An immediate concern of everyone at the moment is the risk of becoming infected with the coronavirus and many questions are asked about a role for nutrition, many supplements are promoted to "boost" immunity. But if we really want to protect ourselves, what should we do. I asked a real expert in Exercise Immunology: Prof Mike Gleeson.

Coronavirus, the flu and common colds

An immediate concern of everyone at the moment is the risk of becoming infected with the coronavirus and as it is still winter in the northern hemisphere there is also the increased risk of picking up other respiratory tract infections such as influenza and the common cold which are also caused by viruses. The symptoms of these illnesses have some similarities but also some differences: Infection with coronavirus is characterized by having a fever and developing a dry tickly cough; having a sore throat and a runny nose are less common. Influenza also causes a fever, aching joints and has symptoms similar to (but usually more severe than) the common cold which include a runny nose, sore throat and sneezing. We generally do not get a fever with the common cold. Most people who get these infections will recover within one to two weeks but they make you feel weaker, tired and generally unwell while symptoms are present. Among the elderly, the infection can be more debilitating, and they have an increased risk of developing more serious complications such as chest (lung) infections, including bronchitis and pneumonia which can be fatal particularly in people with underlying medical conditions such as hypertension, heart disease, lung disease and diabetes.

What is our susceptibility influenced by?

Our immune system protects us against the viruses that cause these infections, but because there is a genetic influence on the efficacy of our immune systems, some people are more prone to infections than others. However, our susceptibility to common infectious diseases is also influenced by what we eat, how much exercise we do, and how well we sleep. In addition, other lifestyle behaviors such as good personal hygiene practices can help to reduce our risk of picking up infections. This article explains the various nutritional, behavioral, and lifestyle strategies that we can easily implement to help minimize our risk of respiratory tract infections.

There are two main factors that influence our chance of picking up a respiratory tract infection: one is the degree of exposure to pathogens like coronavirus and the other is the status of our immune system. We can reduce our risk of infection by doing various things that limit the transmission of infections and there are several behavioral and nutrition strategies that we can do to help make our immune systems more robust.

How can we limit the transmission of infections?

Some practical guidelines for limiting transmission of infections among people are listed below. The most important of these are good hand hygiene and avoiding contact with persons that are infected. Hand washing (using the correct technique for about 30 seconds to ensure all parts of hands are cleaned effectively) with soap and water is effective against most pathogens but does not provide continuous protection. Hand gels containing a minimum of 60% alcohol disinfect effectively, but the protection they provide does not last more than a few minutes, so they need to be applied frequently, and this can cause dry skin and irritation. Other sanitization methods include the use of non-alcohol based antimicrobial hand foams that contain cationic biocides and hydrophobic polymers which are claimed to disinfect hands for up to six hours. However, individuals need to be aware that these products are removed by hand washing and excessive sweating, therefore they also need to be reapplied every few hours.

Behavioral and lifestyle strategies to limit transmission of infections:

• Minimize contact with infected people, young children, animals, and contagious objects.

• Avoid crowded areas and shaking hands.

• Keep your distance from people who are coughing, sneezing, or have a ‘runny nose’, and when appropriate, wear (or ask them to wear) a disposable mask.

• Wash hands regularly and effectively with soap and water, especially before meals, and after direct contact with potentially contagious people, animals, blood, secretions, public places, and bathrooms.

• Use disposable paper towels and limit hand to mouth/nose contact (putting hands to eyes and nose is a major route of viral self-inoculation).

• Carry anti-microbial foam/cream or alcohol-based hand-washing gel with you.

• Do not share drinking bottles, cups, cutlery, towels, etc. with other people.

• When abroad, choose cold beverages from sealed bottles, avoid raw vegetables and undercooked meat. Wash and peel fruit before eating.

Nutritional, behavioral, and lifestyle strategies

The other things that people can do to limit their risk of infection are to adhere to some practical guidelines to maintain robust immunity and limit the impact stress which is known to impair the functioning of our immune system and increase susceptibility to respiratory infections. These guidelines are listed below and relate mostly to nutritional, behavioral, and lifestyle strategies and are based on the findings of numerous research studies. The most effective nutritional strategies to maintain robust immune function are to avoid deficiencies of essential micronutrients, ingest Lactobacillus probiotics on a daily basis, and eat plenty of fruit and vegetables. Probiotics are live bacteria which when ingested in adequate amounts, modify the bacterial population (known as the microbiota) that inhabits our gut and modulate immune function by their interaction with the gut-associated lymphoid tissue, leading to positive effects on the systemic immune system. Some well-controlled studies in children, adults, endurance athletes, and the elderly have indicated that daily probiotic ingestion results in fewer days of respiratory illness and lower severity of infection symptoms.

