Allergies (Respiratory)

Respiratory Allergies (Allergic Rhinitis and Related Conditions)

1. Definition and Overview

Allergic rhinitis is an inflammatory disorder of the nasal mucosa induced by allergen exposure triggering IgE-mediated inflammation. It is an atopic disease characterized by symptoms of nasal congestion, clear rhinorrhea, sneezing, postnasal drip, and nasal pruritis. It affects one in six individuals and is associated with significant morbidity, loss of productivity, and healthcare costs.

Allergic rhinitis is a chronic, IgE-mediated type 2 inflammatory disease that affects both adults and children, imposing a significant financial burden on healthcare systems and negatively impacting patients' quality of life. An estimated 20–40% of people globally have allergic rhinitis, the most common clinical symptom of respiratory allergies.

Allergic rhinitis can be classified as either seasonal (intermittent) or perennial (chronic), with approximately 20% of cases being seasonal, 40% perennial, and 40% with features of both. Seasonal allergic rhinitis (SAR) is fairly easy to identify because of the rapid and reproducible onset and offset of symptoms in association with pollen exposure. Perennial AR is often more difficult to detect than SAR because of the overlap with sinusitis, respiratory infections, and vasomotor rhinitis.

2. Clinical Presentation

Clinically, allergic rhinitis is characterized by four major symptoms: rhinorrhea, sneezing, nasal itching, and nasal congestion. Allergic rhinitis symptoms result in sleep disturbance, fatigue, depressed mood, and cognitive function compromise that impairs quality of life and productivity.

In addition to nasal symptoms, patients with allergic rhinitis may also present with associated allergic conjunctivitis, non-productive cough, Eustachian tube dysfunction, and chronic sinusitis. There may be associated conjunctivitis, postnasal drip, Eustachian tube dysfunction, otitis media, sinusitis, and, in children, dental malocclusions and facial deformities.

Allergic rhinitis is classically characterized by ongoing symptoms of rhinorrhea, nasal congestion and blockage, sneezing, and/or itching of the nose. These symptoms may significantly impact patients' quality of life, often interfering with sleep and contributing to poor academic and work performance.

3. Body Systems Involved

3.1 The Immune System

Knowledge of immune system function is critical for understanding allergic airway disease, including the mechanisms behind allergic rhinitis. Hypersensitivity reactions occur when an exaggerated adaptive immune response is activated. Allergic rhinitis is an example of a type I, immunoglobulin E-mediated hypersensitivity reaction.

In allergic rhinitis, initial allergen exposure and sensitization involves antigen-presenting cells, T and B lymphocytes, and results in the generation of allergen-specific T cells and allergen-specific IgE antibodies. Sensitization results in the generation of allergen-specific IgE that circulates in the peripheral blood and attaches itself on the surface of all mast cells and basophils, including those that home to the nasal mucosa.

On re-exposure to relevant allergens, cross-linking of IgE on mast cells results in the release of mediators of hypersensitivity such as histamine and immediate nasal symptoms. Within hours, there is an infiltration by inflammatory cells, particularly Th2 T lymphocytes, eosinophils, and basophils into nasal mucosal tissue that results in the late-phase allergic response.

Allergen induces Th2 lymphocyte proliferation in persons with allergies with the release of their characteristic combination of cytokines including IL-3, IL-4, IL-5, IL-9, IL-10, and IL-13. These substances promote IgE and mast cell production. Mucosal mast cells that produce IL-4, IL-5, IL-6, and tryptase proliferate in the allergic epithelium.

3.2 Mast Cells

Mast cells are innate immune system effector cells that play a pivotal role in innate immunity and modulating adaptive immunity, rendering them key cells of allergic inflammation. They are typically located in body surfaces exposed to the external environment such as the nasal mucosa. Due to their location in the nasal mucosa, they are in the first line of defense against inhaled substances such as allergens. IgE-dependent activation of mast cells in the nasal mucosa following exposure to allergens in a sensitized individual is a cardinal mechanism in the pathophysiology of allergic rhinitis.

