We all suffer from colds at some time but recent research indicates that a person’s level of physical activity influences their risk of respiratory tract infections such as the common cold, most likely by affecting immune function. Moderate levels of regular exercise seem to reduce our susceptibility to illness compared with an inactive lifestyle, but long hard bouts of exercise and periods of intensified training put athletes at increased risk of colds and ‘flu.
Immune function and infection risk
Infections of the nose, throat, windpipe (trachea) or the two airways that branch from the trachea as it reaches the lungs (bronchi) are the most common infections that people get. These upper respiratory tract infections (URTIs) include the common cold, sinusitis and tonsillitis. More than 90% of URTIs are due to an infection with a virus: the most common ones being human rhinovirus, adenovirus and coronavirus. The more severe and feverish symptoms of ’flu are caused by an influenza virus. The average adult has two to three URTIs each year and young children have twice as many. We are constantly exposed to the viruses that cause these infections, but some people seem more susceptible to catching URTIs than others. Every day our immune system protects us from an army of pathogenic microbes that bombard the body. Immune function is influenced by an individual’s genetic make-up as well other external factors such as stress, poor nutrition, lack of sleep, the normal ageing process, lack of exercise or excessive training . These factors can suppress the immune system, making a person more vulnerable to infection.
Exercise and its effect on the immune system
Exercise can have both a positive and negative effect on the functioning of the immune system and can influence a person’s vulnerability to infection. Researchers have found a link between moderate regular exercise and reduced frequency of URTIs compared with a sedentary lifestyle and also with excessive amounts of exercise and an increased risk of URTIs. A one-year study of over 500 adults found that participating in 1-2 hours of moderate exercise per day was associated with a one third reduction in the risk of getting a URTI compared with individuals that had an inactive lifestyle. Other studies have shown that when 40 minutes of moderate exercise is repeated on daily basis there is a cumulative effect that leads to a long-term improvement in immune response. A study on over 1000 people showed that individuals who exercise two or more days a week have half as many days off school or work due to colds or ‘flu as those who don’t exercise. Other factors that were correlated with a reduction in URTI risk included a high intake of fruit, being married, being male, having a moderate or high level of fitness and having a low level of mental stress.
However, more is not always better in terms of exercise volume as other studies have reported a two-to-six fold increase in risk in developing an URTI in the weeks following marathon (42.2 km) and ultra-marathon (90 km) races. A ‘J’-shaped model (Figure 1) has been used to describe the relationship between the amount of regular physical activity that is undertaken and risk of URTI. The increased sensitivity to infection with high levels of exercise may be due to a depression in immune system function of the individual. Studies have shown that prolonged strenuous bouts of exercise cause a temporary suppression of various immune cell functions and that performing such exercise on a regular basis with limited recovery can result in a longer lasting and more severe depression of immunity. This effect is likely due, in part, to increased levels of stress hormones like adrenaline and cortisol and anti-inflammatory cytokines such as interleukins 6 and 10 that suppress white blood cell functions. After strenuous exercise, athletes enter a brief period of time in which they experience weakened immune resistance and are more susceptible to viral and bacterial infections, in particular URTIs. Another problem for athletes is that their exposure to pathogenic (disease causing) microorganisms in the environment may be higher than normal due to increased rate and depth of breathing during exercise (increasing exposure of the lungs to airborne pathogens), exposure to large crowds and frequent foreign travel. Some of the reported sore throats may not be due to infectious agents but to non-infectious airway inflammation caused by allergies or inhalation of pollutants. However, whatever the cause of respiratory illness symptoms, the worry for athletes is that exercise performance is almost always impaired.
Most people are more susceptible to colds in winter but numerous studies on athletes indicate that they tend to be most susceptible to picking up infections at times close to competition. This usually follows a period of intensive training and added mental stress with the anxiety of wanting to perform well. Even a mild infection can impair an athlete’s ability to perform at the highest level. Preventing infections is therefore very important to them and they can help themselves by ensuring good personal hygiene, good nutrition and minimizing other life stresses. Certain nutritional strategies have also been shown to limit immune depression during exercise and/or reduce URTI risk in athletes. These strategies include avoiding micronutrient deficiencies, ingesting carbohydrate during exercise and taking flavonoid and probiotic supplements.
