bland baby food, bingeing is a risk as well as overeating if you eat too much baby food and adult food.

Fad diets have a couple of things in common. Often they promise a quick fix and make claims that are too good to be true. They often make dramatic statements that are not backed up by scientific evidence. And very often, as we've seen with the crash diets we just talked about, they eliminate whole food groups and make some foods "good" and some foods "bad."

Really, it's about lifestyle changes that will bring weight loss that lasts. Diet and exercise are key. The golden equation is that you need to use more calories than you consume -- by reducing 500 to 1,000 calories per day, you can lose up to two pounds a week, which may be not as dramatic as fad diets, but you will have more success in the long run.

By making simple changes like drinking skim instead of whole milk, you can shave off the weight. It all adds up. You can still have some of your favorite foods -- but remember: It's all about balance. You can give yourself smaller portions less often. It's more about portion control than cutting out whole categories of food. Some helpful tips to keep in mind:

Eat plenty of low-calorie vegetables to help you feel full.
Drink plenty of water so you don't confuse hunger with thirst.
Clear the house of tempting foods.
Stay busy to prevent eating out of boredom.
Eat only from a plate, while seated at a table.
Always eat three meals and one snack daily -- no skipping meals.