Fad diets are often defined by a meal plans which encourage the dieter to either eat only one type of food or totally eliminate another type of food. In the case of the grapefruit diet proponents promise a loss of 10 pounds in approximately 12 days without sending the body into starvation mode. The theory is that the juice from the grapefruit enhances the fat burning process and allows the individual to lose weight much more quickly.

However, there is no documented proof that anyone has ever achieved any long-term success using this diet and there are some significant health risks which may be dangerous for those who have under medical conditions.

Another part of the theory is that grapefruits contain very simple carbohydrates, which are high in fiber and have almost no fat while being low in calories and packed with vitamin C. They reportedly reduce the appetite of the individual who is eating it and promote weight loss by reducing caloric intake.

The plan was first introduced in the 1930s and was designed as a method of jumpstarting a weight loss program in order to encourage people to continue to eat a lower calorie diet by losing approximately 10 pounds in a short period of time. The idea was to eat half a grapefruit before each meal in order to utilize fat burning enzymes.

Some people advocate an individual use this diet for 12 days and take two days off but then return to the grapefruit diet after that. The meal plan is specific with vegetables that are allowed and vegetables that are to be avoided. The list of vegetables which are allowed are high in vitamins and mineral content but the foods which are required are also high in cholesterol.

For instance, every morning's breakfast consists of one half a grapefruit, two eggs and two slices of bacon. Lunch contains meat as does dinner so the individual is eating well over the recommended daily allowance of three to four ounces of meat per day. The diet is also high in protein and fats. And the number of calories is limited to not more than 800 – 1000 calories per day.

Most published material states that the individual will not feel starvation or be deprived even though they are eating well below the 1200 calories per day which most dietitians and researchers believe is the bare minimum amount of calories an individual requires in order to function.

Every person's body has a basic basal metabolic rate which is the number of calories required in order for the heart, brain and lungs to function even when the individual is laying in bed all day. That number usually hovers around 1200 calories per day. When an individual eats 800 calories on the grapefruit diet they are essentially throwing their body into starvation mode.

In fact, the beginning of other high protein diets, sometimes the Atkins diet, may have begun with the strange food choices of the grapefruit diet. In most versions of the plan there are a very small variety of foods that are allowed and dieters are encouraged to drink either water or black coffee all day long.

At this point there are no clinical studies which substantiate the ability of the enzymes in the grapefruit juice to burn fat. Any person who goes on a low carbohydrate high protein diet that has less than 1200 calories will also lose weight, whether or not they are eating grapefruit. Proponents have no explanation of how or why this grapefruit enzyme works and why it is only in grapefruit and not in any other citrus fruit.

There was one study performed in 2006 and funded by the Florida Department of Citrus. During the study researchers found that most participants lost an average of 3 pounds over a 12 week period when they added 4 ounces of grapefruit juice to their meals. Without having controlled for exercise researchers theorize that the addition of the grapefruit juice to an otherwise healthy meal reduced the amount of insulin required and promoted this small weight loss. Some participants also reported having increased their physical activity slightly which could also explain the weight loss.

People who are searching for long-term success should not waste their time on any fad diets but instead, look for a flexible program that meets the individualized needs and behaviors of the person and includes a wide variety of healthy foods. Weight loss programs should also include physical exercise which is the basis for an increase in calorie burn