3.3 The Upper and Lower Respiratory Tract as a Unified Airway

Historically, allergic rhinitis was thought to be a disease process of the nasal airway alone. Still, the development of the unified airway theory has classified it as a component of systemic allergic response, with other associated conditions such as asthma and atopic dermatitis sharing an underlying systemic pathology.

Allergic rhinitis is a common chronic condition that is characterized by inflammation of the nasal mucosa. Although it presents with upper respiratory symptoms, there is a growing body of evidence to suggest that allergic rhinitis may be linked to the development of systemic allergic manifestations that include allergic asthma.

85% to 95% of patients with allergic asthma report rhinitis symptoms, and the severity of the two conditions increases in parallel on exposure to an allergen. Nasal administration of allergens can provoke impaired lower airway airflow in 25% to 30% of individuals and cause airway eosinophilia.

The relationships between rhinitis and asthma can be viewed under the concept that the two conditions may be manifestations of one syndrome, the 'chronic allergic respiratory syndrome,' occurring in two different parts of the respiratory tract. Some interactions arise from the fact that the nasal passages play a major homeostatic role by conditioning inhaled air, but perhaps even more important is the bidirectional interaction that results from the systemic inflammation that is produced after local allergic reactions.

3.4 Systemic Inflammation

Systemic circulation of inflammatory cells allows their infiltration into other tissues where chemoattractant and adhesion molecules already exist. Thus, besides local inflammation, allergic rhinitis also triggers a systemic inflammation, which can in turn augment inflammation in both the upper and lower airways.

The intricate interaction among mediators, cytokines, chemokines, neuropeptides, adhesion molecules, and various cells in the form of a complex network leads to the onset of specific symptoms and the non-specific hyperreactivity of allergic rhinitis. The pathophysiology of allergic rhinitis may be further complicated by the involvement of neural reflex mechanisms and the interaction of allergic and non-allergic stimuli.

4. Contributing and Associated Factors

4.1 Genetic Predisposition

An increasing proportion of the worldwide population is affected by allergic diseases such as allergic rhinitis, atopic dermatitis, and allergic asthma. Allergic diseases are complex and their development involves both environmental and genetic factors. Both environmental and genetic factors influence barrier integrity and sensitization to common environmental allergens. This can lead to a Th2-dominant environment, elevated IgE levels, and allergic phenotypes that affect a variety of organs.

Emerging insights into allergic rhinitis pathophysiology have elucidated the complex interplay between genetic predisposition, environmental factors, and immune system dysregulation. Notably, the role of the epithelial barrier and the microbiome in allergic rhinitis pathogenesis has garnered increasing attention, offering potential targets for novel therapies.

4.2 Common Allergens

Many causative agents have been linked to allergic rhinitis, including pollens, molds, dust mites, and animal dander. A thorough history should include patient questioning regarding a family history of atopic disease, the impact of symptoms on quality of life, and the presence of comorbidities such as asthma, mouth breathing, snoring, sleep apnea, sinus involvement, otitis media, or nasal polyps.

4.3 Environmental Factors

Several studies examining gene-environment interactions have examined diverse environmental determinants, particularly smoking, farming, indoor and occupational environments, air pollution, and diet or nutrition, as factors in allergic disease. Prenatal and postnatal active and passive parental smoking has been associated with adverse respiratory outcomes in children, whereas diversity in diet may reduce a child's allergy outcome.

The increase in allergic diseases is attributed to genetic predisposition, air pollution, climate change, lack of physical activity, and alterations in eating habits.

4.4 The Microbiome

The environmental microbiome represents the entirety of the microbes and their metabolites that we encounter in our environments. A growing body of evidence supports the role of the environmental microbiome in risk for and severity of allergic diseases and asthma.

The environmental microbiome represents a ubiquitous, lifelong exposure to non-self antigens. During the critical window between birth and one year of life, interactions between the early immune system and the environmental microbiome have two consequences: the individual microbiome is populated by environmental microbes, and the immune system is trained regarding which antigens to tolerate. During this time, a diversity of exposures appears largely protective, dramatically decreasing the risk of developing allergic diseases and asthma.

Dysbiosis of the nasopharyngeal microbiome was shown to potentiate respiratory allergic responses, with the predominance of Moraxella sp. leading to induced pulmonary epithelial damage and increased proinflammatory cytokine expression.

