Affecting over 25 million people in the United States, asthma is a common respiratory condition that costs the US over $80 billion annually in healthcare expenses. When not managed properly, asthma can interfere with sleep, cause absence from school/work, and require hospitalization. Long-term uncontrolled respiratory inflammation can cause permanent lung changes and reduce lung function. In severe cases, it can be life-threatening. (1)
What is Asthma
Asthma is an illness characterized by airway hyperreactivity to external stimuli causing chronic inflammation and difficulty breathing. People with asthma can experience asthma exacerbations, or attacks, after being exposed to a trigger that results in an allergic-type immune response within the lungs. During an attack, smooth muscles tighten around the airways, immune cells produce inflammatory chemicals, and cells lining the airways secrete excess mucus. This triad of constriction, inflammation, and mucus production restricts healthy airflow and makes breathing difficult, classifying asthma as an obstructive lung disease.
There are allergic (more common) and non-allergic (less common) forms of asthma, differentiated by symptom triggers and the concentration of IgE, an immune protein produced in response to exposure to an allergen, measured in the blood.
Although there is no cure for asthma, available treatment options effectively manage symptoms and prevent attacks. A conventional treatment approach with prescribed medications is a crucial component of asthma management, but many patients are turning to functional medicine to address underlying causes of asthma, help with symptom management, and reduce the dependence on medications.
How Common is Asthma in Children
Asthma is the most common chronic childhood illness in the United States, affecting an estimated six million children under the age of 18. Asthma is the third leading cause of hospitalization among children under 15, translating to almost 800,000 emergency department visits and over 60,000 inpatient hospitalizations in 2019. While the overall rate of childhood asthma declined between 2001 and 2019, it was still reported that 42.7% of children with asthma had at least one asthma attack in 2020. (1, 2, 3)
Asthma often begins in childhood, and the vast majority of childhood-onset asthma manifests as an allergic type, commonly co-occurring with eczema and hay fever, referred to as the allergic triad. Childhood asthma diagnosed before age 12 likely has a genetic influence. This is supported by the fact that having a parent with asthma increases a child's risk for developing asthma by at least three to six times. (3, 4)
Asthma rates differ by sex across the lifespan. Boys have higher asthma rates and risk of hospitalization compared to girls of the same age before puberty. This trend reverses during adolescence, and females more commonly experience asthma compared to males through adolescence and into adulthood. (4)
How Common is Asthma in Adults
Although most asthma begins in childhood, asthma can manifest for the first time during adulthood. Asthma is more common in adult females until the fifth decade of life, when the female-male asthma burden gap finally narrows. The trend reversal of asthma incidence and prevalence during key reproductive milestones suggests that sex hormones play a role in the pathology of asthma, with estrogen encouraging the inflammatory immune response in the lungs and testosterone suppressing it. (4, 5)
Over 21 million adults over 18 have asthma in the United States. Similar to the pediatric population, the rate of asthma attacks also declined from 2001 through 2019. Still, asthma caused over one million emergency room visits and over 100,000 hospital admissions among adults in 2019, and 40.7% of adults with asthma reported having at least one asthma attack in 2020. (2)
Asthma Signs & Symptoms
The most common asthma symptoms are:
- Shortness of breath,
- Chest tightness and pain,
- Wheezing (a high-pitched whistling/rattling sound) on exhalation.
- When symptoms are uncontrolled, frequent nighttime symptoms will interfere with sleep.
Signs of Worsening Asthma Include:
- More frequent/severe symptoms
- Shortness of breath occurs with minimal physical activity
- Lower than normal peak flow meter readings (see 'Conventional Treatment for Asthma' below for more detail)
- More frequent use of an emergency inhaler
For most, symptoms are episodic, and attacks are triggered by certain exposures, including:
Asthma Allergic Triggers
- Environmental allergens: animal dander, pollen, mold, dust, cockroaches
- Dietary allergens: peanut, soy, egg, dairy, wheat, sulfites, MSG, food dyes
Asthma Non-Allergic Triggers
- Air Pollution: smoke, car exhaust
- Respiratory infections
- Weather: cold air, humidity
- Occupational Exposures: cleaning solutions, paints, aerosols
- Medical conditions: acid reflux
- Medications: aspirin, non-steroidal anti-inflammatories (NSAIDs), beta-blockers
The Link Between Asthma and Gut Health
The respiratory and gastrointestinal (GI) tracts are derived from common tissues during fetal development. It is known that food allergens can cause asthma symptoms in sensitized patients. It is also common for people with asthma to have concurrent GI symptoms, especially reflux.