Probiotic supplements to reduce infection risk
Probiotics are food supplements that contain live microorganisms which when administered in adequate amounts can confer a health benefit on the host. There is now a reasonable body of evidence that regular consumption of probiotics strains that are proven to survive gut transit can modify the population of the gut dwelling bacteria (microbiota) and influence immune function though it should be noted that such effects are dose- and strain-dependent. Probiotics modify the intestinal microbiota such that the numbers of beneficial bacteria increase and usually numbers of species considered harmful are decreased. Such changes have been associated with a range of potential benefits to the health and functioning of the digestive system, as well as modulation of immune function.
Probiotic mechanisms of action
Probiotics have several mechanisms of action. By their growth and metabolism, they help inhibit the growth and reduce potentially harmful effects of other bacteria, antigens, toxins and carcinogens in the gut, but in addition, probiotics are known to interact with the gut-associated lymphoid tissue, leading to positive effects on the innate and even the acquired immune system.
Studies have shown that probiotic intake can improve rates of recovery from rotavirus diarrhoea, increase resistance to enteric pathogens, and promote anti-tumour activity. Some evidence suggests that probiotics may be effective in alleviating some allergic and respiratory disorders in young children. In recent years it has become evident that some probiotics, particularly Lactobacillus (L.) strains, when ingested on a daily basis, can reduce upper respiratory tract infection (URTI) incidence in adults.
Probiotic intervention studies in athletes
Although there are few published studies of the effectiveness of probiotic use in athletes, there is growing interest in examining their potential to help to maintain overall general health, enhance immune function or reduce URTI incidence and symptom severity/duration (West et al., 2009).
In a double-blind, placebo-controlled, cross-over trial in which healthy elite distance runners received the probiotic L. fermentum or placebo daily for 28 days with a 28-day washout period between the initial and the second treatment, athletes (n=20) suffered fewer days of respiratory illness and lower severity of respiratory illness symptoms when taking the daily probiotic. The probiotic treatment also elicited a 2-fold greater change in whole-blood culture interferon-γ production compared with placebo, which may be one mechanism underpinning the positive clinical outcomes. In another study of athletes who presented with fatigue, impaired performance and a deficit in blood CD4+ (T-helper) cell interferon-γ production compared with healthy control athletes, this apparent T cell impairment was reversed following a one-month course of daily probiotic (L. acidophilus) ingestion.
In a study on the effect of a L. casei probiotic supplement on URTI and immune and hormonal changes in soldiers participating in three weeks of commando training followed by a five day combat course, no difference in infection incidence between groups receiving daily probiotic or placebo was reported. The study duration was probably too short for that but did show that the probiotic was associated with better maintenance of salivary immunoglobulin A (IgA) levels: saliva IgA decreased after the combat course in the placebo group, with no change over time in the probiotic group. A recent randomised, placebo controlled trial in 58 Loughborough University athletes reported a lower incidence of URTI episodes during a 4-month winter training period in subjects receiving daily probiotic (L. casei Shirota) compared with placebo and this study also reported better maintenance of salivary IgA in the probiotic group. Importantly, in both athlete and non-athlete populations, falls in saliva IgA have been associated with increased URTI incidence. Another recent study using L. fermentum reported reduced URTI incidence among male but not female athletes during 11 weeks of training.
From the research reviewed here, one cannot reach a solid conclusion of probiotic benefit for sportspeople. Nevertheless, there is now sufficient understanding of the mechanism of action of certain probiotic strains, and enough evidence from trials with highly physically active people to signify that this is a promising area of research with mostly positive indications at present. To date most studies of probiotic interventions in athletes have been relatively small scale and some large scale, double-blind, placebo-controlled trials are needed to confirm likely probiotic benefits for athletes. At Loughborough we are currently conducting such a clinical trial in 250 athletes.
The message from current research is that moderate exercise has a positive effect on the immune system. So to keep colds at bay we should all go out for a brisk walk or participate in sports at least several times per week. Being active on a regular basis also comes with other health benefits including a reduced risk of developing metabolic diseases (e.g. type 2 diabetes) and cardiovascular diseases (e.g. coronary heart disease) later in life. The higher infection risk that comes with very high (some might say excessive) levels of exercise is the small price that athletes pay for being a potential Olympic gold medallist. There is accumulating evidence that the daily ingestion of Lactobacillus probiotics can reduce infection incidence in people with stressful lives like athletes.
Michael Gleeson, Professor of Exercise Biochemistry, School of Sport, Exercise and Health Sciences, Loughborough University, UK
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