Cold & Flu (Upper Respiratory Viral Infections): A Nutrition and Natural-Health Reference
1. Definition and Overview
The common cold and influenza (flu) are the most common syndromes of infection in human beings. Acute upper respiratory tract viral infections (URTIs) are the most common diseases of human beings, with adults having two to five common colds each year and school children having from seven to ten colds per year.
The common cold is an upper respiratory infection caused by a cold virus. The development of a "cold-like illness" (CLI) usually requires infection with an upper respiratory virus such as rhinovirus, influenza virus, respiratory syncytial virus, parainfluenza virus, coronavirus, or adenovirus, among others, and the development of sufficient signs, symptoms, and pathophysiologies to qualify as being ill based on personal and cultural definitions.
Influenza is a highly contagious airborne disease that occurs in seasonal epidemics and manifests as an acute febrile illness with variable degrees of systemic symptoms, ranging from mild fatigue to respiratory failure and death. The influenza virus family contains three main types: A, B, and C. Influenza A occurs more frequently and is more serious; it regularly produces varieties to which populations have no resistance, giving rise to pandemics.
The World Health Organization (WHO) estimates that worldwide, seasonal influenza alone results in 3–5 million cases of severe illness that require hospitalization, and 290,000–650,000 deaths annually.
2. Clinical Presentation
Common Cold
Symptoms of the common cold usually begin 2–3 days after infection and include runny nose, nasal congestion, sneezing, sore throat, postnasal drip, cough, and headache. Additional symptoms may include malaise, fatigue, muscle weakness, or loss of appetite. Fever and extreme exhaustion are more usual in influenza.
Influenza
The systemic symptoms of fever, headache, malaise, myalgia, and anorexia are related to the effects of cytokines released from immune cells; these responses develop rapidly in the first days of infection when the virus is detected by the immune system. New knowledge of the effects of cytokines now helps to explain some of the symptoms of colds and flu that were previously in the realm of folklore rather than medicine — for example, fever, anorexia, malaise, chilliness, headache, and muscle aches and pains.
Distinguishing Features
Common cold symptoms include sore throat, runny nose, nasal congestion, malaise, sneezing, sinusitis, and cough, sometimes accompanied by muscle aches, fatigue, malaise, headache, muscle weakness, or loss of appetite. Fever and extreme exhaustion are more typical of influenza. Flu can develop into high fever, acute bronchitis, encephalitis, and pneumonia. Risk of serious illness is higher in the elderly (over 65), babies and infants, and people with chronic illness.
3. Body Systems Involved
Respiratory Epithelium
The primary site of infection is the nasal epithelium when caused by inhalation of airborne droplets (aerosols), direct contact with infected nasal secretions, or contaminated objects from an infected individual. These viruses gain entry into nasal epithelium of the respiratory tract by binding to specific receptors. All of these viruses trigger inflammatory responses, which lead to observed symptoms. For example, rhinorrhoea and nasal obstruction are caused by increased vascular permeability and stimulation of mucus hypersecretion. Cough is probably caused by irritation from posterior pharyngeal drainage or direct infection of the large airways.
Immune System
The degree of illness occurring during a viral URTI is directly related to the extent of provoked inflammation, which in turn depends on the engagement of antiviral defense systems. The primary mechanism of influenza pathophysiology is a result of lung inflammation and compromise caused by direct viral infection of the respiratory epithelium, combined with the effects of lung inflammation caused by immune responses recruited to handle the spreading virus.
Cardiovascular and Systemic Effects (Influenza)
Inflammation can spread systemically and manifest as multiorgan failure, but these consequences are generally downstream of lung compromise and severe respiratory distress. Some associations have also been observed between influenza virus infection and cardiac sequelae, including increased risk of myocardial disease in the weeks following influenza virus infection; the mechanisms of this, beyond a general inflammatory profile, are still unresolved.
4. Contributing and Associated Factors
Age
Children tend to have more colds than adults. Immunity can be impaired in older people, particularly those who are frail, in those living with obesity, in those who are malnourished, and in those with low intakes of micronutrients. Immune competence can be diminished with ageing, a process called immunosenescence.