Given this, a link between GI health and asthma has been speculated, and the concept of a gut-lung connection has been born. Furthermore, a previous theory that the lungs are a sterile environment has been negated; it is now understood that the lungs house their own unique microbiota, likely influenced by the gut microbiome, that may impact the development of respiratory diseases such as asthma. (25, 27)
Simply stated, the gut-lung connection is the idea that the lungs and the gut microbiome are connected in two directions, and there exists cross-talk between immune cells located in the two organ systems that influence the health of each.
Dysbiosis, an imbalance in microbial composition in either location, can interfere with the healthy cell signaling in these immune pathways, leading to inflammatory, allergic, and further dysbiotic patterns associated with asthma. (26)
Several key dysbiotic patterns in the lungs and GI tract have been associated with asthma risk in people of all ages. Streptococcus, Haemophilus, and Moraxella bacteria predominance in the respiratory tract is associated with an increased risk for asthma symptoms.
Within the gut, both bacterial and fungal dysbiosis have a role in the development of asthma. Within the gut microbiome, Clostridium overgrowth has been associated with an increased risk of asthma, whereas species of Veillonella, Lachnospira, Rothia, Faecalibacterium, and Bifidobacterium bacteria have been associated with a lower incidence of wheezing. (25, 26, 28)
Gut Health Labs
To further explore how gut health may be contributing to asthma symptoms, several functional medicine tests are commonly recommended:
Comprehensive Stool Analysis: These in-depth stool analyses measure important gut health and function markers. The majority of the test is dedicated to identifying and quantifying commensal and pathogenic microorganisms (bacteria, yeast, parasites, and viruses) living in the large intestine. Analyzing these bacteria helps us better understand how dysbiosis may contribute to GI health, allergies, and asthma. Additionally, markers of intestinal inflammation, digestion/absorption, and bacterial metabolites are measured. Cumulatively, this test gives specific data markers that help diagnose underlying causes of symptoms and helps to guide treatment recommendations.
Organic Acid Testing: Organic acids are byproducts of the metabolic reactions constantly happening within the body. The organic acid test (OAT) measures organic acids through a simple urine collection and can give additional information regarding gut dysbiosis. This test also measures vitamins and antioxidants essential in immune pathways that may be suboptimal and contribute to hypersensitivities and allergies. Many argue that OAT testing is the best method for assessing fungal overgrowth.
Food Intolerances: Food allergies and sensitivities differ in the antibodies (immune proteins) involved in creating an unwanted immune response; allergies are IgE-mediated, and sensitivities are mediated by IgG and IgA antibodies. Identifying and eliminating food allergies and sensitivities is essential in removing triggers responsible for asthma exacerbations, reducing inflammation in the airways and the gut, and removing disruptors of a healthy gut microbiome.
Environmental Exposure Labs
An Inhalant Allergens Panel measures IgE antibodies to the most common allergens, including mites, molds, weeds, trees, grasses, and animal dander. Results from this test can help patients narrow down asthma triggers so they can be avoided.
Conventional Treatment for Asthma
Asthma treatment emphasizes the reduction of symptom severity and functional limitations caused by respiratory symptoms; and risk reduction related to future asthma attacks, lung function, and medication side effects (8). Asthma is considered to be well managed when (10):
- Peak flow meter readings are higher than 80% of personal best
- The short-acting beta-agonist inhaler is used less than twice weekly
- Waking from symptoms occurs less than twice monthly
- No more than one burst of oral steroids has been used in the last year
Different types of asthma are classified by the frequency of symptoms, lung function, and frequency of medication use for symptom management. This is important because pharmaceutical management of asthma is done in a stepwise approach based on the type of asthma you have. This article will not discuss all pharmaceutical options but will give a comprehensive overview of the most common types and forms of medications utilized in managing asthma. Before we dive in, though, let's first discuss the peak flow meter and Asthma Action Plan.
Peak Flow and Asthma Action Plan
A peak flow meter is a device that measures lung function. When asthma is under good control, and no symptoms are present, a "personal best" peak flow reading should be measured. Routine measurement of peak flow, and comparison to your personal best, will facilitate early recognition of worsening asthma and dictate proper steps for asthma management.