4.5 Early-Life Factors

Mode of delivery and breastfeeding play a role in shaping a child's gut microbiome. Early-life viral infections have been reported to provide protective disease effects. The atopic march reflects a progression of allergic disease, from atopic dermatitis, food allergy, allergic rhinitis, and asthma from infancy to adolescence. The origins of these diseases are complex, though understanding of genetic and environmental risk factors can shed light on possible preventative measures.

5. Nutrients, Herbs, and Natural Ingredients

5.1 Quercetin

Traditional Use

Quercetin is a flavonoid found naturally in foods such as onions, apples, capers, and berries, and has been used in various traditional botanical systems as a component of anti-inflammatory and anti-allergic plant preparations, though its use as an isolated compound is a modern development.

Scientific Evidence

Quercetin inhibits mast cell degranulation, reduces the production of histamine and pro-inflammatory cytokines, and restores homeostasis of the immune system by modulating the Th1/Th2 and Treg/Th17 balances. Additionally, its antioxidant properties help to dampen oxidative stress, a critical factor in the pathophysiology of allergic diseases.

In vitro studies have consistently demonstrated quercetin's ability to suppress allergic reactions. In vivo studies, particularly in murine models of allergic rhinitis, have confirmed its efficacy in relieving symptoms such as nasal itching, sneezing, rhinorrhea, and congestion, and dampening type 2 mucosal inflammation. Preclinical evidence also supports its therapeutic potential in asthma, conjunctivitis, atopic dermatitis, and food allergies.

However, human studies are still scarce, as only two clinical trials have investigated quercetin as a monotherapy. Both studies reported promising results, including symptom reduction and improved quality of life, though larger, randomized trials are needed to validate these findings.

Quercetin stabilizes mast cells, inhibits Lyn/PLCγ pathways, and improves rhinitis symptoms in small randomized trials using bioavailable formulations. A 2022 randomized, placebo-controlled, double-blind parallel-group clinical trial (Yamada et al., European Review for Medical and Pharmacological Sciences, PMID: 35776034) investigated a quercetin-containing supplement on allergic reactions, providing one of the few controlled human trial data points available.

Evidence strength: Current evidence supports quercetin's effectiveness in reducing oxidative stress and improving clinical outcomes in allergic rhinitis through dual antioxidant and anti-inflammatory mechanisms. While most evidence derives from animal studies, quercetin shows promise as a safe adjuvant therapy. Large-scale human clinical trials using high-bioavailability formulations are needed to establish standardized clinical guidance.

5.2 Vitamin D

Traditional Use

Vitamin D is obtained primarily through sunlight exposure and dietary sources such as fatty fish, egg yolks, and fortified foods. It has no specific traditional herbal use in the context of allergy management, but its role as an essential nutrient has long been recognized in multiple cultural health traditions emphasizing sun exposure and cod liver oil consumption.

Scientific Evidence

Vitamin D is known for its essential role in calcium metabolism and bone health; however, accumulating evidence suggests its critical involvement in immune system regulation. Vitamin D exerts immunomodulatory effects through its active form, 1,25-dihydroxyvitamin D, which interacts with the vitamin D receptor expressed in various immune cells.

In the adaptive immune system, vitamin D plays a crucial role in balancing pro-inflammatory and anti-inflammatory responses. It promotes regulatory T cell (Treg) differentiation, which helps suppress inflammatory responses and maintain immune homeostasis. Moreover, vitamin D inhibits the production of pro-inflammatory cytokines such as interleukin-6, tumor necrosis factor-α, and IL-17 while enhancing the secretion of anti-inflammatory cytokines like IL-10.

Furthermore, vitamin D supplementation has been reported to enhance the efficacy of conventional allergic rhinitis therapies, such as antihistamines, steroids, and immunotherapy, supporting symptom control and improved quality of life.

Evidence strength: Omega-3 fatty acids and vitamin D have both been studied in this context; vitamin D contributes to immune tolerance and epithelial integrity, although supplementation trials remain heterogeneous. Vitamin D supplements may be an effective adjunctive therapy for allergic rhinitis. However, although clinical studies have suggested that concomitant vitamin D therapy may be effective, individual studies vary in terms of conditions and degree of efficacy, and the usefulness of vitamin D supplementation remains unclear overall.