Nutritional Status and Micronutrient Deficiency
Malnutrition or a diet lacking in one or more nutrients can impair the production and activity of immune cells and antibodies. Animal studies have found that deficiencies in zinc, selenium, iron, copper, folic acid, and vitamins A, B6, C, D, and E can alter immune responses. These nutrients help the immune system in several ways: working as an antioxidant to protect healthy cells, supporting growth and activity of immune cells, and producing antibodies. Epidemiological studies find that those who are poorly nourished are at greater risk of bacterial, viral, and other infections.
Obesity
Obesity increases both the risk and severity of RTI. Being overweight is associated with decreased immune response to influenza virus and decreased response to flu vaccines. Obesity is associated with low-grade chronic inflammation. Fat tissue produces adipocytokines that can promote inflammatory processes. Research is early, but obesity has also been identified as an independent risk factor for the influenza virus, possibly due to the impaired function of T-cells, a type of white blood cell.
Psychological Stress
Factors associated with greater risk of respiratory illnesses after virus exposure include smoking, ingesting an inadequate level of vitamin C, and chronic psychological stress. Many modifiable factors influence the immune response. These include stress, physical fitness, frailty, body fatness, and diet.
Sleep
Sleep quality is thought to be an important predictor of immunity and in turn susceptibility to the common cold. Whether sleep duration and efficiency in the weeks preceding viral exposure are associated with cold susceptibility has been specifically examined. In a well-designed rhinovirus challenge study, 153 healthy men and women volunteers, ages 21–55, reported their sleep duration and efficiency for 14 consecutive days; participants were then administered nasal drops containing a rhinovirus, quarantined, and monitored for five days. There was a graded association with average sleep duration, with those averaging fewer than 7 hours being nearly three times more likely to develop a cold.
Physical Activity and Social Factors
Factors associated with decreased risk of respiratory illness after virus exposure included social integration, social support, physical activity, adequate and efficient sleep, and moderate alcohol intake. Host factors including age, BMI, physical activity, and diet contribute significantly to immune responses and should be included in human clinical trials that assess immune endpoints.
Transmission Factors
Colds spread by airborne droplets when a person coughs or sneezes, or by hand contact. RTIs are usually spread through contact with others who are infected, for example from exposure to coughs, sneezes, and even breath. RTIs spread rapidly when people are in confined spaces: schools, large office blocks, public transportation, etc.
5. Nutrients, Herbs, and Natural Ingredients
5.1 Vitamin C (Ascorbic Acid)
Traditional Use
Vitamin C has been proposed for treating respiratory infections since it was isolated in the 1930s. It became particularly popular in the 1970s when Nobel laureate Linus Pauling concluded from earlier placebo-controlled trials that vitamin C would prevent and alleviate the common cold; over two dozen new trials were undertaken thereafter. Vitamin C has been widely sold and used as a preventive and therapeutic agent.
Scientific Evidence
Regular ingestion of vitamin C had no effect on common cold incidence in the ordinary population, based on 29 trial comparisons involving 11,306 participants. However, with respect to duration and severity, thirty-one comparisons examined the effect of regular vitamin C on common cold duration (9,745 episodes). In adults, the duration of colds was reduced by 8% and in children by 14%. In children, 1 to 2 g/day vitamin C shortened colds by 18%. The severity of colds was also reduced by regular vitamin C administration.
Seven comparisons examined the effect of therapeutic vitamin C (3,249 episodes). No consistent effect of vitamin C was seen on the duration or severity of colds in the therapeutic trials.
The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use. However, evidence suggests that it could be justified in people exposed to brief periods of severe physical exercise or cold environments.
Evidence strength: Moderate to strong evidence that regular supplementation modestly reduces cold duration and severity; no consistent evidence for prevention in the general population or for therapeutic use after symptom onset (Cochrane systematic review, 2013).
5.2 Zinc
Traditional Use
Zinc lozenges for the common cold were popularized in the 1980s following an initial trial suggesting that direct contact of zinc with the oropharyngeal mucosa could inhibit viral replication locally. In laboratory studies, zinc inhibited the replication of respiratory viruses and enhanced the effect of interferons. Non-immune mechanisms have also been proposed to explain the effect of zinc lozenges on the common cold.
