The Blood Type Diet

The theory

With the blood type diet, firstly you need to know what your blood type is:  O (the most common), A (second most common), B (one in ten people), or AB (the most rare).  Depending on your blood type you should eat from a specific list of foods. Each blood group has a very detailed list of foods that must be avoided, based on the premise that, each blood type has its own unique antigen marker that reacts in a negative way with certain foods (an antigen is something that identifies 'alien' substances).

The plan

If you follow a specific diet based on your blood type, you'll improve digestion and lose weight. Each of the four blood types evolved at different times of human evolution:  O is the oldest (hunter-gatherer); A evolved 15,000 years ago (the cultivator when civilizations settled down); B a bit later (nomadic people); and AB much more recently (and so best adapted to modern eating).  Also, different blood types have varying levels of stomach acidity and digestive enzymes.  Finally different blood types are advised to do different types of exercise - yoga types won't do well with jogging, and vice versa.

The blood type diet claims:
O: can eat high meat diets, no dairy or grains - very like the Atkins diet
A: is vegetarian, eating grains, pulses nuts and seeds
B: is the only one to really do well on dairy foods, but nuts and seeds are disallowed
AB: can eat a combination of the diets advised for the previous two blood types.

Pros

If you like following a set list of foods of what you can and can not eat, this diet may be for you.
The diets totally discourage junk food.  By focussing on our evolution it encourages us to eat in tune with how the human body evolved which is no bad thing (it is true we are not designed for high sugar, caffeine laden, and processed-food rich diets).
Specific guidelines are given for foods, amounts and timing of meals.
Weight loss is likely to result from restricting your food intake.

Cons

Each of the four plans unnecessarily eliminates specific groups of foods, which could result in nutrient deficiency if not handled expertly.
There is a lack of medical research to back up this approach.

Duration

Indefinitely.

Checklist

What the pros say

Most dieticians loath the blood type diet approach as it is deemed to be unscientific.  It probably works for adherents simply because by cutting back on many foods, calories are also cut back leading to weight loss.