Some physicians and nutritionists argue that Dr. D'Adamo's theory about lectins lacks solid scientific support. These critics point out that the research that has been done on lectins has been performed mostly in test tubes. Therefore, it is not yet known what, if any, physiological effects lectins have in humans. Furthermore, many food lectins are destroyed by cooking and/or digestive enzymes, so many critics argue that the number of lectins absorbed intact through the digestive system is minimal. Other critics point out that Dr. D'Adamo's emphasis on the ABO blood-typing system is somewhat arbitrary. In a book review, Alan Gaby, MD, points out that the ABO system is only one of many different blood-typing methods, and to date, more than 30 unique markers have been identified on the surface of red blood cells. Consequently, if Dr. D'Adamo had based his diet on a different marker, his diet recommendations may have been very different.
Most critics believe the diet is associated with no real health hazards. However, critics caution that people with Type O blood may increase their risk of heart disease by adhering to Dr. D'Adamo's Type O diet recommendations. Registered dietitians caution against classifying foods into "good" and "bad" categories, advocating instead the idea that "all foods fit" into a healthy diet in moderation. Restricting certain foods or food groups altogether makes it difficult to eat the balanced diet that most health professionals recommend.
Although most critics concede that the Blood Type Diet produces weight loss in some people, they argue that this diet is merely a calorie-restricted diet. As with any other low calorie diet, weight loss is likely to occur.