Scientific Evidence
A systematic review was restricted to trials examining the therapeutic effect of zinc lozenges on natural common cold infections, requiring a concurrent placebo group because clinically relevant common cold outcomes are largely subjective. Combining the P-values of all 13 comparisons provides very strong evidence that the zinc lozenge and placebo groups differ. However, the benefit of zinc is restricted to trials where the dose was greater than 75 mg/day. A significant effect by zinc lozenges is seen in three high-dose trials where zinc acetate was used and in five high-dose trials which used zinc salts other than acetate.
The evidence is very strong that zinc lozenges may shorten the duration of colds by approximately 33%. The optimal composition of zinc lozenges should be investigated. Nevertheless, the current evidence of efficacy for zinc lozenges, in particular zinc acetate lozenges, is so strong that common cold patients may be encouraged to try them.
Although high-dose zinc acetate lozenges can substantially shorten the duration of various common cold symptoms, many zinc lozenges on the US market either have too low a dose of zinc or contain ingredients that tightly bind to zinc ions, such as citric acid.
Greater reductions in the duration of symptoms occurred with higher doses than with lower doses, and zinc acetate reduced the duration of symptoms whereas other formulations showed no effect.
Evidence strength: Strong evidence from multiple randomized, placebo-controlled trials and meta-analyses that zinc lozenges at >75 mg/day elemental zinc, begun within 24 hours of symptom onset, reduce cold duration. Evidence is specific to lozenge formulations; evidence for zinc syrup or tablets is weaker and inconsistent.
5.3 Vitamin D
Traditional Use
Vitamin D is a fat-soluble nutrient produced endogenously upon sun exposure. Populations living at high latitudes and those with limited sun exposure have historically exhibited higher rates of respiratory infections during winter months, leading to speculation about a vitamin D–immune connection long before scientific investigation.
Scientific Evidence
Vitamin D deficiency is associated with an increased risk of infections of bacterial and viral origin through decreased innate immunity. Vitamin D's role in regulating the immune system has led scientists to explore whether vitamin D deficiency contributes to autoimmune diseases, and whether vitamin D supplements could help boost the body's defenses against infectious disease, such as tuberculosis and seasonal flu.
Previous RCTs and meta-analyses have reported inconsistent findings about the preventive effect of vitamin D supplements on acute respiratory infections. One systematic review in 2015 concluded that there was no efficacy of vitamin D supplementation for the prevention of childhood ARIs. Patients with vitamin D deficiency and those not receiving bolus doses experienced the most benefit in trials where benefit was observed.
Evidence strength: Preliminary and mixed. Observational data link low vitamin D status to greater infection susceptibility. Multiple RCTs and meta-analyses show inconsistent results; the strongest signal is among individuals who are vitamin D–deficient, where supplementation may be most beneficial.
5.4 Echinacea
Traditional Use
Echinacea species (primarily E. purpurea, E. angustifolia, and E. pallida) were used extensively by several North American Indigenous peoples for the treatment of colds, infections, and sore throats before being adopted into Western herbal medicine in the late 19th century. Echinacea has antiviral activity against influenza viruses in vitro and has traditionally been used for treatment of colds and flu.
Scientific Evidence
Previous reviews suggest that echinacea might have a preventative benefit for upper respiratory tract infections, including the common cold, but whether any reported effect is clinically meaningful is debatable. Elderberry may reduce the duration and severity of symptoms in individuals taking elderberry as a treatment for influenza-like symptoms.
A 2023 systematic review and analysis concluded that echinacea may improve cold symptoms; however, the quality of the evidence was low to moderate. Taking echinacea for a short amount of time is probably safe, but some people may experience gastrointestinal side effects.
Various studies demonstrated that use of this plant may be a complementary treatment of respiratory tract infections. In a randomized, double-blind, placebo-controlled trial, echinacea reduced the total number of cold episodes, cumulated episode days within the group, and pain-killer medicated episodes; inhibited virally confirmed colds; and especially prevented enveloped virus infections. It showed maximal effects on recurrent infections, and preventive effects increased with therapy compliance.