The Asthma Action Plan (AAP) is a written plan developed with one's doctor and is a critical component of asthma management. Everyone with asthma should have an AAP and update it yearly. Components of the AAP include asthma severity determined by peak flow, medications and dosages, and instructions for managing asthma attacks. AAPs are associated with better disease control and a lower risk of disease complications. (6, 7)
Bronchodilators stimulate smooth muscle relaxation, causing the airways to become wider to facilitate breathing. The predominant bronchodilators utilized in the asthma treatment algorithm are short-acting beta-agonists (SABA) and long-acting beta-agonists (LABA). The most notorious asthma medication, albuterol, falls under the classification of a SABA. SABAs act quickly and are utilized as rescue medications; they should be taken at first signs of asthma symptoms. A LABA might be added to a treatment plan, usually in combination with a steroid, when more persistent asthma symptoms are present. They have a more prolonged onset of action, so they are never used for immediate symptom relief but keep airways open longer than SABAs.
Anti-Inflammatories (corticosteroids/steroids) help to control and prevent asthma symptoms by reducing inflammation and mucus inside the airways. Steroids can be inhaled by an inhaler or taken orally in pill or liquid form. Inhaled corticosteroids (ICS) are the mainstay of persistent asthma treatment and are usually the first medication to be added to a SABA. An inhaler spacer can ensure that the ICS is inhaled, not swallowed, to prevent common side effects like sore throat, hoarseness of voice, and oral fungal overgrowth. Rinsing your mouth after administration can also reduce the risk of side effects.
Oral corticosteroids are utilized in acute asthma exacerbations that are not responding to other asthma medications or for cases of severe asthma. Corticosteroid side effects are more serious when taken orally and include mood swings, weight gain, high blood pressure, increased risk of infection due to immune suppression, adrenal suppression, and osteoporosis.
- Leukotriene Receptor Antagonists (LTRA): Taken in pill form, LTRAs block the effect of leukotrienes, inflammatory chemicals made by immune cells during an allergic response. The most common LTRA prescribed is montelukast.
- Biologics: Reserved for severe and persistent allergic asthma, biologics act by working on different aspects of the immune system to interrupt the inflammatory pathways responsible for causing asthma symptoms.
For acute exacerbations of asthma symptoms, you should follow your AAP for steps in taking additional rescue medications, contacting your doctor, or going to the emergency department. If you use a quick-relief inhaler more than two days a week for symptom management, a step up in the asthma treatment algorithm may be necessary. Oxygen therapy, magnesium sulfate, and intravenous corticosteroids may be utilized for severe attacks managed in the emergency department.
Functional Medicine Treatment for Asthma
Functional medicine's treatment goal for asthma is to help patients reduce the frequency of asthma attacks, increase lung function, and reduce the need for pharmaceutical intervention. Multiple therapeutic approaches need to be addressed to achieve this, including; lifestyle, nutrition, herbs, and supplements.
The first step in the root cause treatment approach to asthma is identifying and reducing triggers. This is also a very important aspect of the conventional treatment approach to asthma, as frequent exposure to triggers will cause symptoms even when taking the appropriate medications.
Airborne triggers are often difficult to avoid entirely, but some steps can be taken to reduce exposure. Removal of surfaces where allergens easily collect in the home (i.e., carpets, curtains, upholstered furniture) is an easy first step. When this is not possible, prioritizing weekly cleaning of surfaces, furniture, and bedding is critical, and installing a high-quality air purifier (i.e., HEPA filter) can help decrease overall airborne allergen load. This tool can help to identify and remove indoor triggers to improve indoor air quality.
Tobacco smoke contains thousands of chemicals that irritate the airways and can trigger asthma attacks. For smokers, smoking cessation is important to reduce airway inflammation and protect lung function. Everyone with asthma should avoid exposure to secondhand smoke.
Various breathing and stress management techniques have also been studied as adjunctive asthma therapies. A 2019 Cochrane review indicated that yoga is a form of exercise with a low risk of adverse effects and may improve quality of life and reduce medication usage.
A 2020 Cochrane review concluded that breathing exercises, including Buteyko breathing, pranayama breathing, and the Papworth Method, positively affect the quality of life, reduce hyperventilation symptoms, and improve lung function. Biofeedback may help reduce symptom severity and bronchodilator use (11, 12).
