Fad diets generally cause a lot of weight loss quickly, usually because calories are very restricted on these diets. A person can stick to a low calorie diet for a short while and see some positive results, but it is difficult (and unhealthy) to stick to a very low calorie diet long-term. Also, these diets often focus on a particular food or food group or avoid a particular food group (think grapefruit diet, low carb diet, the cookie diet) and limit or provide too much of a certain nutrient or nutrients. Fad diets are generally not balanced diets. Our bodies are made to thrive on a balanced diet with food from a variety of food groups. For the best results and the healthiest body, it is best for one to eat a balanced diet and stay away from fads.