The efficacy of Echinacea species is dubious based on identified studies; over 2,000 persons were given the treatment, but equal amounts of studies of good quality found positive and negative results.
Recent NCCIH-sponsored research suggests that the activity of echinacea extracts is influenced by soil conditions that affect the plant's bacterial community.
Evidence strength: Low to moderate. Results across clinical trials are mixed, with heterogeneity attributed to different species, plant parts, extraction methods, and dosing regimens. Echinacea purpurea is the most consistently studied species and shows the most promising results.
5.5 Elderberry (Sambucus nigra)
Traditional Use
Black elderberry has a documented history of use in European folk medicine for fevers, colds, and influenza-like illnesses, with berries and flowers prepared as syrups, teas, and decoctions. It was used in this way across Central and Northern Europe for centuries prior to any formal scientific evaluation.
Scientific Evidence
Some preliminary research suggests that elderberry may relieve symptoms of flu or other upper respiratory infections. A small number of studies in people have evaluated elderberry for flu and other upper respiratory infections; however, conclusive evidence from high-quality clinical trials is lacking. A 2020 review of studies concluded that although there is a hypothetical benefit of elderberry for the treatment of viral infections, there isn't enough conclusive evidence from high-quality clinical trials.
A 2019 meta-analysis of four studies found that supplementation with black elderberry (Sambucus nigra) was significantly effective at reducing the total duration and severity of upper respiratory symptoms, compared with placebo. A 2016 double-blind, randomized controlled trial evaluating elderberry in air travelers suggested that elderberry may reduce cold symptom duration and severity.
The studies were extremely small (three had fewer than 65 participants), short (follow-up periods of 16 days or less), and four out of five were funded by the manufacturer of the product being tested. The largest study in the meta-analysis did not test elderberry alone, but rather a product containing a mix of ingredients, including echinacea. Several studies also had problems with selective reporting of outcomes and unreliable research methods.
Evidence strength: Preliminary. Available trials show a directional benefit for symptom duration and severity, but are small, short, and methodologically limited. High-quality, independent trials are lacking.
5.6 Garlic (Allium sativum)
Traditional Use
Garlic (Allium sativum) has been used for centuries in traditional remedies for colds and support of the immune system. It features in Ayurvedic medicine, traditional Chinese medicine, and European folk herbalism for respiratory ailments, typically consumed raw, cooked, or as a macerated preparation.
Scientific Evidence
One randomized trial randomly assigned 146 participants to either an allicin-containing garlic capsule or a placebo once daily for 12 weeks. The trial reported 24 occurrences of the common cold in the garlic intervention group compared with 65 in the placebo group (P < 0.001), resulting in fewer days of illness in the garlic group compared with the placebo group (111 versus 366). The number of days to recovery from an occurrence of the common cold was similar in both groups.
Only one trial met the inclusion criteria, therefore limited conclusions can be drawn. The trial relied on self-reported episodes of the common cold but was of reasonable quality in terms of randomisation and allocation concealment. Adverse effects included rash and odour. There is insufficient clinical trial evidence regarding the effects of garlic in preventing or treating the common cold.
Although the study was reported as a double-blind trial, proof of blinding was not provided. The active treatment was described as "an allicin-containing garlic supplement" dosed at "one capsule daily," with no further information on extraction methods, phytochemical composition, or amount of garlic.
Evidence strength: Very weak. Only a single qualifying human trial has been identified by the Cochrane review; results are promising but the evidence base is insufficient to support clinical recommendations.
5.7 Probiotics
Traditional Use
Fermented foods containing live cultures — such as yogurt, kefir, kimchi, and miso — have been part of traditional diets across multiple cultures for thousands of years and have been associated anecdotally with improved health and resistance to illness.
Scientific Evidence
In the 2015 Cochrane systematic review, probiotics were better than placebo in reducing the number of participants experiencing episodes of acute URTI, the mean duration of an episode of acute URTI, antibiotic use, and cold-related school absence. This indicates that probiotics may be more beneficial than placebo for preventing acute URTIs.
