Proponents of the blood type diet claim that people with type A blood benefit most from a vegan diet. However, a new study found no link between diet and blood type. The researchers instead suggest that plant-based diets are beneficial for people of all blood types.

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Staying healthy generally involves exercising regularly and following a nutritious diet. While the Centers for Disease Control and Prevention (CDC) recommend a balanced diet of fruits, vegetables, lean meat, and poultry, the blood type diet suggests that nutritional needs depend mainly on a person’s blood type.

The blood type diet tailors an individual’s eating patterns to specific food items to maximize the health benefits. However, a 2013 review in The American Journal of Clinical Nutrition found that these claims lacked scientific evidence.

When looking at cardiometabolic factors, or a person’s chance of stroke, diabetes, and heart disease, the findings of a 2014 study did not support tailoring diets to blood types.

Although the results did show that people on the Type A diet — which involves eating high amounts of grains, fruits, and vegetables — had a lower body mass index (BMI) and waist circumference, as well as reduced blood pressure, cholesterol, and fat, these improvements in risk factors were not dependent on blood type.

Despite insufficient evidence to support blood type diets, some people believe that catering to blood type can lower disease risk.

The appeal for blood type diets may actually come from the health benefits associated with a plant-based diet. Researchers have linked these diets to a lower BMI and a reduced risk of obesity, diabetes, and heart disease.

A recent study in JAMA Network Open found that low fat vegan diets helped lower insulin resistance and boost metabolism, which aided weight management. Based on these results, researchers affiliated with the Physicians Committee for Responsible Medicine reanalyzed the study’s data to determine whether blood type played a role.

The current study was a subset of the JAMA Network Open study, and it focused exclusively on the participants belonging to the intervention group in the 16-week trial.

The findings appear in the Journal of the Academy of Nutrition and Dietetics.

The trial recruited a total of 244 adult men and women from Washington, D.C., with a BMI between 28 and 40. None of the participants had a history of diabetes, drug abuse, pregnancy, or lactation, and none were currently on a vegan diet.

The researchers assigned half of the participants to follow a strict, low fat vegan diet, while the other half did not make any changes to their diet. The participants self-reported what they ate during the 16-week trial.

The vegan group also attended weekly classes on dietary information, which health professionals led. The researchers advised all of the participants to continue their regular exercise habits.

The measurement of cardiometabolic risk factors took place at the start and end of the trial, following a 10-to-12-hour overnight water-only fast.

After this, the researchers replicated the method but included blood typing. The secondary analysis involved a total of 68 participants.

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At the start of the trial, body weight and low-density lipoprotein (LDL) cholesterol levels were higher in people with blood type A than in those with other blood types. In contrast, body weight and LDL cholesterol levels were lower in people with blood type O than in those with other blood types.

There were no significant differences between blood type and the changes that resulted from the vegan diet.

After 16 weeks on the low fat vegan diet, there were no statistically significant differences in the average change in body weight between the blood type groups. People with blood type A lost an average of 5.7 kilograms (kg) compared with 7.0 kg for people with other blood types. The average weight loss for people with blood type O and those with other blood types was 7.1 kg and 6.2 kg, respectively.

The researchers also found no significant evidence of a difference in the decrease in average cholesterol between the groups either. They reported a decrease of 17.2 milligrams per deciliter (mg/dl) in the type A group versus 18.3 mg/dl in the other participants, and a decrease of 17.4 mg/dl for the type O group versus 18.4 mg/dl.

“Although the intervention diet was similar to that recommended by D’Adamo [a proponent of the blood type diet] for individuals with blood type A and specifically recommended against for those with type O, there were no associations between these blood types and the outcomes of the dietary intervention,” concluded the authors.

There were multiple strengths to the study, starting with ruling out possible confounders that could influence the results.

For instance, to exclude physical activity as a factor, participants maintained their regular exercise routine during the 16-week trial. Also, all participants started the trial simultaneously, which ruled out seasonal changes in diet.

According to the researchers, 16 weeks was a suitable time for participants to acclimate and adhere to the study. There was also a low drop-out rate.

The participants did not have access to ready-prepared meals but could prepare food at home or opt for take-out. This method made the findings more applicable to real-life conditions.

However, there were major limitations regarding representation.

Not as many participants with blood types AB and B were involved in the trial. As a result, the team had to combine blood groups. Also, participants in the study were health-conscious and may have been more motivated to adhere to a low fat vegan diet. Consequently, the participants may not be representative of the general population.

Overall, the researchers say that their data add to the current body of work consistently finding a lack of strong evidence for the blood type diet.

“These studies, like the present one, suggest that dietary changes, especially increased intake of fruits, vegetables, and grains, are beneficial not only to individuals with blood type A but also to all individuals regardless of blood type, and that there is no apparent value of limiting these healthful diet changes to a specific set of individuals based on ABO blood group.”