'Vigorous' RCT debunks blood type diet

By Nikki Hancocks

- Last updated on GMT

Getty | Kubra Cavus
Getty | Kubra Cavus

Related tags diet Cardiovascular disease Blood

A newly published intervention study has added weight to those debunking the celeb-lauded 'blood type diet', concluding that blood type is not associated with the effects of a plant-based diet on body weight, body fat, plasma lipid concentrations, or glycemic control.

Studies reporting links between specific blood types and certain health issues, such as coronary heart disease​ and ischemic heart disease risk and blood type O​, have raised the idea that individuals with different blood groups will benefit from different diets. In 1996, a popular book by D’Adamo recommended diets high in fruits, vegetables, and grains for type A individuals, meat-oriented diets for type O individuals, and other diet variants for the other blood types. Since then, the diet has been touted by a number of celebrities​ including Elizabeth Hurley, Demi Moore, and Cheryl Cole. 

A number of studies have gone some way to confuting the validity this diet but researchers with the Physicians Committee for Responsible Medicine - a nonprofit of 12,000 doctors - wanted to conduct a 'rigorous' intervention RCT.

Their study, published in Journal of the Academy of Nutrition and Dietetics,​ is based on a randomised control trial whose main findings were published in JAMA Network Open​ on Nov. 30. That trial randomly assigned overweight participants with no history of diabetes to an intervention or control group​ on a 1:1 ratio for 16 weeks.

The intervention group was asked to avoid animal products while focusing on consuming fruits, vegetables, grains, and legumes, and to limit added oils, nuts, and seeds. Daily dietary fat intake was limited to 20 to 30 g. No meals were provided. Vitamin B-12 was supplemented (500 μg/day). Participants in the control group were asked to maintain their current diets for the duration of the study, which included animal products. 

The key finding is that a plant-based diet ramps up metabolism as measured by an increase in after-meal calorie burn of 18.7%, on average, for the intervention group over the control.

To consider a potential connection between blood type​ and diet, the tea conducted a secondary analysis among intervention-group participants. Researchers considered whether the effects of a plant-based dietary intervention on body weight​, blood lipids​, and glycemic control​ are associated with ABO blood type. The 'blood type diet' recommends a mainly plant-based diet for those with blood type A, while it recommends a diet heavy in meat for people with blood type O.

"We found that blood type made no difference," ​says study author Neal Barnard, MD, president of the Physicians Committee. "While the blood type diet​ says that a plant-based diet should be better for blood type A and less so for blood type O, it turned out to be beneficial for people of all blood types, and there was no evidence that meaty diets are good for anyone.

"Our research shows that all blood types benefit equally from a vegan diet​ based on the consumption of fruits and vegetables, legumes and whole grains, looking specifically at weight loss​ and cardiometabolic health in overweight adults,​" he says.

Main outcomes that were measured were body weight, fat mass, visceral fat volume, blood lipids, fasting plasma glucose, and HbA1c. T-tests compared participants with blood type A to all other participants (non-A), and individuals with blood type O to all other participants (non-O).

There were no significant differences in any outcome between individuals of blood type A and non-A, or between individuals of blood type O and non-O. Mean body weight change was -5.7 kg for blood type A participants and -7.0 kg for non-A participants, and was -7.1 kg for type O participants and -6.2 kg for non-O participants. Mean total cholesterol decreased 17.2 mg/dl in the type A group and 18.3 mg/dl for non-A participants, and decreased 17.4 mg/dl among type O participants and 18.4 mg/dl for non-O participants.

Research background

A 2013 systematic review​ identified no published observational or intervention studies that had investigated the health effects of diets based on blood type.

A 2014 report​ from the Toronto Nutrigenomics and Health study (N = 1,455) described a cross-sectional examination of young adults ranging in age from 20 to 29 years. Using food frequency questionnaires, diet scores were calculated to determine relative adherence to each of the four blood type diets recommended by D’Adamo.

Regardless of blood type, participants in the highest tertile of the type-A diet score (who consumed more fruit, vegetables, and grains, and less meat) had better cardiometabolic outcomes (ie, lower BMI, waist circumference, serum cholesterol and triglyceride levels, and insulin resistance) compared with individuals not adhering as closely to the type-A diet.

Greater adherence to the type AB diet (emphasizing fruits, vegetables, and dairy products) was associated with lower blood pressure, total cholesterol and triglyceride levels, insulin, and insulin resistance, regardless of the participants’ blood type. Greater adherence to the type O diet (emphasizing meat, fruits, and vegetables, and de-emphasizing grains) was associated with lower triglyceride levels, regardless of the participants’ blood type. The authors concluded that, although adherence to certain dietary practices was associated with changes in certain physical variables, these associations were not related to an individual’s blood type.

The Toronto Healthy Diet Study​, a 6-month trial using a dietary intervention that emphasised greater intake of plant-derived foods, included a secondary analysis assessing whether relative adherence to blood type diets influenced the cardiometabolic effects of the intervention.

A food frequency questionaire​ was completed at baseline and 6 months after the dietary intervention by 576 participants. Although associations between diet adherence and cardiometabolic risk factors were observed, the blood types of the participants exerted no apparent effect.

The current report states: "The present study used a more vigorous intervention, excluding all animal products, along with weekly classes to maximize adherence, achieving clinically important and statistically significant effects within each blood group. Even so, there was no significant difference in clinical effect across blood groups. This does not discount the possibility that the effects of diet on health may be dependent on other genetic components."

Source: Journal of the Academy of Nutrition and Dietetics

Barnard. N. D., et al

"Blood Type Is Not Associated with Changes in Cardiometabolic Outcomes in Response to a Plant-Based Dietary Intervention"


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