A subsequent updated Cochrane analysis found that the probiotics intervention was better than placebo or no treatment (rate ratio 0.79, 95% CI 0.71 to 0.89; P < 0.001; 2,032 participants).
Evidence strength: Moderate, with consistent but heterogeneous evidence from multiple RCTs and two Cochrane reviews. Results vary by strain, dose, and population; the overall quality of evidence is rated low to moderate by Cochrane reviewers due to methodological limitations in included trials.
5.8 Vitamin A
Traditional and Scientific Context
Reviews on vitamin A suggest that the effect on respiratory infections could differ by age and circumstance, and that excessive amounts could actually increase the risk of infection in children who are already healthy, without vitamin A deficiency or malnourishment.
Evidence strength: Preliminary and context-dependent. Evidence primarily supports vitamin A's role in maintaining mucosal barrier integrity; supplementation in non-deficient populations may not confer benefit and could carry risk.
5.9 Other Natural Ingredients Under Investigation
Honey
Honey has been used across cultures for millennia as a remedy for sore throats and coughs. Complementary approaches that have shown some promise include honey as a nighttime cough remedy for children.
Ginseng
There is inconsistent evidence for the Asian ginseng (Panax ginseng) and North American ginseng (Panax quinquefolius) in the prevention of the common cold.
South African Geranium (Pelargonium sidoides)
Complementary health approaches that have been advocated for preventing or treating colds or influenza include South African geranium, among others. Clinical trials have evaluated root extract preparations (EPs 7630) for acute bronchitis and URTI, with some positive results, though evidence quality remains moderate.
6. Dietary and Lifestyle Factors
Overall Diet Quality
Nutrition is one of multiple factors that determines the immune response, and good nutrition is important in supporting the immune response. Immunity can be impaired in older people, particularly those who are frail, in those living with obesity, in those who are malnourished, and in those with low intakes of micronutrients. Nutritional inadequacy also allows dysregulated inflammation and oxidative stress, contributing to frailty and to poor outcome from infection.
Sleep Duration and Quality
Sleep quality is thought to be an important predictor of immunity and in turn susceptibility to the common cold. As described above, short sleep duration (fewer than 7 hours) is associated with substantially higher susceptibility to rhinovirus-induced cold in challenge studies.
Physical Activity
Physical activity was associated with decreased risk of respiratory illness after virus exposure in studies that tracked psychosocial and behavioral predictors of cold susceptibility. Host factors including age, BMI, physical activity, and diet contributed significantly to the relationship between immune cell activation markers and cold and flu status.
Smoking and Environmental Toxins
Environmental toxins such as smoke and other particles contributing to air pollution and excessive alcohol can impair or suppress the normal activity of immune cells. Smoking was identified as a factor associated with greater risk of respiratory illnesses after virus exposure in controlled challenge studies.
Hygiene Practices
A number of standard public health practices have been developed to help limit the spread and impact of respiratory viruses, such as regular hand washing, avoiding those showing symptoms of infection, and covering coughs. Hand hygiene reduces the spread of viruses that cause cold illnesses.
Social Connection
Social integration and social support were associated with decreased risk of respiratory illness in prospective studies where participants were exposed to rhinovirus or influenza virus under controlled conditions.
7. Evidence Summary Table
- Vitamin C (regular supplementation): Modestly reduces cold duration (8% adults, 14% children); does not prevent colds in the general population; may benefit those under extreme physical or cold stress. Evidence: Strong for duration reduction (Cochrane); weak for prevention in general population.
- Zinc lozenges (>75 mg/day elemental zinc): Reduces cold duration by approximately 33% when started within 24 hours of symptom onset. Evidence: Strong from multiple systematic reviews and meta-analyses; specific to lozenge form at adequate dose.
- Vitamin D: Deficiency associated with higher infection susceptibility; supplementation trials show mixed results; greatest benefit in deficient individuals. Evidence: Preliminary to moderate; inconsistent across RCTs and meta-analyses.