5.3 Omega-3 Polyunsaturated Fatty Acids

Traditional Use

Dietary sources of omega-3 fatty acids — including oily fish such as salmon, mackerel, and herring, as well as flaxseeds — have historically formed a central part of Nordic, Japanese coastal, and Mediterranean culinary traditions. These traditions associated fish-rich diets with general vitality and respiratory health, though specific anti-allergic applications were not formally recognized in pre-modern medicine.

Scientific Evidence

Omega-3 fatty acids modulate Th2 responses, promote regulatory T cells, and generate specialized pro-resolving mediators, with modest clinical benefits observed in pregnancy and early life.

Changes in dietary fat intake — specifically increases in n-6 PUFAs and decreases in n-3 PUFAs — through increased production of prostaglandin E2, may have contributed to the increase in the prevalence of asthma. PGE2 suppresses T-helper (Th)1 and increases Th2 phenotype, thus reducing IFN-gamma.

Evidence strength: Evidence from human clinical trials in the context of allergic rhinitis specifically is preliminary and the effects are characterized as modest. The mechanistic evidence from in vitro and animal studies is stronger than the clinical trial evidence.

5.4 Probiotics

Traditional Use

Fermented foods containing live microorganisms — including yogurt, kefir, kimchi, miso, and traditionally fermented vegetables — have been central to the diets of numerous cultures for millennia, valued for digestive and general health benefits. Their application specifically to allergy is a modern extrapolation from this traditional knowledge.

Scientific Evidence

Recent research suggests that the gut microbiome and immune regulation play important roles in allergic diseases. Probiotics may help by balancing gut bacteria and reducing inflammation.

A 2022 systematic review and meta-analysis published in PMC (Zeng et al.) that searched MEDLINE, Embase, and the Cochrane Central Register through June 2021 included 28 controlled studies. The results showed that probiotics significantly relieved allergic rhinitis symptoms (standardized mean difference −0.29, 95% CI [−0.44, −0.13]; p = 0.0003) and decreased Rhinoconjunctivitis Quality of Life Questionnaire scores compared with the control group, while also increasing the Th1/Th2 ratio.

Results across multiple studies indicate that probiotics, particularly Lactobacillus and Bifidobacterium, appear to prevent allergy recurrences, alleviate the severity of symptoms, and improve the quality of life of patients with allergic rhinitis. This occurs through immune system modulation via the induction of cytokine production, which causes a dominant Th1 response in allergic patients by modulating the Th1/Th2 balance.

Evidence strength: The use of probiotic bacteria could be an effective and safe way to prevent and/or treat allergic rhinitis, but its underlying mechanisms remain unclear. Therefore, clinical studies using probiotics and dietary intervention should be the focus of future research to enable more widespread use. Heterogeneity across probiotic strains, doses, and study populations makes universal recommendations difficult. The 2022 meta-analysis showing statistically significant effect sizes represents among the strongest level of evidence for any single supplement in this condition.

5.5 Butterbur (Petasites hybridus)

Traditional Use

Butterbur is a herbaceous perennial plant native to Europe and parts of Asia. In European folk medicine, particularly in Germany and Switzerland, preparations from the rhizome and leaves of Petasites hybridus were historically used for coughs, bronchial spasms, headache, and febrile conditions. It was described in texts of German herbalism and was listed among plants used for respiratory complaints.

Scientific Evidence

One study demonstrated that butterbur (Ze 339; 2 tablets, 3 times per day for 1 week) improved day and nighttime nasal symptoms, decreased nasal resistance as verified by rhinomanometry, and decreased anti-inflammatory mediators (histamine, LTB4, and cysteinyl-LT) in nasal fluids and serum. Petasins (petasin, isopetasin, and neopetasin) are believed to be the pharmacologically active components of butterbur extracts.

In a two-week, double-blind, placebo-controlled study of 186 people with intermittent allergic rhinitis, the use of butterbur at a dose of three standardized tablets daily, or one tablet daily, reduced allergy symptoms compared with placebo. Significantly greater benefits were seen in the higher dose group. Such dose-dependency is taken as a confirming sign that a treatment really works.

