Understanding the Myth: Why Can't Type O Blood Have Coffee? According to the Blood Type Diet

October 7, 2025 •

4 min read

Despite its enduring popularity, a systematic review published in the American Journal of Clinical Nutrition found no scientific evidence to support the health claims of the blood type diet, including the theory for why can't type O blood have coffee?. This nutritional framework, devised by naturopath Peter D'Adamo, suggests specific foods, like coffee for Type Os, are incompatible with one's biology based on their blood group.

Quick Summary

The popular Blood Type Diet suggests Type O individuals avoid coffee due to purported effects on adrenaline and stomach acid. However, this theory is not supported by credible scientific evidence. Any perceived health benefits are likely tied to the diet’s emphasis on whole foods and exercise, rather than blood type.

Key Points

Diet Lacks Scientific Proof: The entire Blood Type Diet, which includes the claim about coffee for Type O individuals, is considered a fad diet with no supporting scientific evidence.
Coffee and Adrenaline: The diet’s rationale is that caffeine further raises already high adrenaline levels in Type Os, but this link is unproven and speculative.
Lectins Aren't the Issue: The theory that lectins in food interact harmfully with specific blood types is incorrect; cooking typically destroys lectins, and most affect all blood types similarly.
Personal Tolerance Varies: Individual caffeine tolerance is based on genetics and personal physiology, not blood type, so any sensitivity is unique to the person.
Health Benefits Are Misattributed: Any positive health outcomes from following the Blood Type Diet are likely due to its general emphasis on whole foods, not the blood type-specific rules.

The Blood Type Diet Explained

The claim that Type O blood individuals should avoid coffee originates from the Blood Type Diet, a nutritional plan developed by naturopathic physician Peter D'Adamo in the 1990s. The core idea of this diet is that a person's blood type (O, A, B, or AB) represents their ancestors' eating patterns—hunter-gatherers for Type O, agrarian for Type A, and so on. D'Adamo theorizes that certain foods contain proteins called lectins that can interact negatively with a person's specific blood type antigens, causing adverse health effects like illness and weight gain. According to this model, eating and exercising in line with your blood type is the key to optimal health.

The Justification for Restricting Coffee for Type O

For Type O individuals, D'Adamo outlines a dietary regimen emphasizing lean meat and high-intensity exercise, reflecting a 'hunter-gatherer' ancestry. Within this framework, coffee is placed on the 'avoid' list for two main reasons:

Adrenaline Regulation: D'Adamo's theory posits that Type O individuals already have naturally high levels of adrenaline and noradrenaline. Since caffeine is a stimulant that further elevates these stress hormones, it is deemed detrimental, potentially leading to anxiety and other nervous system issues.
Digestive System Sensitivity: The diet also suggests that Type O individuals are prone to higher-than-average stomach acid production. The acidity of coffee, combined with this supposedly high stomach acid, is claimed to increase the risk of peptic ulcers.

The Overwhelming Scientific Counter-Argument

Despite its popularity, the entire premise of the Blood Type Diet has been widely and consistently debunked by the mainstream scientific and medical communities. Multiple comprehensive reviews have found no scientific evidence to support D'Adamo's claims. For example, a 2013 review of 1,415 references in the American Journal of Clinical Nutrition found no evidence to support the purported health benefits of the blood type diet.

Experts point out several critical flaws in the diet's underlying assumptions:

Lectins Are Not the Problem: The claim that specific lectins in foods cause blood cell agglutination (clumping) based on blood type is unfounded. While some lectins can cause issues in their raw form, cooking typically denatures and destroys them, rendering them harmless. Moreover, most lectins react with all blood types, not just specific ones.
Any Health Benefits Are Incidental: Health professionals suggest that any positive outcomes experienced by people following the diet are likely a result of its general healthy-eating principles. The plan encourages consuming whole, unprocessed foods and regular exercise, which are beneficial for all individuals, regardless of blood type. The diet also restricts processed foods, added sugars, and unhealthy fats, which naturally leads to weight loss and improved health.
Evolutionary Theory is Faulty: The ancestral claims linking blood types to hunter-gatherer or agrarian diets are not supported by evolutionary science. The timeline of blood type evolution is debated and not correlated with specific dietary patterns in the way D'Adamo suggests.

