Why is it unwise to avoid certain categories of foods all together or to only eat some categories of foods as suggested by fad diets?

Fad diets can deny a person nutritional requirements by avoiding certain foods perceived as fattening foods. Even such foods as low-density lipids (that are considered unhealthy in large amounts) are required by the body but in minimal amounts. People on fad diets are more likely to experience symptoms such as dizziness, fainting, and muscle cramps.