In another double-blind study, 330 people were given either butterbur extract (one tablet three times daily), the antihistamine fexofenadine, or placebo. The results showed that butterbur and fexofenadine were equally effective, and both were more effective than placebo.

A randomized, placebo-controlled study demonstrated that in sensitized patients, butterbur protected against adenosine monophosphate-induced nasal responsiveness during grass pollen season. In contrast, a more recent study found no significant effect of butterbur (50 mg twice daily for 2 weeks) on peak nasal inspiratory flow, total nasal symptom score, eye symptom score, or quality of life in patients with intermittent allergic rhinitis, compared with placebo.

Evidence strength: Evidence is mixed. Several well-designed RCTs support symptom reduction, but at least one negative RCT exists. Unrefined butterbur contains pyrrolizidine alkaloids, which are hepatotoxic; only PA-free standardized extracts have been evaluated in published clinical trials.

5.6 Stinging Nettle (Urtica dioica)

Traditional Use

Stinging nettle (Urtica dioica) has been used medicinally for centuries, traditionally valued as a natural antihistamine by blocking the body's ability to produce histamine. Its use as a food and medicine spans European, Middle Eastern, and Native American traditions, where the leaves were consumed as a cooked vegetable or prepared as infusions for respiratory, joint, and urinary complaints. The German Commission E recognizes nettle leaf as a traditional herbal remedy.

Scientific Evidence

A randomized double-blind clinical trial examined the benefit of stinging nettle in the management of clinical and laboratory signs and symptoms of allergic rhinitis. 74 patients with signs and symptoms of allergic rhinitis and a positive skin prick test were selected and randomly divided into two groups who received Urtica dioica 150 mg (Urtidin® tablet) or placebo for one month. Their signs and symptoms, eosinophil percentage on nasal smear, serum IgE, and interleukin IL-4, IL-5, and interferon-γ levels were recorded.

Based on the Sino-Nasal Outcome Test 22 (SNOT-22), a significant improvement in clinical symptom severity was observed in both groups (P < .001). Furthermore, a statistically significant reduction in mean nasal smear eosinophil count was observed after treatment with nettle (P < .01).

In a survey of patients with perennial allergic rhinitis, stinging nettle (Urtica dioica) was identified as the most commonly used herbal supplement, reported by 12.6% of patients.

Evidence strength: Preliminary. The trial cited above had only 40 completers, limiting statistical power. Larger, multi-center, adequately powered RCTs are lacking. The evidence base is currently insufficient to make strong clinical recommendations.

5.7 Spirulina

Traditional Use

The use of spirulina dates back to the 16th century, when the Aztec peoples consumed it as part of their diet. Spirulina represents a blue-green alga that is produced and commercialized as a dietary supplement for modulating immune functions, as well as ameliorating a variety of diseases. Its history of use as an anti-allergic agent, however, is primarily a contemporary clinical extrapolation rather than a traditional pharmacological use.

Scientific Evidence

It has been well documented that spirulina exhibits anti-inflammatory properties by inhibiting the release of histamine from mast cells. In a randomized, double-blind placebo-controlled trial, individuals with allergic rhinitis were fed daily either placebo or spirulina for 12 weeks. Peripheral blood mononuclear cells were isolated before and after the spirulina feeding and levels of cytokines (IL-4, IFN-γ, and IL-2) were measured. The study showed that high dose of spirulina significantly reduced IL-4 levels by 32%, demonstrating the protective effects of this microalga toward allergic rhinitis.

The active ingredient found in spirulina responsible for its anti-inflammatory activities is C-phycocyanin, a pigment commonly found in blue-green algae. It is known that C-phycocyanin can selectively inhibit the activity of cyclooxygenase-2, an enzyme responsible for prostaglandin biosynthesis.

A mixed-methods systematic review identified spirulina among a set of supplements with promising evidence for allergic rhinitis.

Evidence strength: Limited but positive. Clinical evidence comes from a small number of controlled trials with modest sample sizes. The IL-4 reduction finding is mechanistically plausible but requires replication in larger trials.

