Do Hazelnuts Have Side Effects? Understanding the Risks and Allergic Reactions

November 11, 2025 •

4 min read

Nut allergies, particularly to tree nuts like hazelnuts, are among the most common food allergies in many countries. For most people, hazelnuts are a healthy and delicious snack, but for those with sensitivities, the proteins in hazelnuts can trigger allergic reactions, ranging from mild Oral Allergy Syndrome to severe anaphylaxis. Do hazelnuts have side effects? Yes, and understanding them is key to safe consumption.

Quick Summary

Hazelnuts can cause allergic reactions in sensitive individuals, ranging from mild oral symptoms to severe anaphylaxis. Overconsumption can also lead to digestive issues like bloating and gas, as well as weight gain due to their high calorie density. People with birch pollen allergies often experience cross-reactivity with hazelnuts.

Key Points

Allergic Reactions: Hazelnuts are a common tree nut allergen that can cause symptoms from mild oral itching to severe, life-threatening anaphylaxis.
Oral Allergy Syndrome (OAS): A milder, secondary allergy related to birch pollen can cause symptoms in the mouth and throat. Roasting may reduce the allergenicity for some individuals with OAS.
Digestive Upset: High fiber content in hazelnuts can cause bloating, gas, and stomach issues if eaten in large quantities, especially for those with sensitive digestive systems or IBS.
Risk of Weight Gain: Despite healthy fats, hazelnuts are high in calories. Exceeding recommended portion sizes (1-2 ounces) can contribute to unwanted weight gain.
Mineral Absorption Concerns: Phytic acid in hazelnuts can slightly inhibit the absorption of certain minerals like zinc and iron, but this is a minor concern in a balanced diet.
Pregnancy Safety: Hazelnuts are generally safe for pregnant women in food amounts, but moderation is advised, and those with allergies or concerns should consult a doctor.

The Most Serious Risk: Hazelnut Allergies

For many, the most significant risk associated with hazelnuts is a tree nut allergy. Hazelnut allergy is a prevalent issue, particularly in Europe and the United States. Symptoms can appear within minutes to a few hours after ingestion and vary significantly in severity.

Primary Hazelnut Allergy: Severe Reactions

This type of allergy is often associated with more severe, systemic reactions, including potentially life-threatening anaphylaxis. The proteins responsible, such as Cor a 9 and Cor a 14, are stable and resistant to heat, meaning cooked or roasted hazelnuts can still trigger a reaction. These severe allergies are more common in children and can affect the entire body, causing hives, swelling, and difficulty breathing.

Secondary Hazelnut Allergy: Oral Allergy Syndrome (OAS)

Many adults experience a milder, more localized reaction known as Oral Allergy Syndrome (OAS) or pollen food syndrome. This occurs due to a cross-reactivity between proteins in tree pollen, especially birch pollen, and similar proteins found in raw hazelnuts, specifically Cor a 1. For these individuals, symptoms are typically confined to the mouth and throat and include itching, tingling, or swelling. Roasting hazelnuts can often destroy the responsible protein, making cooked versions safe for some with OAS.

Common Hazelnut Allergy Symptoms

Mild to Moderate: Itching or tingling in the mouth, hives, swelling of the lips, tongue, or face, abdominal pain, nausea, and diarrhea.
Severe (Anaphylaxis): Constriction of airways, swollen tongue or throat, weak pulse, dizziness, and shock.

Digestive Issues and Hazelnut Consumption

Even for those without an allergy, consuming too many hazelnuts can lead to digestive discomfort. Hazelnuts are rich in dietary fiber, which is generally beneficial for gut health and regularity. However, a sudden or excessive increase in fiber intake can cause bloating, gas, and an upset stomach, especially if your body is not accustomed to it.

For individuals with Irritable Bowel Syndrome (IBS) or other digestive sensitivities, hazelnuts contain fermentable carbohydrates known as FODMAPs, specifically galacto-oligosaccharides (GOS). To avoid triggering symptoms like cramping and bloating, people with FODMAP sensitivity should limit their intake. For example, a low FODMAP serving is considered around 10 hazelnuts.

Weight Gain and High Calorie Density

Hazelnuts, like other nuts, are nutrient-dense but also high in calories and fat. While the fats are primarily heart-healthy monounsaturated fats, consuming large quantities can easily lead to a significant increase in daily calorie intake and contribute to weight gain. A typical serving size is about 1 to 2 ounces, which is roughly 15 to 20 whole hazelnuts. Overeating this calorically dense food can counteract any weight management efforts.