Obesity is a risk factor for developing asthma and poor asthma control, as the increased weight increases pressure within the chest. Weight loss interventions emphasizing diet quality and physical activity help to improve asthma outcomes in the overweight/obese population, suggesting that maintaining an ideal body weight may be beneficial for preventing and treating asthma.
Avoidance of Food Triggers: It is well known that food allergies and sensitivities can trigger acute asthma attacks and contribute to systemic and respiratory inflammation and mucus secretion. Elimination-Reintroduction diets are a useful diagnostic and therapeutic tool, either on their own or in conjunction with food sensitivity testing. Eliminating food additives such as tartrazine, benzoate, sulfur dioxide, MSG, and sulfites is equally important, as these increase asthma symptoms. (13, 14, 15, 16)
Sodium Restriction: Evidence suggests increased salt intake increases lung reactivity and mortality from asthma. Sodium restriction may help to thin mucus secretions to decrease airway restriction. Trials have also shown particular benefit of salt restriction in exercise-induced asthma, correlating the intervention with improved lung function measurements. (17, 18, 19)
High Antioxidant Diet: A whole foods diet emphasizing fruit and vegetable intake, high in antioxidants, is associated with better respiratory health and reduced asthma exacerbations.
Dietary Fiber: Prebiotic fiber supports gut health by feeding the healthy microbiota in a process called fermentation. Short-chain fatty acids (SCFAs) are a byproduct of this fermentation process and are known to confer many health benefits. SCFAs are believed to be important in decreasing asthma risk by how they can regulate the immune system, fuel the activities of beneficial gut bacteria, and reduce inflammation systemically and locally to the intestines. SCFAs are measured on a comprehensive stool analysis and can be optimized with dietary fiber intake if suboptimal.
Herbs & Supplements
Magnesium: Magnesium acts to relax smooth muscle, has an anti-inflammatory effect and decreases response to histamine (a compound released in allergic immune responses). Magnesium deficiency is associated with more severe asthma symptoms and exacerbates some unwanted side effects of asthma medications. Research studies have concluded that oral magnesium supplementation improves symptom control and quality of life. (20, 21, 22)
Vitamin D: Vitamin D supplementation is linked to the reduction of asthma attacks, hospital visits, and the use of oral steroids in the management of acute exacerbations, especially in the use of children.
Fish Oil: Omega-3 fatty acids helps to improve airway reactivity to triggers and respiratory function. This study proved that dietary supplementation with n–3 PUFA reduces bronchial inflammation even after low-dose allergen challenges. (23, 24)
Boswellia: Boswellia is an herb with anti-inflammatory and anti-allergy properties that has been shown to improve asthma symptoms and reduce the use of ICS and LABA medications for asthma control.
Cordyceps: Cordyceps Sinensis is an adaptogenic herb commonly used in traditional Chinese medicine to "strengthen" the lungs and immune system. Studies show that cordyceps improves the patient's quality of life by reducing asthma-related symptoms, onset frequency, and severity. It significantly reduced asthma-induced inflammation at the 3-month intervention period, as demonstrated by decreased expression of inflammatory biomarkers.
Homeopathic Allergen Drops: Homeopathic allergen drops are safe and effective daily drops that are specific to the zone the patient is currently living in. These drops are homeopathic sublingual immunotherapy that helps reduce the body's reaction to allergens by providing micro-doses of what plants/flora the person is allergic to so the body becomes conditioned to tolerate said allergens and reduce its allergic response. While these don't directly treat asthma, they can help offset inflammation in the respiratory tract and reduce IgE antibodies.
Probiotics: Oral probiotics contain live cultures of bacteria and yeasts to restore a healthy gut microbiome balance. Evidence suggests that probiotics can help regulate the gut-lung axis, decrease inflammation, and reduce sensitization to allergens. If your stool culture determines that dysbiosis is present, probiotics can help reintroduce desired bacterial cultures and crowd out the unwanted bacteria that may be causing problems.
Asthma is characterized by chronic lung inflammation and acute exacerbations of airway narrowing and mucus production, resulting in difficulty breathing. As asthma can be life-threatening if not managed correctly, a conventional pharmaceutical approach to treatment, outlined by an Asthma Treatment Plan, is important to have in place. Presently, available conventional treatments are aimed at reducing symptoms but cannot completely prevent exacerbation or cure asthma.