5.8 Curcumin

Traditional Use

Curcumin is the primary polyphenolic compound in turmeric (Curcuma longa), a rhizome that has been used in Ayurvedic and traditional Chinese medicine for more than two thousand years. In Ayurveda, turmeric preparations were used for respiratory conditions, nasal congestion, and inflammatory states. Traditional formulations included decoctions, milk preparations (golden milk), and topical applications.

Scientific Evidence

Curcumin inhibits NF-κB/MAPK signaling, enhances barrier function, and improves allergic rhinitis and dermatitis despite limited bioavailability. These mechanisms are relevant to the Th2-dominant inflammatory environment that characterizes allergic rhinitis. The key limitation of curcumin as a supplement is its poor aqueous solubility and low systemic bioavailability when consumed in standard formulations; various encapsulation and phospholipid-complexed delivery systems have been studied to address this.

Evidence strength: Mechanistic and preclinical evidence is substantial. Clinical trial evidence in allergic rhinitis specifically remains limited, and the heterogeneity of preparations (standard curcumin vs. bioavailability-enhanced forms) complicates interpretation of results.

5.9 Ginger (Zingiber officinale)

Traditional Use

Ginger has been used across South Asian, East Asian, and Middle Eastern traditional medicine systems for thousands of years to address respiratory and inflammatory conditions. In Ayurveda, it is classified as a warming herb (ushna) used for respiratory congestion and inflammatory conditions of the mucous membranes. Traditional Chinese medicine employs both fresh ginger (sheng jiang) for dispersing cold and dried ginger (gan jiang) for warming internal conditions. In Unani medicine, ginger preparations were used for catarrhal states of the respiratory tract.

Scientific Evidence

Ginger constituents — [6]-gingerol and [6]-shogaol — modulate Th1/Th2 balance, mast-cell activity, and oxidative stress, with early clinical evidence in rhinitis and asthma.

Evidence strength: Largely preclinical and mechanistic. Clinical trial evidence in allergic rhinitis is early-stage. The mechanistic plausibility is established, but robust human clinical data specific to respiratory allergy are not yet available.

5.10 Vitamin C

Traditional Use

Vitamin C-rich foods — including citrus fruits, rose hips, and certain vegetables — have historically been used across many cultural traditions to address respiratory ailments and catarrhal conditions, though pre-modern use was associated with the whole foods rather than any isolated compound.

Scientific Evidence

A systematic review identified vitamin C among supplements with promising evidence for allergic rhinitis. Vitamin C is proposed to reduce histamine levels in blood and has antioxidant activity that may limit oxidative stress in the inflamed nasal mucosa. The NIH Office of Dietary Supplements acknowledges the role of vitamin C in immune function, though its specific efficacy in allergic rhinitis requires further controlled trial evidence.

Evidence strength: Preliminary. Evidence from RCTs in allergic rhinitis specifically is limited. Vitamin C's established role in immune function provides mechanistic rationale, but effect sizes and optimal dosing in the context of respiratory allergy are not definitively established.

5.11 Bromelain

Traditional Use

Bromelain is a proteolytic enzyme complex extracted primarily from pineapple (Ananas comosus) stem and fruit. The pineapple plant has been used in traditional Central and South American medicine for anti-inflammatory and digestive purposes. Its specific application to allergic rhinitis, however, is a contemporary nutritional medicine application rather than a historically documented traditional use.

Scientific Evidence

A mixed-methods systematic review identified bromelain (referred to as part of a broader group of plant-based extracts) among a number of individual health supplements identified as potentially having a beneficial effect on allergic rhinitis. Bromelain has been proposed to exert anti-inflammatory effects by reducing the production of pro-inflammatory prostaglandins and modulating T-cell differentiation. The NCCIH (NIH) notes that most bromelain research is in vitro or animal-based, and robust human clinical trial evidence for its use in allergic rhinitis specifically is limited.

Evidence strength: Weak to preliminary. Most evidence is mechanistic or derived from in vitro/animal studies. Bromelain is sometimes studied in combination with quercetin or other compounds rather than as a standalone intervention for respiratory allergy.

