This page covers walnuts (Juglans spp., family Juglandaceae), including English walnut (Juglans regia), black walnut (Juglans nigra), and California walnut (Juglans californica).
A note on “California walnut”: walnuts sold as “California walnuts” are usually English walnuts (Juglans regia) grown in California. The native California black walnuts (Juglans californica and Juglans hindsii) are rarely eaten. The FDA’s January 2025 list names English/Persian walnut, black walnut, California walnut and heartnut (Japanese walnut) separately, and all of them must be declared on US labels.
Botanical Characteristics of Walnuts (Juglans spp.)
- Scientific Name: Juglans regia, Juglans nigra, Juglans californica
- Family: Juglandaceae (Walnut family)
- Common Names: English walnut, Persian walnut, Black walnut, California walnut
- Native Regions:
- Juglans regia (English walnut) → Europe, Middle East, Central Asia.
- Juglans nigra (Black walnut) → North America.
- Juglans californica (California walnut) → Western United States.
Tree and Nut Description
- Walnut trees are deciduous, growing 10–30 meters tall.
- The nuts are enclosed in a hard shell, surrounded by a green husk.
- Walnuts are among the most common tree nut allergens, with very high cross-reactivity to pecans; co-allergy with other tree nuts is common.
Walnuts as a Food Allergen
Walnuts are classified as a major food allergen by the U.S. FDA, requiring mandatory labeling.
Allergenic Proteins in Walnuts
| Allergen | Protein family | Molecular weight (kDa) | Clinical relevance |
|---|---|---|---|
| Jug r 1 | 2S albumin | 15–16 | Major allergen; closely related to pecan Car i 1 |
| Jug r 2 | Vicilin (7S globulin) with alpha-hairpinin peptides | 44 | Storage-protein allergen |
| Jug r 3 | Non-specific lipid transfer protein (nsLTP1) | 9 | Stable; relevant in LTP sensitization (often Mediterranean) |
| Jug r 4 | 11S globulin (legumin) | 58.1 | Storage-protein allergen |
| Jug r 5 | PR-10 (Bet v 1-related) | 20 | Birch-pollen related; heat-labile; usually mild |
| Jug r 6 | Vicilin-like (7S globulin) | 47 | Storage-protein allergen |
| Jug r 7 | Profilin | 13 | Pollen-related panallergen |
| Jug r 8 | Non-specific lipid transfer protein (nsLTP2) | 9 | LTP |
| Jug r 9 | Phospholipase D alpha 1 | 92 | Minor allergen |
| Jug n 1, Jug n 2, Jug n 4 (black walnut) | 2S albumin, vicilin, legumin | — | Black walnut storage-protein allergens |
Cross-Reactivity
- Walnuts have very high cross-reactivity with pecans, through closely related storage proteins (Jug r 1, Jug r 2 and Jug r 4 and their pecan counterparts).
- Walnut vicilin (Jug r 2) is related to peanut vicilin (Ara h 1), but walnut–peanut and walnut–almond overlap is mostly co-sensitization rather than true cross-reactivity.
- LTP (Jug r 3) cross-reacts with Rosaceae fruits (peach, apple, cherry).
- Jug r 5 is a PR-10 (Bet v 1-related) protein linked to birch-pollen allergy; it is destroyed by heat and usually causes mild oral symptoms.
Detailed epitope (amino-acid) maps previously shown here have been removed because they could not be matched to published studies. Exact epitope positions vary between studies and between patients.
Stability of Walnut Allergens
| Allergen | Heat stability | Digestive stability | Effect of processing |
|---|---|---|---|
| Jug r 1 | High | High | Survives roasting and digestion |
| Jug r 2 | Moderate to high | Moderate | Largely survives roasting |
| Jug r 3 | High | High | Survives cooking and digestion |
| Jug r 4 | Moderate to high | Moderate | Largely survives roasting |
| Jug r 5 | Low | Low | Usually destroyed by roasting or cooking |
- Walnut allergens like Jug r 1 and Jug r 3 remain stable, meaning roasted walnuts can still cause reactions.
- Cooking destroys Jug r 5 (PR-10), but the storage proteins (Jug r 1, Jug r 2, Jug r 4) survive roasting.
Comparison with Other Tree Nuts
| Nut | Key allergens (WHO/IUIS) | Strongest clinical cross-reactivity | Stability | Anaphylaxis risk |
|---|---|---|---|---|
| Almond | Pru du 6 (legumin), Pru du 3 (LTP); Pru du 1 (PR-10) | Among the least cross-reactive tree nuts | Storage proteins and LTP stable; PR-10 labile | High |
| Brazil nut | Ber e 1 (2S albumin), Ber e 2 (legumin) | Often an isolated allergy | Stable | High |
| Cashew | Ana o 3 (2S albumin), Ana o 1 (vicilin), Ana o 2 (legumin) | Pistachio — very high | Stable | High |
| Hazelnut | Cor a 9 (legumin), Cor a 14 (2S albumin); Cor a 1 (PR-10) | Birch pollen (Cor a 1, usually mild) | Cor a 9/14 stable; Cor a 1 labile | High with Cor a 9 or Cor a 14 sensitization |
| Macadamia | Mac i 1 (vicilin), Mac i 2 (legumin) | Often an isolated allergy | Stable | High |
| Pecan | Car i 1 (2S albumin), Car i 2 (vicilin), Car i 4 (legumin) | Walnut — very high | Stable | High |
| Pine nut | Pin p 1 (2S albumin) | Often an isolated allergy | Stable | High — many reported reactions are anaphylaxis |
| Pistachio | Pis v 1 (2S albumin), Pis v 2 and Pis v 5 (legumin), Pis v 3 (vicilin) | Cashew — very high | Stable | High |
| Walnut | Jug r 1 (2S albumin), Jug r 2 (vicilin), Jug r 4 (legumin); Jug r 3 (LTP) | Pecan — very high | Stable | High |
- Walnut allergens are structurally closest to pecan allergens, making walnut-pecan cross-reactivity highly likely.
- Individuals with LTP syndrome should avoid walnuts due to Jug r 3.
Conclusion
- Walnuts contain potent allergens (Jug r 1, Jug r 3) that cause severe IgE-mediated reactions.
- Cross-reactivity with pecans is very high; co-allergy with almonds and peanuts is common but mostly separate sensitization.
- Roasting does not eliminate allergenicity.
- Anaphylaxis risk requires strict avoidance and emergency preparedness.
Anaphylaxis: act fast. After an exposure, any breathing symptom (wheeze, repeated cough, throat tightness, hoarse voice), fainting or dizziness, or symptoms in two or more body systems (for example hives plus vomiting) is anaphylaxis — even without hives.
- Give epinephrine right away (auto-injector or nasal spray). Do not wait to see if symptoms get better.
- Call 911, even if symptoms improve.
- Lie flat with legs raised (sit up if breathing is hard). Give a second dose after 5–15 minutes if there is no improvement.
Antihistamines and steroids do not treat anaphylaxis and must never delay epinephrine. Carry two doses and follow the written emergency action plan from your allergist.