The B positive blood type diet is a lectin-avoidance eating plan that asks you to skip chicken, lentils, corn, tomatoes, and wheat and to favor lamb, venison, leafy greens, and low-fat dairy. It rests on a hypothesis, not on clinical evidence: no controlled trial has shown that eating by blood type improves health. A 2013 review in the American Journal of Clinical Nutrition found zero studies supporting the claimed benefits, and a 2014 PLOS ONE analysis of 1,455 adults found the four diets produced health markers regardless of blood type.
The Lectin Mechanism, Explained One Level Deeper
The diet's founder, naturopath Peter D'Adamo, published Eat Right 4 Your Type in 1996. His premise is that ABO blood group antigens act as lectin receptors. Dietary lectins, he argues, bind to those antigens and trigger agglutination, meaning red cells clumping together.
In the B positive version, the claimed triggers are lectins in chicken, peanuts, lentils, corn, buckwheat, and wheat. The claimed safe foods are lamb, goat, venison, and most vegetables. The underlying logic is that B antigen evolved in populations under different dietary pressure, so B carriers supposedly digest certain foods better.
The biology is real but the leap is not. Lectins do bind carbohydrate structures on red cells, and some plant lectins agglutinate blood in a laboratory dish. That is a test-tube observation at concentrations far above anything a meal delivers. Human digestion denatures most dietary lectins before they reach the bloodstream, and the gut lining blocks the rest.
In plain terms: the mechanism exists in petri dishes, not in people eating dinner.
How Many People This Affects and What the Numbers Say
B positive is one of the rarer ABO types. Roughly 9 percent of Americans are B positive and about 2 percent are B negative, per Red Cross blood distribution data. Globally, B ranges from about 4 percent in parts of South America to over 25 percent in parts of India and Central Asia.
The evidence base is small and negative. D'Adamo's 1996 book has sold millions of copies, yet the peer-reviewed literature contains no randomized controlled trial of blood-type diets. The 2013 review, led by researchers at the University of Toronto, screened 1,415 references and found not one study meeting inclusion criteria.
The 2014 PLOS ONE study is the closest thing to a test. Researchers followed 1,455 adults for up to 75 days on the four blood-type diets and measured cholesterol, triglycerides, insulin, and weight. Diets labeled beneficial for type B did improve some markers, but the same pattern held across all blood types, which means the diet's identity was not driving the result.
B Positive Food Lists: What You Eat and Avoid
The B positive plan divides food into highly beneficial, neutral, and avoid categories. The table below condenses the most consequential items, because these choices drive both the cost and the nutritional risk.
| Category | Claimed beneficial | Claimed neutral | Claimed avoid |
|---|---|---|---|
| Meat | Lamb, venison, goat | Beef, turkey, liver | Chicken, pork, duck |
| Seafood | Cod, salmon, mackerel | Tuna, halibut | Shellfish, anchovy |
| Legumes | None listed | Kidney beans, navy beans | Lentils, chickpeas, peanuts |
| Grains | None listed | Rice, oats, spelt | Wheat, corn, buckwheat |
| Vegetables | Kale, collards, beets | Broccoli, carrots | Tomatoes, corn, pumpkin |
| Dairy | Low-fat yogurt, kefir | Eggs, goat cheese | Blue cheese, ice cream |
| Exercise | Moderate, 3 to 5 sessions weekly | — | — |
The chicken exclusion is the counterintuitive one. Chicken breast is among the leanest, cheapest, most broadly recommended proteins in every major dietary guideline. Removing it pushes B positive dieters toward lamb, which sells for roughly 40 to 60 percent more per pound at US retail than chicken, and toward venison, which is seasonal and scarce.
The practical effect is a diet that costs more and narrows protein variety without any demonstrated benefit.
The Timeline: What Changes, and When It Does Not
Adherents often report feeling better within weeks. That timeline is real, but the cause is usually the elimination of ultra-processed food, not the blood type. Here is what the evidence supports over time.
| Timeframe | What typically happens | What the evidence supports |
|---|---|---|
| Week 1 to 2 | Digestive changes reported by many dieters | Reduced processed food intake, not blood type |
| Week 3 to 6 | Weight loss of 2 to 5 pounds commonly reported | Calorie deficit and water shift from cutting wheat and corn |
| Month 2 to 3 | Improved energy and appetite reports | Higher vegetable and fiber intake |
| Month 3 to 6 | Cholesterol and triglyceride changes | Driven by diet quality, independent of ABO type |
| Month 6 to 12 | Sustainability problems emerge | Restriction reduces adherence; no blood-type effect appears |
| Beyond 12 months | Nutrient gaps possible (calcium, B12, iron) | Risk rises if dairy and meat variety stay restricted |
The key correction: the timeline tracks eating behavior, not antigen biology. The same 2014 data showed beneficial-diet scores improving markers in every blood group.
Where the Science Stands: Established, Hypothetical, and False
Established: blood group antigens are real, inherited, and clinically important for transfusion and pregnancy. Established: diet quality strongly affects cardiometabolic markers, per decades of randomized trials.
Hypothetical: that ABO type dictates which foods suit a person. This is D'Adamo's model and has never been tested in a controlled human trial.
Not supported: that lectins from a normal meal cause harmful agglutination in the body. Also not supported: that blood type predicts exercise response. Public health guidance from the CDC lists no blood-type-based diet among recommended eating patterns.
A professional caveat worth naming: the 2014 study was observational in design, not randomized. That means even its negative findings cannot prove the diet fails. It simply found no blood-type-specific signal. Absence of evidence is not evidence of absence, but after three decades and millions of books sold, the burden of proof sits with the diet.
Red Flags: When to Stop or Never Start
- You have kidney disease or take warfarin. Lamb, leafy greens, and shifting vitamin K intake can destabilize anticoagulation.
- You are pregnant or breastfeeding. Restricting dairy, legumes, and grains can cut calcium, folate, and B12 below recommended levels.
- You have type 1 or type 2 diabetes on insulin. Large swings in carbohydrate sources require dose adjustment.
- You have a history of disordered eating. Food rules organized around identity markers can reinforce restriction.
- You take levothyroxine or iron supplements. Timing against dairy and high-fiber meals matters more than blood type.
If any of these apply, talk to a clinician before starting.
What to Actually Do Instead
If the appeal of the B positive diet is structure and a food list, keep the structure and drop the blood-type premise. Build meals around lean protein, vegetables, beans, and whole grains in proportions you can sustain. Most adults need roughly 0.8 grams of protein per kilogram of body weight daily, which for a 150-pound adult is about 55 grams.
Three concrete moves:
- Rotate chicken, fish, eggs, and legumes as protein anchors instead of eliminating any one of them.
- Keep low-fat yogurt or fortified plant milk to protect calcium intake near 1,000 milligrams daily.
- Get a complete blood count and a B12 or ferritin check if you have followed a restrictive version for more than six months.
The B positive blood type diet is not dangerous for most healthy adults in the short term, but it is unsupported, expensive, and harder to sustain than a straightforward whole-food pattern. Your blood type matters at a transfusion. It does not decide what belongs on your plate.
Limitations of This Evidence
Two limits deserve honesty. First, the negative evidence is observational and short. The 2014 study ran up to 75 days, which is far shorter than the years over which diet affects disease risk. Second, no trial has tested D'Adamo's specific protocol with a control group assigned to a non-matching diet. The strongest statement the science supports is that blood type has never been shown to modify diet response. That is a genuine gap, and anyone claiming otherwise, in either direction, is overstating what we know.