6. Dietary and Lifestyle Factors

6.1 The Mediterranean Diet

The Mediterranean diet, which includes a lot of fruits and vegetables, whole grains, legumes, nuts, olive oil, and fish, has been linked to a variety of health benefits, including a lower risk of chronic and allergic disease.

A cross-sectional study in Mexican children found that adherence to a Mediterranean dietary pattern was inversely associated with asthma ever (OR = 0.60), wheezing ever (OR = 0.64), and rhinitis ever (OR = 0.41). These findings suggest a protective effect of following a healthy dietary pattern on asthma and allergic rhinitis in children.

A study in Cretan children suggests a beneficial effect of commonly consumed fruits, vegetables and nuts, and of high adherence to a traditional Mediterranean diet during childhood on symptoms of asthma and rhinitis. Diet may explain the relative lack of allergic symptoms in this population.

However, the evidence is not uniformly positive. In a systematic review, the majority of studies assessing the effect of the Mediterranean diet on allergies in childhood found no significant association. These findings coincide with Castro-Rodriguez's review that reported no significant effect of the Mediterranean diet on preventing atopic eczema, rhinitis, or atopy. The Mediterranean diet in children may prevent asthma or wheeze, but randomized controlled trials are lacking.

6.2 Western Diet and Processed Foods

Modern diets are usually based on ultra-processed foods and low consumption of fruits and vegetables, which leads to insufficient intake of dietary fiber, yet excessive intake of sugar, saturated fats, and omega-6 unsaturated fats. This diet is strongly associated with an increase in allergic diseases, as the lack of dietary fiber, as well as the intake of excess sugar, omega-6 unsaturated fats, and saturated fats may increase inflammation in the body, which may trigger or exacerbate these diseases.

Recent studies showed that caloric dense foods, processed foods, red meats, and sweets — the key components of Western diet — are important diet-related risk factors in asthma prevalence and its symptom severity. The adoption of a dietary pattern during childhood based on fast food, Western diet, and processed meat is related to the development of asthma in adult life.

6.3 Dietary Diversity and Early Life

Diversity in diet may reduce a child's allergy outcome. The intake of grapes, oranges, apples, and fresh tomatoes — the main local products in Crete — had no association with atopy but was protective for wheezing and rhinitis in a cross-sectional study of children.

6.4 Dietary Antioxidant Intake

The growth of airways during childhood may be vulnerable to oxidative exposures; suboptimal antioxidant status during this critical period might result in oxidative airway damage, reductions in airway compliance, or both. Suboptimal dietary intake of antioxidant vitamins, especially vitamins A, C, and E and the carotenoids, as well as other antioxidants, has been implicated in this process.

6.5 Environmental and Lifestyle Exposures

In children sensitized to fungi, their presence in schools can greatly exacerbate asthma symptoms. Built environment factors, including age of the school building, humidity, and presence of air-conditioning systems, appear to shape the indoor dust microbiome, including both bacterial and fungal changes, and impact severity of asthma symptoms.

Gene-environment interaction studies have examined diverse environmental determinants, particularly smoking, farming, indoor and occupational environments, air pollution, and diet or nutrition. Microbiota and stress, especially during the prenatal or early postnatal period, are additional relevant factors.

6.6 The Gut–Airway Axis and Dietary Fiber

The relationship between gut microbiome composition and respiratory allergy risk is increasingly studied. Recent research suggests that the gut microbiome and immune regulation play important roles in allergic diseases, and that probiotics may help by balancing gut bacteria and reducing inflammation. Dietary patterns that support gut microbial diversity — including high-fiber, plant-rich diets — are proposed to support regulatory immune responses relevant to allergic disease, though direct interventional data in allergic rhinitis are limited.

6.7 The Hygiene Hypothesis and Microbial Exposure

The prevalence of allergic diseases appears to be increasing over the past century, suggesting that environmental factors — rather than changing genetics — are primarily responsible for this change. The hygiene hypothesis proposes that reduced early-life microbial exposure in industrialized settings impairs normal immune tolerance development, increasing the risk of atopic and allergic conditions. During the critical window between birth and 1 year of life, interactions between the early immune system and the environmental microbiome train the immune system regarding which antigens to tolerate, and a diversity of exposures appears largely protective, dramatically decreasing the risk of developing allergic diseases and asthma.

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