Fad diets promise quick, easy weight loss, which gets many people to jump on the bandwagon. Unfortunately, this type of weight loss is unhealthy and very difficult to maintain. Here's why:
Weight loss is best when it is slow and steady. Losing about 1-2 pounds per week by decreasing one's Calories intake by 500-1000/day (or exercising more, or a combination of the two) is the best. It requires changes to your diet and lifestyle, but the changes can be small. You can take it at your own pace, learning to eat smaller portions and make healthier choices, and increase your physical activity. On the contrary, fad diets are a drastic shift and causes a lot of water loss and muscle loss (in addition to fat), that can often trigger yo-yo dieting and a regain of weight (maybe even higher than you were before).
Diets are a bad idea. The term diet, as it is commonly used, to describe a short-term fast or change of behavior from your norm, is a recipe for disaster. It promises unrealistic effects and makes you an emotional rollercoaster. Rather than eating mindfully and intuitively, listening to your body for cues on what and when to eat, you look to a book or some other external cue to make those decisions. In doing this, you lose contact with your body and its needs.
Fad diets restrict major macronutrients. The most popular fad diets are low in carbohydrates (think Atkins, Dukan), but there is always a significant restriction on the intake of healthy foods and nutrients that are important to maintain a healthy body. Let's take a low-carb diet for example. You have to limit your intake of fruit, starchy vegetables, grains (even whole/unrefined ones like quinoa, brown rice), etc. In doing so, you are missing out on the antioxidants, vitamins, and minerals found in these foods. When on a fad diet, you may need to take a multivitamin--another indicator of its inadequacy.
To learn more about fad diets, listen to my podcast episode on this topic at: http://nutritionallyspeaking.org/fad-diets-debunked/