The blood type diet, popularised by naturopath Peter D'Adamo in his 1996 book "Eat Right 4 Your Type," claims that your blood type (A, B, AB, or O) determines which foods you should eat for optimal health.
The book became a massive bestseller, and the diet still has devoted followers today. But despite its popularity, the scientific evidence tells a very different story.
The Theory
D'Adamo's hypothesis is based on the idea that different blood types evolved at different points in human history, and that the foods available during each era are the ones we're genetically programmed to eat:
Type O - "The Hunter"
Supposedly the oldest blood type. Recommended: high-protein diet with meat, fish, vegetables. Avoid: grains, legumes, dairy.
Type A - "The Agrarian"
Evolved with agriculture. Recommended: vegetarian diet with grains, legumes, vegetables. Avoid: meat, dairy.
Type B - "The Nomad"
Evolved in nomadic populations. Recommended: balanced diet including meat, dairy, grains, vegetables. Avoid: chicken, corn, wheat.
Type AB - "The Enigma"
Most recent blood type. Recommended: combination of A and B guidelines. Avoid: caffeine, alcohol, smoked meats.
The Problem: No Evidence It Works
The blood type diet sounds scientific. It has evolutionary logic, specific recommendations, and millions of satisfied followers. But when researchers actually tested whether it works, they found nothing.
The Definitive Study
In 2014, researchers at the University of Toronto conducted a systematic review of all available evidence on the blood type diet. They analysed data from 1,455 participants and found "no evidence to support the blood type diet theory." People who followed the diets recommended for their blood type showed no better health outcomes than those who didn't.
A follow-up study in 2018 looked specifically at cardiovascular risk factors. Again, no association was found between blood type-specific diets and health outcomes.
Why Some People Feel Better
Many blood type diet followers report feeling healthier. This isn't surprising, but it's not because of blood type matching:
- Any structured diet beats no structure - Simply paying attention to what you eat tends to improve diet quality
- Reduced processed foods - Most versions of the diet emphasise whole foods
- Placebo effect - Believing a diet will work has genuine psychological benefits
- Vegetable increase - Most versions emphasise more vegetables
In other words, people feel better because they're eating more thoughtfully - not because they matched their diet to their blood type.
The Evolutionary Argument Doesn't Hold Up
D'Adamo's theory also has fundamental problems with its evolutionary claims:
- Blood types are older than claimed - Type A isn't a recent agricultural adaptation; it's found in other primates and predates modern humans
- Geographic distribution doesn't match - If type O was optimal for meat-eating, we'd expect it to dominate in pastoral societies, but it doesn't
- The lectin theory is unsupported - D'Adamo claims certain food lectins react with blood type antigens, but this hasn't been demonstrated in controlled studies
The Verdict
Despite being a bestseller for nearly 30 years, the blood type diet has never been validated by peer-reviewed research. Multiple studies have specifically looked for the effect and found nothing. This is as close to "debunked" as nutrition science gets.
What Actually Matters for Diet
While blood type doesn't determine your optimal diet, plenty of things do:
- Your preferences - A diet you'll actually follow beats a "perfect" diet you won't
- Your activity level - Active people need more calories and often more carbohydrates
- Your health conditions - Diabetes, heart disease, and other conditions have genuine dietary implications
- Your microbiome - Gut bacteria may influence how you respond to certain foods (research ongoing)
- Your genetics - But not blood type; other genetic factors may influence things like lactose tolerance or caffeine metabolism
The Bottom Line
The blood type diet is an appealing idea with no scientific support. If you've tried it and felt better, the improvement came from eating more whole foods and paying attention to your diet - not from matching your meals to your blood type. Save yourself the complexity and just focus on eating more vegetables, adequate protein, and fewer processed foods.
References
- Wang, J., et al. (2014). ABO Genotype, 'Blood-Type' Diet and Cardiometabolic Risk Factors. PLOS ONE
- Cusack, L., et al. (2013). Blood type diets lack supporting evidence: a systematic review. American Journal of Clinical Nutrition
- De Roos, N.M., et al. (2018). The Blood Type Diets Have No Effect on Cardiometabolic Risk Factors. British Journal of Nutrition
