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SAN ANTONIO -- There areplenty of diets, trendy workouts and modern procedures aimed at helping people slim down. One dieting theory takes a skin deep -- or vein deep -- look at weight loss.

Dr. Peter D'Adamo popularized choosing diets based off an individuals blood type in his book 'Eat Right 4 Your Type.'

The book suggests that people should look at their blood type and follow the dietary guidelines of their ancestors.

For instance, type O is the most popular blood type. It is believed to have first been born 30,000 years ago. According to the diet, type O people are hunters and should follow a high-protein diet -- like a caveman diet.

Type A blood people are cultivators, according to the diet. Therefore, anybody with type A blood should eat mostly vegetables.

The theory is believable, but critics say the diet lacks evidence and clinical trials.

This diet seems pretty restricted in a few parts, a dietitian at the University of Texas Health and Science Center told KENS 5. I see it as a short term weight loss.

As a national best seller, D'Adamo's book sparked a lot of curiosity, and has people poking around to find out their blood type.

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