Potential Effects on Mineral Absorption

Hazelnuts contain phytic acid, a natural compound that can bind to minerals like zinc, iron, and calcium, reducing their absorption. For most people on a balanced diet, this effect is negligible. However, if a significant portion of a person's mineral intake comes from nuts and other plant-based foods, and they are consumed in very large amounts, it is a factor to consider.

Other Considerations: Contamination and Pregnancy

Though rare and not an inherent property of the nut itself, food safety concerns can arise from processing and handling. One report mentions a botulism outbreak linked to contaminated yogurt with hazelnuts, highlighting the importance of safe food handling practices.

For pregnant and breastfeeding women, hazelnuts are generally considered safe in food amounts. In fact, they offer beneficial nutrients like folic acid. However, moderation is key, and women with known nut allergies should avoid them entirely. Some sources also advise against raw or salted nuts due to potential foodborne illness risks.

Comparison of Hazelnut Benefits vs. Risks


Feature	Benefits	Potential Risks (with excessive intake)
Nutrition	Rich in monounsaturated fats, fiber, vitamin E, antioxidants, magnesium, and protein.	High calorie density leading to weight gain if portions are not controlled.
Heart Health	Can lower LDL ('bad') cholesterol and raise HDL ('good') cholesterol.	Negligible, provided moderation is practiced.
Digestion	Fiber promotes healthy bowel function and prevents constipation.	High fiber can cause gas, bloating, and upset stomach in large amounts.
Allergy	Not applicable.	Can cause mild to severe allergic reactions, including anaphylaxis, in sensitive individuals.
Mineral Absorption	Rich in essential minerals like iron, zinc, and calcium.	Phytic acid can reduce the absorption of these minerals if consumed excessively.

What to Do if You Experience Hazelnut Side Effects

If you believe you are experiencing side effects from eating hazelnuts, consider these steps:

For allergic reactions: If symptoms are mild (e.g., mouth itchiness), stop eating immediately. For severe symptoms like difficulty breathing or swelling, seek immediate emergency medical help.
For digestive issues: Reduce your portion size. Stick to the recommended 1-2 ounces daily, or less if you have FODMAP sensitivity. Gradually increase your fiber intake from various sources over time.
For weight concerns: Practice strict portion control. An ounce (around 20 nuts) is a sensible serving size. Integrate hazelnuts into meals rather than mindlessly snacking on large quantities.
Consult a professional: If you have ongoing symptoms, a known nut allergy, or are concerned about interactions with medications or medical conditions, consult a doctor or registered dietitian.

Conclusion: Moderation and Awareness are Crucial

Hazelnuts are undeniably a nutritious and flavorful addition to a healthy diet, offering a wealth of monounsaturated fats, vitamins, and minerals. However, the key to safely enjoying their benefits lies in moderation and a heightened awareness of potential side effects. The most critical risk is a severe allergic reaction, which can be life-threatening for sensitive individuals. For the general population, paying attention to portion sizes can prevent digestive discomfort and unwanted weight gain due to their high calorie content. By consuming hazelnuts in moderation and being mindful of your body's response, most people can enjoy them without issue. For specific medical concerns, especially allergies, consulting a healthcare provider is essential.

Frequently Asked Questions

Yes, hazelnut is a common tree nut allergen that can cause allergic reactions ranging from mild Oral Allergy Syndrome (OAS) to severe, life-threatening anaphylaxis in sensitive individuals.

Consuming an excessive number of hazelnuts can cause digestive discomfort, including gas, bloating, and stomach upset, primarily due to their high fiber content. Individuals with IBS or FODMAP sensitivity may experience worse symptoms.

Hazelnuts are calorie-dense, and overconsuming them can lead to weight gain. However, when eaten in moderation as part of a balanced diet (typically 1-2 ounces), their healthy fats and fiber can help with satiety without causing weight gain.

A recommended daily intake is typically 1 to 2 ounces, or about 15-20 whole hazelnuts. Limiting your portion size is the best way to enjoy their benefits while minimizing the risk of digestive issues or excess calorie intake.

Yes, hazelnuts are safe for pregnant and breastfeeding women when consumed in moderation in food amounts. They are a source of beneficial nutrients like folic acid, but women with nut allergies should avoid them entirely.

People with birch pollen allergy may experience cross-reactivity with hazelnuts, leading to Oral Allergy Syndrome (OAS). Symptoms are often milder and confined to the mouth. Roasting the hazelnuts may denature the protein responsible for this reaction, making them easier to tolerate for some.

Hazelnuts contain phytic acid, which can bind with certain minerals and reduce their absorption. For most individuals on a varied diet, this effect is insignificant. It only becomes a consideration with extremely high consumption levels or a diet heavily reliant on nuts and seeds.