1. Why Are Walnut Allergy Reports Rising and Labeling Rules Changing in Japan?
In this context, "nuts" refers to a group including peanuts and tree nuts like walnuts. A national survey by Japan’s Consumer Affairs Agency showed that reported cases of immediate allergic symptoms from walnuts rose from 40 in fiscal year 2012 to 74 in 2015, and 251 in 2018. [1] Cases of life-threatening shock also increased, from 4 to 7, and then to 42. [1] In 2018, walnuts were the 4th most common cause of food allergies, behind eggs, milk, and wheat, while peanuts were 5th with 247 cases. [1] These numbers represent reported medical visits, not the total number of patients or the percentage of the population affected.
Japan’s mandatory labeling system began in 2001 with 5 items, including peanuts. [1] The Consumer Affairs Agency noted the need for caution due to the sharp increase in allergies to tree nuts, led by walnuts. [1] Yamanashi Prefecture guides that walnuts were added to the mandatory list in March 2023, bringing the total to 8 items, and cashew nuts were added in April 2026, bringing the total to 9 items. [2]
2. How Does the Body React to Protein Shapes Instead of Food Names in Allergies?
Food allergies occur when the body views proteins in food as foreign objects and reacts hypersensitively. [1] While the general mechanism involving IgE antibodies and mast cells is covered elsewhere, the specific trigger is crucial here. IgE antibodies bind to specific shapes of parts of proteins, not to the name of the food itself. [5] This means that even different foods can trigger the same IgE antibodies if their protein shapes are similar. [5] Whether similar shapes exist in other nuts is the central question for the rest of this discussion.
3. Why Are Some Allergen Proteins in Peanuts Stable Against Heat and Digestion?
The main allergens in peanuts are storage proteins found in the seeds. Ara h 2, 6, and 7 belong to the "2S albumin" group, while Ara h 1 and 3 belong to another group. [4] The "Ara h" prefix comes from the scientific name for peanuts, Arachis hypogaea. [4] 2S albumins are reported to be stable against both heating and enzymes that break down proteins. [4] Ara h 2 and Ara h 6 extracted from peanuts roasted at 140°C retain their original 3D structure, and IgE reactions are mainly directed toward this correctly folded shape. [3] However, some reports indicate that binding decreases with excessive heating, such as at 110°C for 15 minutes. [3] This stability is often discussed in relation to severe systemic symptoms, but it cannot be said to be the only reason for severity. [4]
4. How Similar Are the Proteins in Cashews and Pistachios?
Cashew nuts and pistachios are both seeds from trees in the Anacardiaceae family. Their respective 2S albumins (Ana o 3 for cashew, Pis v 1 for pistachio) have amino acid sequences that are about 70% similar. [5] In experiments using patient serum, cashew proteins almost completely blocked IgE binding. [5] This result suggests that IgE antibodies targeting cashews may also bind to pistachio proteins. [5] Cashews are a mandatory label item while pistachios are a recommended label item. [2] However, no source used here states that this specific study was the direct reason for the labeling rules.
5. Can Nuts Be Similar Without Being Related, and Different Even When Related?
Botanically, peanuts are legumes, not tree nuts (hard nuts), though they are grouped in the "nut and seed" category in food classification. [4] In mouse experiments, mice sensitized only to cashews reacted strongly to walnuts. [6] This was attributed to a protein in walnuts similar to vicilin (Jug r 2). [6] At the T-cell level, cross-reactivity was seen among cashews, walnuts, and peanuts. [6] These are mouse results and may not apply to humans. [6] In reports analyzing human serum, peanut allergen Ara h 2 shared IgE binding sites with almonds and Brazil nuts, but not with cashew or hazelnut allergens. [7] Thus, within the same nut group, similarity depends on the specific protein.
6. Did Babies Who Ate Peanuts Early Have Fewer Allergies in the LEAP Study?
The LEAP study involved 640 infants aged 4 to 11 months in the UK who had severe eczema or egg allergy. [8] They were randomly divided into groups that either ate peanuts at least 3 times a week or avoided them. [8] At 60 months (5 years), food challenge tests showed that 17.2% of the avoidance group and 3.2% of the eating group tested positive for peanut allergy. [8] The positive rate in the eating group was about 1/5 of the avoidance group. [8] This was a study of high-risk infants under expert supervision. It does not mean whole nuts should be given at home. [8] Due to risks of choking and severe reactions, any introduction must be discussed with a doctor. [8]
Safety note: This article does not support giving pieces of nuts at home based on self-judgment. There are risks of choking and allergic reactions. Always consult a doctor before changing a child’s diet regarding allergens.
7. What Food Label Information Can You Check at Home?
If you want to investigate further, you can look at the ingredient lists on snack or bread packages at home. Check the area near the ingredients for phrases like "(contains ...)" and see which food names are listed. You can compare whether the wording has changed between old and new packages. Official information from the Consumer Affairs Agency and local governments is available in Japanese. [1] This article is not medical advice; consult a doctor for symptoms or dietary questions. [1]