- Echinacea: May reduce cold episodes and duration; results mixed depending on species, preparation, and dose. Evidence: Low to moderate; heterogeneous across trials.
- Elderberry: May reduce symptom duration and severity during acute illness; does not appear to prevent colds. Evidence: Preliminary; trials are small, short, and often industry-funded.
- Garlic: One small qualifying RCT suggests fewer cold episodes during prophylaxis. Evidence: Very weak; only one eligible trial identified by Cochrane review.
- Probiotics: Multiple trials and Cochrane reviews support reduced URTI incidence and duration. Evidence: Moderate; quality of evidence rated low to moderate in Cochrane analyses.
- Vitamin A: Supports mucosal barrier integrity; excessive supplementation in non-deficient populations may increase infection risk. Evidence: Preliminary; context-dependent.
- Ginseng: Inconsistent evidence from clinical trials. Evidence: Weak and inconsistent.
References
- Common cold and influenza — PMC (Elsevier/COVID-19 resource)
- Understanding the symptoms of the common cold and influenza — The Lancet Infectious Diseases (2005)
- Understanding the symptoms of the common cold and influenza — PubMed (Eccles, 2005)
- Etiology of the common cold: Modulating factors — PMC
- Influenza virus-related critical illness: pathophysiology and epidemiology — PMC
- Influenza: Background, Etiology, Pathophysiology — Medscape
- Nutrition and immunity: lessons for COVID-19 — PMC
- Nutrition and Immunity — Harvard T.H. Chan School of Public Health, The Nutrition Source
- In vitro Production of IL-6 and IFN-γ is Influenced by Dietary Variables — PMC
- Optimal Nutritional Status for a Well-Functioning Immune System — PMC
- Vitamin C for preventing and treating the common cold — Cochrane (Hemilä & Chalker, 2013)
- Vitamin C reduces the severity of common colds: a meta-analysis — PMC
- Zinc Lozenges May Shorten the Duration of Colds: A Systematic Review — PMC
- Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate — PMC (Hemilä, 2017)
- Zinc for the treatment of the common cold: a systematic review and meta-analysis of RCTs — PMC
- The effectiveness of high dose zinc acetate lozenges on various common cold symptoms: a meta-analysis — PMC
- Zinc acetate lozenges for treating the common cold: an individual patient data meta-analysis — PMC
- The Common Cold and Complementary Health Approaches: What the Science Says — NCCIH/NIH
- Echinacea: Usefulness and Safety — NCCIH/NIH
- Colds, Flu, and Complementary Health Approaches — NCCIH/NIH
- Select Dietary Supplement Ingredients for Preserving and Protecting the Immune System: A Systematic Review — PMC
- Effect of an Echinacea-Based Hot Drink Versus Oseltamivir in Influenza Treatment — PMC
- Self-Care for Common Colds: The Pivotal Role of Vitamin D, Vitamin C, Zinc, and Echinacea — PMC
- Role of Vitamin D Deficiency in Increased Susceptibility to Respiratory Infections Among Children: A Systematic Review — PMC
- Efficacy of Vitamin D Supplements in Prevention of Acute Respiratory Infection: A Meta-Analysis — PMC
- Association Between Vitamin D and Influenza: Meta-Analysis and Systematic Review — Frontiers in Nutrition
- Garlic for the common cold — Cochrane (Lissiman et al., 2014)
- Garlic for the common cold — PMC (Cochrane Review full text)
- Viral Upper Respiratory Infection — PMC/NIH
- Probiotics for preventing acute upper respiratory tract infections — Cochrane (Hao et al., 2015)
- Probiotics for preventing acute upper respiratory tract infections (updated Cochrane review) — PMC (Zhao et al., 2022)
- Sleep Habits and Susceptibility to the Common Cold — PMC (Cohen et al., 2009)
- Psychosocial Vulnerabilities to Upper Respiratory Infectious Illness — PMC
- Echinacea and elderberry — should they be used against upper respiratory tract infections during pregnancy? — PMC
- Complementary and Integrative Health Approaches to Travel Wellness — CDC Yellow Book
Go back