Polyphenols and supplements

Some studies suggest that regular consumption of fruits and plant polyphenol supplements (e.g., quercetin) or beverages that contain high amounts of polyphenols (e.g., non-alcoholic beer and green tea) can also reduce common cold incidence. Ensuring that the individual has adequate vitamin D may also be helpful, and supplementation with vitamin D3 (1,000-4,000 IU/day or 25-100 micrograms (µg)/day) may be warranted for some people, especially in the winter months for those living at latitudes of 48°North (equivalent to Paris, France and the USA-Canada border) and above since the skin is unable to form vitamin D between the months of October through to March because the sunlight is not strong enough. Many other nutrition supplements, including β-glucan, echinacea, glutamine, colostrum and others, are on sale with claims that they can boost the immune system, but the scientific evidence that any of these are effective in preventing infections is not compelling. When respiratory illness symptoms begin, there is some evidence that taking zinc lozenges (>75 mg zinc/day; high ionic zinc content) or certain herbal supplements (e.g., echinacea, ginseng, kaloba) can reduce the number of days that illness symptoms last for. However, these may not be any more effective than treating illness symptoms with over-the-counter cold remedies.

Nutritional and behavioral strategies to help maintain robust immunity

• If you participate in regular exercise, avoid very prolonged training sessions (longer than two hours) and excessive periods of intensified training as this can depress your immunity.

• Wear appropriate outdoor clothing in inclement weather, and avoid getting cold and wet after exercise.

• Get adequate sleep (at least seven hours per night is recommended). Missing a single night of sleep has little effect on immune function at rest or after exercise, but respiratory illness episodes are more prevalent in those who regularly experience low sleep quantity (less than seven hours per night) and poor sleep quality (frequent awakenings).

• Keep other life stresses to a minimum.

• Ensure adequate dietary energy, protein, and essential micronutrient intake.

• Vitamin D plays an important role in promoting immunity, and this is a concern as vitamin D insufficiency is common in people especially in situations where exposure to natural sunlight is limited (e.g., during the winter months or when living or working mostly indoors). A vitamin D3 supplement (1,000-4,000 IU/day or 25-100 µg/day) may be beneficial to optimize immune function from October to April in Northern hemisphere countries.

• Avoid crash dieting and rapid weight loss. Care should be taken to ensure adequate protein (and micronutrient) intakes during periods of intentional weight loss, as individuals undergoing weight reduction are likely to be more prone to infection. In general, a broad-range multivitamin/mineral supplement is the best choice to support a restricted food intake, and this may also be suitable when travelling abroad in situations where food choices and quality may be limited.

• Eat several different fruits daily at least five times per week as regular fruit intake is associated with a lower incidence of the common cold.

• If you plan to do a prolonged (90 minutes or more) moderate to high intensity exercise session, ensure adequate carbohydrate intake before and during exercise in order to limit the extent and severity of exercise-induced immune depression. Ingesting about 40 g carbohydrate per hour of exercise during prolonged workouts maintains blood sugar levels and lowers circulating stress hormones and so helps to limit immune function depression. A 500 mL bottle of a sports drink usually contains 30-40 g of carbohydrate.

• The consumption of beverages during exercise not only helps prevent dehydration (which is associated with an increased stress hormone response) but also helps to maintain saliva flow rate during exercise. Saliva contains several proteins with antimicrobial properties including immunoglobulin A, lysozyme, a-amylase, and defensins. Saliva secretion usually falls during exercise, but regular fluid intake (water is fine) during exercise can prevent this.

• The efficacy of most so-called dietary immunostimulants has not been confirmed. However, there is limited evidence that some flavonoids (e.g., quercetin at a dose of 1 g/day) or flavonoid containing beverages (e.g., non-alcoholic beer, green tea), and Lactobacillus and/or Bifidobacterium probiotics (daily doses of ~1010 live bacteria) can reduce respiratory infection incidence in physically active people or those under stress. Another potential benefit of probiotics could be a reduced risk of gastrointestinal infections – a particular concern when travelling abroad.

• High daily doses (up to 1000 mg) of vitamin C are not generally justified, but individuals engaged in intensive training and/or cold environments may gain some benefit for preventing respiratory infections.

• Avoid strenuous exercise for a few days when experiencing upper respiratory symptoms like sore throat, sneezing, runny, or congested nose. Avoid all exercise other than walking when experiencing symptoms like muscle/joint pain and headache, a chesty cough, fever (indicated by a resting body temperature of 38-40°C), and generalized feeling of malaise, diarrhea, or vomiting. Some light exercise like walking may be beneficial to help avoid fluid accumulation in the lungs which increases the risk of developing dangerous complications like pneumonia.

• You may be asked to self-isolate for up to 14 days if you experience respiratory illness symptoms and this advice is generally for the protection of others by limiting virus transmission. After your symptoms resolve, start to do some light to moderate exercise around the home and the garden.

Professor Michael Gleeson

This article was written by Professor Michael Gleeson of Loughborough University who is a past president of the International Society of Exercise and Immunology and has many years of research experience in keeping athletes healthy. He retired 4 years ago but is still an active science writer and he has recently released an evidence-based healthy lifestyle guidebook for the general public entitled Eat, Move, Sleep, Repeat published by Meyer and Meyer Sport. Much of the information in this article and additional advice about how to eat healthily, improve sleep quality, lose weight, maintain healthy senses, tissues and organs and exercises to improve health and avoid both acute and chronic illness can be found in his book. Eat, Move, Sleep, Repeat is available in paperback or as an eBook from Amazon.