It is now supported that both lung and gut microbiota impact asthma development and severity. Considering the vital role of the microbiome in influencing immune responses, targeting the gut-lung axis may be a strategy to prevent asthma and exacerbation.
Working with a functional medicine provider to identify root causes of underlying respiratory inflammation and implementing lifestyle modifications and natural supplementation may be beneficial in reducing asthma symptom frequency and severity, dependence on medications, and risk of pharmaceutical side effects.
Lab Tests in This Article
1. Asthma Facts. (2022, April). Asthma and Allergy Foundation of America. https://www.aafa.org/asthma-facts/
2. Most Recent National Asthma Data. (2022, May 25). Centers for Disease Control and Prevention (CDC). https://www.cdc.gov/asthma/most_recent_national_asthma_data.htm
3. Asthma and Children Fact Sheet. (2020, October). American Lung Association. https://www.lung.org/lung-health-diseases/lung-disease-lookup/asthma/learn-about-asthma/asthma-children-facts-sheet
4. Dharmage, S. C., Perret, J. L., & Custovic, A. (2019). Epidemiology of Asthma in Children and Adults. Frontiers in Pediatrics, 7. https://doi.org/10.3389/fped.2019.00246
5. Fuseini, H., & Newcomb, D. C. (2017). Mechanisms Driving Gender Differences in Asthma. Current Allergy and Asthma Reports, 17(3). https://doi.org/10.1007/s11882-017-0686-1
6. Yawn, B. (2009, May 1). The New Asthma Guidelines. American Family Physician. https://www.aafp.org/pubs/afp/issues/2009/0501/p727.html
7. Peak Flow Meter: What It Is & How To Use It. (2020, December 15). Cleveland Clinic. https://my.clevelandclinic.org/health/articles/4298-peak-flow-meter
8. Asthma Care Quick Reference: Diagnosing and Managing Asthma. (2012, January 9). National Heart, Lung, and Blood Institute. https://www.nhlbi.nih.gov/resources/asthma-care-quick-reference-diagnosing-and-managing
9. How Is Asthma Treated? | AAFA.org. (2021, June). Asthma and Allergy Foundation of America. https://www.aafa.org/asthma-treatment/
10. Guidelines for the Diagnosis and Management of Asthma 2007 (EPR-3). (2020, December 3). National Heart, Lung, and Blood Institute. https://www.nhlbi.nih.gov/health-topics/guidelines-for-diagnosis-management-of-asthma
11. Coen, B. L., Conran, P. B., McGrady, A., & Nelson, L. (1996). Effects of Biofeedback-Assisted Relaxation on Asthma Severity and Immune Function. Pediatric Asthma, Allergy & Immunology, 10(2), 71–78. https://doi.org/10.1089/pai.1996.10.71
12. Kern-Buell, C. L., McGrady, A. V., Conran, P. B., & Nelson, L. A. (2000). Asthma severity, psychophysiological indicators of arousal, and immune function in asthma patients undergoing biofeedback-assisted relaxation. Appl Psychophysiol Biofeedback, 25(2), 79–91. https://doi.org/10.1023/a:1009562708112
13. Foong, R. X., du Toit, G., & Fox, A. T. (2017). Asthma, Food Allergy, and How They Relate to Each Other. Frontiers in Pediatrics, 5. https://doi.org/10.3389/fped.2017.00089
14. di Palmo, E., Gallucci, M., Cipriani, F., Bertelli, L., Giannetti, A., & Ricci, G. (2019). Asthma and Food Allergy: Which Risks? Medicina, 55(9), 509. https://doi.org/10.3390/medicina55090509
15. Mathison, D. A., Stevenson, D. D., & Simon, R. A. (1985). Precipitating Factors in Asthma. Chest, 87(1), 50S-54S. https://doi.org/10.1378/chest.87.1.50s
16. Freedman, B. J. (1977). Asthma induced by sulphur dioxide, benzoate and tartrazine contained in orange drinks. Clin. Allergy, 7(5), 407–415. https://doi.org/10.1111/j.1365-2222.1977.tb01471.x
17. Pogson, Z., & McKeever, T. (2011). Dietary sodium manipulation and asthma. Cochrane Database of Systematic Reviews. https://doi.org/10.1002/14651858.cd000436.pub3
