The underlying hypothesis of blood type diets is that people with different blood types digest lectins differently, and that if people eat food that is not compatible with their blood type, they will experience many health problems. The blood type diet was named by the British Dietetic Association as one of the 'Top 5 Celeb Diets to Avoid in 2019'.

In other words, those who were eating the 'right' diet for their blood type did not show different biomarker values on average compared to those eating the 'wrong' diet. While there were significant differences in some biomarkers between these groups, there was no significant interaction between diet and biomarkers. Based on one's diet each person was classified as tending to follow the blood-type diet recommended for O, A, or B. In what was apparently the first study testing whether there was any benefit to eating the 'right' diet according to one's blood type, a study published in 2014 compared 'biomarkers' such as body mass index, blood pressure, and serum cholesterol and insulin among young people, and assessed their diets over a period of a month. The consensus among dietitians, physicians, and scientists is that these diets are unsupported by scientific evidence.

These diets are based on the notion that blood type, according to the ABO blood group system, is the most important factor in determining a healthy diet, and each author recommends a distinct diet for each blood type.

The blood type diets are fad diets advocated by several authors, the most prominent of whom is Peter J.

Diet o negative blood type