Comparing the Blood Type Diet Claims vs. Scientific Consensus


Feature	Blood Type Diet Claim	Scientific Consensus
Basis	Blood type determines specific dietary needs based on ancestral eating patterns and lectin interactions.	The diet is based on pseudo-scientific theories lacking credible evidence and is considered a fad.
Coffee for Type O	Avoid coffee due to purportedly high baseline adrenaline and potential stomach acid issues.	No scientific evidence suggests Type O individuals should avoid coffee based on their blood type. Personal tolerance to caffeine varies, not by blood type.
Lectins	Incompatible lectins cause blood clumping and disease in specific blood types.	Most lectins react with all blood types and are destroyed by cooking. The theory of blood type-specific agglutination is not supported.
Health Outcomes	Following the diet improves digestion, energy, and prevents disease.	Any health improvements are likely due to the diet's general promotion of whole foods and exercise, not blood type-specific rules.

What This Means for Your Coffee Habits

Ultimately, whether or not you should drink coffee has nothing to do with your blood type. The decision should be based on your personal health, lifestyle, and how your body individually responds to caffeine. Some people may be more sensitive to caffeine than others, regardless of their blood type, experiencing jitters, increased heart rate, or sleep disruption from excessive consumption.

If you find that coffee causes you problems like anxiety or indigestion, limiting or avoiding it is a sensible choice. This decision, however, is a matter of individual physiology and personal preference, not a consequence of having Type O blood.

Alternatives to Coffee

For those who enjoy the ritual of a warm morning beverage but want to limit caffeine, there are several delicious and healthy alternatives available:

Herbal Teas: Options like chamomile, peppermint, or ginger tea offer calming or invigorating effects without caffeine.
Chicory Coffee: Made from the roasted and ground root of the chicory plant, this option provides a robust, coffee-like flavor without any caffeine.
Golden Milk: A warm, spiced drink made with turmeric, ginger, and milk (dairy or non-dairy), known for its anti-inflammatory properties.
Green Tea or Matcha: For a lower, more controlled dose of caffeine, green tea and matcha provide antioxidants and a more gentle boost.

Conclusion: Making Informed Nutritional Choices

The idea that people with Type O blood can't have coffee is a central tenet of the Blood Type Diet, a hypothesis that has been repeatedly disproven by scientific research. Instead of relying on an unvalidated dietary model, individuals are better served by focusing on a well-balanced diet rich in whole foods, managing caffeine intake based on personal tolerance, and incorporating regular exercise. Your blood type is a fundamental biological marker, but it does not dictate your nutritional needs or your ability to enjoy a morning cup of coffee. Consulting a registered dietitian or other healthcare professional is the best approach for creating a personalized and evidence-based nutrition plan.

Source for Further Reading: Blood type diets lack supporting evidence: a systematic review

Frequently Asked Questions

No, the Blood Type Diet is not scientifically proven. Multiple reviews, including one in the American Journal of Clinical Nutrition, found no evidence supporting the diet's claims linking blood type to optimal nutrition.

The diet's creator, Peter D'Adamo, claimed that Type O individuals have naturally high stress hormones (adrenaline) and that coffee's stimulating effects would exacerbate this. He also claimed the acidity of coffee could contribute to stomach ulcers in Type Os.

No, the lectin theory is largely unfounded. Cooking typically denatures and destroys lectins, and scientific evidence does not support the idea that specific lectins cause blood clumping in a blood type-specific manner.

Assuming no pre-existing caffeine sensitivity, a Type O person will experience the same effects from coffee as anyone else. There are no known adverse effects based solely on their blood type.

People who experience anxiety, insomnia, or digestive discomfort after consuming caffeine might benefit from reducing or eliminating coffee. These sensitivities are individual and not tied to blood type.

Excellent coffee alternatives include chicory root beverages, herbal teas like peppermint or chamomile, golden milk made with turmeric, and lower-caffeine options like green tea or matcha.

The most reliable method is to consult with a registered dietitian or healthcare professional. They can provide evidence-based recommendations considering your overall health, genetics, and personal needs, not just your blood type.