18. Carey, O. J., Locke, C., & Cookson, J. B. (1993). Effect of alterations of dietary sodium on the severity of asthma in men. Thorax, 48(7), 714–718. https://doi.org/10.1136/thx.48.7.714
19. Mickleborough, T. D., & Fogarty, A. (2006). Dietary sodium intake and asthma: an epidemiological and clinical review. International Journal of Clinical Practice, 60(12), 1616–1624. https://doi.org/10.1111/j.1742-1241.2006.01103
20. Kılıc, H., Kanbay, A., Karalezlı, A., Babaoglu, E., Hasanoglu, H., Erel, O., & Ates, C. (2018). The Relationship between Hypomagnesemia and Pulmonary Function Tests in Patients with Chronic Asthma. Medical Principles and Practice, 27(2), 139–144. https://doi.org/10.1159/000487760
21. Gontijo-Amaral, C., Ribeiro, M. A. G. O., Gontijo, L. S. C., Condino-Neto, A., & Ribeiro, J. D. (2006). Oral magnesium supplementation in asthmatic children: a double-blind randomized placebo-controlled trial. European Journal of Clinical Nutrition, 61(1), 54–60. https://doi.org/10.1038/sj.ejcn.1602475
22. Kazaks, A. G., Uriu-Adams, J. Y., Albertson, T. E., Shenoy, S. F., & Stern, J. S. (2010). Effect of Oral Magnesium Supplementation on Measures of Airway Resistance and Subjective Assessment of Asthma Control and Quality of Life in Men and Women with Mild to Moderate Asthma: A Randomized Placebo Controlled Trial. Journal of Asthma, 47(1), 83–92. https://doi.org/10.3109/02770900903331127
23. Schubert, R., Kitz, R., Beermann, C., Rose, M., Lieb, A., Sommerer, P., Moskovits, J., Alberternst, H., Böhles, H., Schulze, J., & Zielen, S. (2008). Effect of n–3 Polyunsaturated Fatty Acids in Asthma after Low-Dose Allergen Challenge. International Archives of Allergy and Immunology, 148(4), 321–329. https://doi.org/10.1159/000170386
24. Dry, J., & Vincent, D. (1991). Effect of a Fish Oil Diet on Asthma: Results of a 1-Year Double-Blind Study. International Archives of Allergy and Immunology, 95(2–3), 156–157. https://doi.org/10.1159/000235421
25. Hansen, R., Gerasimidis, K., & Turner, S. (2019). Asthma causation and the gastrointestinal microbiome and metabolome: Might there be a signal, or is it just noise? Journal of Allergy and Clinical Immunology, 144(2), 401–403. https://doi.org/10.1016/j.jaci.2019.04.026
26. Barcik, W., Boutin, R. C., Sokolowska, M., & Finlay, B. B. (2020). The Role of Lung and Gut Microbiota in the Pathology of Asthma. Immunity, 52(2), 241–255. https://doi.org/10.1016/j.immuni.2020.01.007
27. Hufnagl, K., Pali-Schöll, I., Roth-Walter, F., & Jensen-Jarolim, E. (2020). Dysbiosis of the gut and lung microbiome has a role in asthma. Seminars in Immunopathology, 42(1), 75–93. https://doi.org/10.1007/s00281-019-00775-y
28. Arrieta, M. C., Stiemsma, L. T., Dimitriu, P. A., Thorson, L., Russell, S., Yurist-Doutsch, S., Kuzeljevic, B., Gold, M. J., Britton, H. M., Lefebvre, D. L., Subbarao, P., Mandhane, P., Becker, A., McNagny, K. M., Sears, M. R., Kollmann, T., Mohn, W. W., Turvey, S. E., & Brett Finlay, B. (2015). Early infancy microbial and metabolic alterations affect risk of childhood asthma. Science Translational Medicine, 7(307). https://doi.org/10.1126/scitranslmed.aab2271
29. Tecklenburg, S. L., Mickleborough, T. D., Fly, A. D., Bai, Y., & Stager, J. M. (2007). Ascorbic acid supplementation attenuates exercise-induced bronchoconstriction in patients with asthma. Respiratory Medicine, 101(8), 1770–1778. https://doi.org/10.1016/j.rmed.2007.02.014
30. Kaur, B., Rowe, B. H., & Stovold, E. (2009). Vitamin C supplementation for asthma. Cochrane Database of Systematic Reviews. https://doi.org/10.1002/14651858.cd000993.pub3