On the Blood Type Diet, created by Peter J. D'Adamo, people with type O blood should avoid grains, beans and dairy products, reports WebMD. Instead, they should eat a high-protein diet filled with lean meats, fish and vegetables.
D'Adamo also states that people with type O blood are more prone to gastrointestinal issues and should supplement their diets accordingly, writes WebMD. To help speed weight loss, they should also engage in vigorous aerobic activity, such as jogging or biking.
According to D'Adamo, type O blood is the first and oldest blood type. People with this blood type are more aggressive but also more prone to metabolic disorders with higher levels of stomach acid. He states that people with type O blood are likely to have iodine deficiencies, but recommends that they avoid iodine supplements and instead eat saltwater fish and kelp to help regulate the thyroid gland.
There is no scientific evidence supporting the Blood Type Diet or D'Adamo's allegations, cautions WebMD. The diet generally encourages followers to avoid processed foods and carbohydrates, which can aid in weight loss, but this has not been linked specifically to the diet. It also fails to take into account medical history, chronic conditions, cholesterol and heart disease. The diet is also very restrictive, with D'Adamo recommending expensive substitutes and his own line of supplements.